Anthrax

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Anthrax (Greek Άνθραξ for coal) is an acute infectious disease in humans and animals that is caused by the bacterium Bacillus anthracis and is highly lethal in some forms. Anthrax is one of only a few bacteria that can form long lived spores. When the bacteria s life cycle is threatened by factors such as lack of food caused by their host dying or by a change of temperature, the bacteria turn themselves into a dormant spores to wait for another host to continue their life cycle.

When breathing, ingesting or getting anthrax spores in a cut in the skin, these spores reactivate themselves and multiply in their new host very rapidly. The anthrax spores in the soil are very tough, can live many decades and are known to occur on all continents except Antarctica. Anthrax most commonly occurs in wild and domestic grass eating mammals (ruminants) who ingest or breathe in the spores while eating grass. Anthrax can also infect humans when they are exposed to dead infected animals, eat tissue from infected animals, or are exposed to a high density of anthrax spores from an animal’s fur, hide, or wool. Anthrax spores can be grown outside the body and used as a biological weapon. Anthrax cannot spread directly from human to human; but anthrax spores can be transported by human clothing, shoes, among others. If a person dies of anthrax their body can be a very dangerous source of anthrax spores. The word anthrax is the Greek word for coal, the germ’s name is derived from anthrakitis, the Greek word for anthracite, in reference to the black skin lesions victims develop in a cutaneous skin infection.

Anthrax, caused by Bacillus anthracis, is thought to have originated in Egypt around 1250 BC. Described as being a disease affecting horses, camels and sheep, anthrax had an impact on great civilizations, such as the Greek and Roman. It was described clinically for the first time by Maret in 1752 and Fournier in 1769. In 1877, based upon his studies with Bacillus anthracis, Robert Koch was able to demonstrate what became known as Koch’s postulates. In 1881, Louis Pasteur worked to create a vaccine for anthrax, which he was able to test with success in animals. In 1900, due to the great amount of knowledge gathered during the 1800s, anthrax cases were well documented in the US, UK and Germany. In 1944, penicillin was first used to treat anthrax. The first commercial vaccine to prevent anthrax in humans was created in 1950s. In the past 10 years there have been a few reported cases in the US, specifically in 2006 in NYC, 2009 in Connecticut and in 2011 in Florida. Anthrax has also been used throughout history as a biologic weapon and there has been efforts to create and enforce legislation to avoid disastrous outbreaks of the disease. For that, a Convention on the Prohibition of the Development, Production, and Stockpiling of Biological and Toxin Weapons and on Their Destruction was created and later ratified in April of 1972, with more than 100 nations signing it, including Iraq, the United States, and the Soviet Union.

The genetic material of Bacillus anthracis is coded within 1 chromosome and 2 plasmids, which are fundamental for its toxicity. The spores of B. anthracis are the infectious form and can remain dormant in the environment for decades. The disease may be transmitted through the skin, gastrointestinal or respiratory systems. The bacterium causes disease through 2 mechanisms: toxemia and bacterial infection.^[1] B. anthracis begins to produce toxins within hours of germination.^[2] Protective antigen (PA) and edema factor (EF) combine to form edema toxin (ET), and PA and lethal factor (LF) combine to form lethal toxin (LT), the active toxins. Bacterial toxins have a direct cytotoxic effect by interfering with cellular pathways, being also responsible for weakening the immune system, so the initial systemic infection may occur. Anthrax lesions at any site are characterized by lymphadenopathy, extensive edema, necrosis and confluent exudate containing macrophages and neutrophils. If not stopped, the infection may affect different organs, causing septicemia and potentially death.

The causative agent of anthrax is B. anthracis, a nonmotile, Gram-positive, aerobic or facultatively anaerobic, endospore-forming, rod-shaped bacterium. The spores of B. anthracis, which can remain dormant in the environment for decades, are the infectious form, but this vegetative B. anthracis rarely causes disease.^[3] The Bacillus may enter the body through the skin, lungs, gastrointestinal system or by injection, after which they will travel to the lymph nodes. The virulence factors will facilitate the translocation of the toxins to the cytosol. The natural reservoirs of Bacillus anthracis include humans, mammals, herbivores, reptiles, and birds.

The differential diagnosis of anthrax includes a wide range of infectious and non-infectious conditions. Depending on the mode of anthrax exposure in the patient (cutaneous, ingestion, inhalation or injection), there will be different forms of the disease.^[4] A history of exposure to contaminated animal materials, occupational exposure, and living in an endemic area, is crucial when considering the diagnosis of anthrax. Additional tests to isolate Bacillus anthracis are required to differentiate anthrax from other diagnoses, thereby confirming the correct etiologic agent.

Incidence of the natural disease in humans is dependent on the level of exposure to affected animals and, for any one country, national incidence data for non-industrial cases reflect the livestock situation. Human case rates for anthrax are highest in Africa and Central and Southern Asia. While, statistically, in Northern Europe and countries with similar epidemiological situations, there is one human cutaneous case per every 10 livestock carcasses butchered, there can be some 10 human cutaneous and enteric cases per single carcass butchered in Africa, India, and the Southern Russian Federation.^[4]

Risk factors for contracting anthrax include handling of livestock or livestock products, playing animal hide drums, working in a laboratory researching anthrax, and traveling to an endemic region such as Central and South America, Sub-Saharan Africa, Central and southwestern Asia, Southern and eastern Europe, or the Caribbean. Risk factors for anthrax in the setting of bioterrorism are: working as a mail handler, military personnel, or response worker.^[4]

The natural history of anthrax depends on the mode of exposure to the disease (cutaneous, ingestion, inhalation, or injection). In cutaneous anthrax, a small painless skin sore develops into a blister and later into a skin ulcer, with a central black area. The resolution of the lesion takes several weeks, depending on its size, location and severity. The anthrax lesions might lead to scarring and contractures. Inhalation anthrax is characterized by a mild initial phase of nonspecific symptoms, that is followed by sudden development of dyspnea, cyanosis, disorientation with coma, and death.^[4] In oropharyngeal anthrax, the lesion is generally localized in the oral cavity. This type may progress rapidly, and edema around the lymph nodes may result in extensive swelling of the neck and anterior chest wall.^[4] The gastrointestinal anthrax lesions may occur anywhere within the gastrointestinal tract, potentially bleeding, and lead to fatal hemorrhage. Some cases are complicated by massive ascites,shock and death.^[4] The prognosis of anthrax depends on the form of the disease, how early it is diagnosed, the strain of bacteria, the patient’s age and his health condition. Pulmonary anthrax has the highest mortality rate.^[5]

The symptoms of anthrax infection depend on the mode of anthrax exposure in the patient (cutaneous, ingestion, inhalation, injection). The cutaneous type of anthrax is characterized by a skin blister that evolves into an ulcer with a black center, muscle pains, fever, and vomiting. The gastrointestinal type may include symptoms of fever, chills, sore throat, painful swallowing, and abdominal pain. The symptoms of the inhalation type of anthrax are fever, chills, fatigue, sore throat, and shortness of breath. The symptoms of the injection type are usually similar to those of cutaneous anthrax; however, the disease may spread through the body faster. The symptoms of the injection type of anthrax include fever, chills, skin ulcer, and subcutaneous or muscular abscess. A history of exposure to contaminated animal materials, occupational exposure, and living in an endemic area is crucial when considering a diagnosis of anthrax.

The physical findings of anthrax infection depend on the mode of anthrax exposure in the patient (cutaneous, ingestion, inhalation, injection). Common findings associated with cutaneous anthrax infection include fever, tachycardia, skin rash with formation of a typical scar, edema and lymphadenopathy; with gastrointestinal anthrax infection include fever, tachycardia, mucosal ulcer and edema in case of oropharyngeal lesion and edema and pallor in more severe cases; with inhalation anthrax infection includes: fever, tachycardia, bradypnea in severe cases, pallor, cyanosis and decreased heart and lung sounds in the presence of pleural effusion; and with anthrax infection due to injection include fever, typical skin scar at the site of injection, edema and subcutaneous and/or muscular abscess.

Initial evaluation of patients suspected of having anthrax should be similar to the standard evaluation for patients with an acute febrile illness and should have an emphasis on obtaining pre-treatment blood and other appropriate cultures.^[6] When systemic anthrax is present, abnormalities in laboratory tests include anemia, thrombocytopenia, and leukocytosis particularly in the latter stages of the disease. Other laboratory findings are hyponatremia, increased BUN, elevated transaminase levels, hypoalbuminemia, and elevated troponin. Cell cultures from the initial skin lesion, blood, CSF, or pleural fluid can identify the organism and possibly the toxins. In injection anthrax, the typical laboratory finding is an inflammatory pattern with a low CRP. A normal PT/PTT at admission does not exclude coagulopathy nor DIC.

Chest X-ray is a sensitive diagnostic test for inhalation anthrax. Chest X-ray abnormalities associated with inhalation anthrax include mediastinal widening, paratracheal fullness, pleural effusions, parenchymal infiltrates, and mediastinal lymphadenopathy.

The chest CT scan findings in anthrax include mediastinal widening, hyperdense lymph nodes, and edema of the mediastinal fat.

Several studies are used for the diagnosis and monitoring of anthrax. The polymerase chain reaction (PCR) test is ordered to confirm the virulence of the organism. In addition, lumbar puncture should be performed on admission when it is not contraindicated to search for the organism in the cerebrospinal fluid (CSF) and to to exclude other alternative diagnoses. Other diagnostic studies include an electrocardiogram and an echocardiogram to assess possible complications of anthrax such as atrial fibrillation and pericardial effusion.

Medical therapy of anthrax infection includes antibiotic and antitoxin drugs. Uncomplicated cutaneous anthrax is treated with a single oral antimicrobial drug for a duration of 7-10 days for naturally acquired anthrax and 60 days for bioterrorism-related exposure. In case of systemic anthrax without meningitis, the initial treatment should include ≥2 antimicrobial drugs for ≥2 weeks or until the patient is clinically stable, whichever is longer. In case of systemic anthrax with suspected or confirmed meningitis, the initial treatment should include ≥3 antimicrobial drugs for ≥2 weeks or until the patient is clinically stable, whichever is longer. Once patients with systemic illness who were exposed to aerosolized spores have completed initial combination treatment, they should be transitioned to single-agent oral treatment to prevent relapse from surviving B. anthracis spores. These patients should be monitored at all times to evaluate the need for supportive care measures, such as hemodynamic support, mechanical ventilation, corticosteroids, procedures, and surgical interventions in certain occasions. An antitoxin should be added to combination antibiotic treatment. Currently there are 2 antitoxins in the CDC Strategic National Stockpile: raxibacumab and Anthrax Immune Globulin Intravenous (AIGIV).^[6]

Prevention of anthrax infection can be achieved with post-exposure prophylaxis antibiotics and vaccination. The US Advisory Committee on Immunization Practices recommended 60 days of antibiotic drug prophylaxis for immediate protection and a 3-dose series of Anthrax Vaccine Adsorbed (AVA) for long-term protection. Postexposure prophylaxis of asymptomatic persons should start as soon as possible after exposure because its effectiveness decreases with delay in implementation. Everyone exposed to aerosolized B. anthracis spores should receive a full 60 days of post-exposure prophylaxis antimicrobial drugs, whether they are unvaccinated, partially vaccinated, or fully vaccinated. Protective measures should also be implemented to prevent the transmission of the disease.^[7]

Given the morbidity and mortality associated with Anthrax infection, and the low cost of safe and effective antibiotics, current pharmacotherapy to treat anthrax is relatively cost-effective.

Given the safety and efficacy of current vaccines and antibiotics, there are not a large number of new agents under development to treat anthrax.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Anthrax, caused by Bacillus anthracis, is thought to have originated in Egypt around 1250 BC. Described as being a disease affecting horses, camels and sheep, anthrax had an impact on great civilizations, such as the Greek and Roman. It was described clinically for the first time by Maret in 1752 and Fournier in 1769. In 1877, based upon his studies with Bacillus anthracis, Robert Koch was able to demonstrate what became known as Koch’s postulates. In 1881, Louis Pasteur worked to create a vaccine for anthrax, which he was able to test with success in animals. In 1900, due to the great amount of knowledge gathered during the 1800s, anthrax cases were well documented in the US, UK and Germany. In 1944, penicillin was first used to treat anthrax. The first commercial vaccine to prevent anthrax in humans was created in 1950s. In the past 10 years there have been a few reported cases in the US, specifically in 2006 in NYC, 2009 in Connecticut and in 2011 in Florida. Anthrax has also been used throughout history as a biologic weapon and there has been efforts to create and enforce legislation to avoid disastrous outbreaks of the disease. For that, a Convention on the Prohibition of the Development, Production, and Stockpiling of Biological and Toxin Weapons and on Their Destruction was created and later ratified in April of 1972, with more than 100 nations signing it, including Iraq, the United States, and the Soviet Union.

Anthrax is thought to have originated in Egypt and Mesopotamia. Many scholars think that in Moses’ time, during the 10 plagues of Egypt, anthrax may have caused what was known as the fifth plague, described as a sickness affecting horses, cattle, sheep, camels and oxen. Ancient Greece and Rome were also well acquainted with anthrax, and this is illustrated in many of the ancient writings of the most famous scholars from those times. For example, many scholars think anthrax was depicted by Homer in the Iliad from 1230 BC and by Virgil in 70-90 BC. Some even suggest that anthrax may have contributed to the fall of Rome. The first clinical descriptions of cutaneous anthrax were given by Maret in 1752 and Fournier in 1769. Before this, anthrax had only been described through historical accounts.^[1]

In 1877, scientist Robert Koch studied Bacillus anthracis, the bacterium that causes anthrax. He discovered that the bacteria formed spores that were able to survive for very long periods of time and in many different environments. Koch decided to use anthrax bacteria in one of his most important historical experiments, in which he isolated and grew Bacillus anthracis in pure culture and injected animals with the bacteria. Using what he observed in this study, he described how the microbe he injected into the animals caused the disease. From these studies, he was also able to determine the life cycle of the anthrax bacteria and was able to demonstrate what became known as Koch’s postulates, which demonstrate a causal relationship between a specific microorganism and a disease.^[1]

During the 1800s, doctors saw cases of anthrax but did not yet have a diagnosis for the disease. During this time, the organism that causes anthrax had not yet been discovered, but doctors had noticed a link between the disease and the animal hair industry. Because of this, the disease became known as “wool sorters disease.” By the middle of the century, early researchers had associated the disease with the presence of rod-shaped bodies that were seen in the blood of infected animals. These bodies were eventually identified as bacteria and given the name Bacillus anthracis.^[1]

In 1881, Louis Pasteur, another prominent scientist, took Koch’s work a step further, trying to fully prove how anthrax was spread and how it made people or animals sick. Pasteur also worked to create a vaccine for anthrax. In his experiment, Pasteur gave 25 animals two shots of an anthrax vaccine he had created with weakened anthrax bacteria. After he gave both rounds of the vaccine to these animals, he injected them with live anthrax bacteria. He also injected live bacteria into 25 other animals that had not been vaccinated. Each of the vaccinated animals survived, while the 25 that were not vaccinated died.^[1]

Much knowledge was gained about anthrax in the 1800s. As a result, animal and human cases of anthrax in the United States, Britain, and Germany were well documented in the early 1900s. However, there were still places where anthrax cases hadn’t been documented, such as Russia, Asia, India and Africa. Because of the high number of contaminated animal products imported from these countries, however, it was known that anthrax had to be widespread in these regions.^[1]

In 1937, Max Sterne successfully created the anthrax live spore vaccine for animals. This vaccine is still used in animals in most countries. Because of the introduction of routine vaccination of animals against anthrax and the improvements in animal product processing procedures, the number of cases of anthrax in humans declined. This decline was so significant that during the entire 20th century there were only 18 cases of inhalation anthrax in the United States.^[1]

Penicillin had been discovered in 1928, but it wasn’t until 1944 when it was first used to treat anthrax. Penicillin became the drug of choice for treating anthrax, and it replaced all previous therapies, such as serum therapies and chemotherapies.^[1]

In 1950, the first anthrax vaccine for humans was created. This anthrax vaccine was tested in a group of goat hair mill workers. Volunteers were given either the vaccine or a placebo (a shot that does not have the vaccine in it). The volunteers were then followed over a 2-year period. This study determined that the vaccine was 92.5% effective in preventing cutaneous anthrax. After the study, the vaccine was made available to people working in goat hair processing mills in the United States.^[1]

In 1970, an updated human anthrax vaccine was released, replacing the 1950s vaccine. This is essentially the same vaccine used today.^[1]

In 2006, a drum-maker from New York City got sick while on tour with a dance troupe in Pennsylvania. He had just returned from Africa with four goat skins that he planned to use to make drums. He said that when he processed the goat skins to remove the hair, he did not use chemicals on the skins to kill germs or wear protection while handing the skins. He also reported that while he processed the skins, hair and dust particles floated into the air. Four days after he last had contact with the goat skins, he began having breathing problems and was hospitalized. Five days later he was diagnosed with inhalation anthrax. Public health investigators determined he had been exposed to anthrax while processing the goat skins he brought home from Africa. When he scraped the hair from the skins, the anthrax spores were released into the air and he breathed them in. The spores got into his lungs and caused him to become ill. It was the first time in 30 years that a case of naturally acquired anthrax was reported in the United States.^[1]

A woman in Connecticut was diagnosed with gastrointestinal anthrax. Public health investigators learned that the woman had participated in a drumming event the day before she became ill. The drums used at the event and the event space were all tested for contamination with anthrax spores. Two animal skin drums were found to have anthrax spores on them, and spores were also found in the room where the drumming took place, and in other rooms in the building. Investigators determined that the spores were released into the air while the contaminated drums were played. After 2 months in the hospital, the woman recovered and was released from the hospital.^[1]

In 2010, a small outbreak of anthrax occurred in the United Kingdom and Germany. All of the patients who came to the hospital were illicit drug users who had used heroin before having symptoms. Anthrax in these patients did not look like typical cutaneous anthrax. Many had swelling and infection of the deeper layers of skin but they didn’t have a raised sore with a black center – the tell-tale sign of cutaneous anthrax. Doctors recognized this anthrax as a new type of anthrax, calling it injection anthrax. Doctors wondered where the anthrax spores came from and how they were injected into the drug users. While no anthrax was found in the heroin itself, the evidence gathered by epidemiologists strongly suggested that was anthrax was in the heroin. Public health officials believe that the anthrax spores were in the heroin and that when the patients injected the drug into their bodies, they also injected anthrax spores.^[1]

A retired Florida man and his wife traveled for 3 weeks on a cross-country trip that took them through Wyoming, Montana and the Dakotas. They visited many state parks. The man got sick when they arrived in Minnesota. He went to the emergency room complaining of flu-like symptoms and was originally diagnosed with community-acquired pneumonia. A doctor, who had grown up on a cattle farm and was familiar with anthrax, felt that this diagnosis was not right and ordered more tests. The tests found bacteria in his blood that looked like anthrax bacteria. The samples of his blood were then sent to the Minnesota Public Health laboratory, where his anthrax illness was confirmed. Because the doctors at the hospital were able to quickly diagnose anthrax, the patient got treatment immediately, including a specialized antitoxin (anthrax immunoglobulin) rushed in by the Centers for Disease Control and Prevention. After 3 weeks in the hospital, the patient fully recovered and was sent home.^[1]

A case of naturally occurring inhalation anthrax is very rare in the United States, so to rule out any possible bioterrorism threats, the FBI was called in to investigate the case. The FBI determined that the man had inhaled the anthrax spores in a natural environment and there was no threat to anyone else.^[1]

In 1800, the work of scientist Robert Koch led to the development of more modern microbiology experiments. This increase in more sophisticated experiments also created the knowledge of how to grow and produce large stocks of specific germs.

In 1900, the first deliberate uses of anthrax as an act of aggression were recorded in the early decades of the 1900s, during World War I. Betwwen 1914 and 1918, there is evidence that the German army used anthrax to secretly infect livestock and animal feed traded to the Allied Nations by neutral partners. An example of this undercover biological warfare was the infection of Argentinian livestock intended for trade with the allied forces, resulting in the death of 200 mules in 1917 and 1918. In 1925, after the many chemical and biological horrors of WWI, a diplomatic attempt was made to limit the use and creation of this kind of warfare. The Geneva Protocol for the Prevention of the Use in War of Asphyxiating, Poisonous or other Gases and Bacteriological Methods of Warfare was created. This treaty was a great step in trying to stop the use of biologic agents during war. However, it did not specifically outlaw the research or production of biologic agents. Many countries agreed to the treaty but then created amendments to allow for use of biologic weapons during retaliation. After the Geneva Convention, interest in anthrax mostly focused on preventing disease in livestock and on improving the Pasteur vaccine.

In 1932, Japan began producing anthrax to be used as a weapon and conducted research with biological weapons in Japanese-occupied Manchuria. During this time, prisoners were infected with anthrax and other deadly diseases. It was later discovered that during this program, the Japanese attacked at least 11 Chinese cities with anthrax and other biological agents by spraying them directly onto homes from aircraft. In 1942, a bioweapons program was started in the United States. The United States conducted experiments with anthrax, among other biologic agents, at testing sites in Mississippi and Utah. More than 5,000 bombs were filled with anthrax in preparation for a response to any possible attacks from Germany. Great Britain also began to experiment with anthrax for bioweapons on a small island off the coast of Scotland called Gruinard Island. They tested the widespread release of anthrax by releasing bombs containing the germ over the island, where 80 sheep had been placed. All of the sheep died from anthrax. One of the most important findings from this experiment was how long anthrax stays in the environment after a release. The island remained uninhabitable until 1986, when Great Britain decided to decontaminate it by killing all of the anthrax spores. After a year of soaking the island in a mixture of formaldehyde and seawater, the island was considered disinfected.

In 1950, during the Korean War, U.S. bioweapon programs were expanded. This expansion included the creation of a program to develop vaccines and treatments to protect troops against biological agents.

In 1960, the United States possessed a large collection of bioweapons, including many types of bacteria, fungi, and toxins. During the late 1960s, there was growing concern, internationally, about the use of biological weapons and the ineffectiveness of the Geneva Protocol. In July of 1968, Great Britain submitted a proposal to the Committee on Disarmament of the United Nations, which would prohibit the development, production, and stockpiling of biological agents. This proposal also outlined the need for inspections for alleged violators. Several months later, The Warsaw Pact nations submitted a similar proposal. In 1969, President Nixon terminated the U.S. bioweapons program through an executive order. This executive order stopped offensive bioweapon research and production of the weapons, and it also called for destruction of the arsenal. The United States also adopted the policy to never use any biological or toxic weapons under any circumstances. After this, research efforts in the United States became solely directed toward the creation of defensive methods like vaccines, treatments, and diagnostic tests for potential biologic threats.

The 1972 Convention on the Prohibition of the Development, Production, and Stockpiling of Biological and Toxin Weapons and on Their Destruction was later created after the proposals of Great Britain and the Warsaw Pact nations. This treaty prohibited the development, possession, and stockpiling of pathogens or toxins. The treaty also required parties to destroy stockpiles of bioweapons within 9 months of signing the treaty. The treaty was ratified in April of 1972, with more than 100 nations signing it, including Iraq, the United States, and the Soviet Union.

Between 1971 and 1972, the United States destroyed pathogens and stockpiles of biologic weapons. Small amounts of certain pathogens were kept so they could be used to test new treatments and vaccines.

In April and May of 1979, an unusual outbreak of anthrax was reported in the city of Sverdlovsk, USSR.However, reports of this outbreak did not begin to surface in Western news until early 1980. Later that year, articles in Soviet medical, veterinary, and legal journals described the outbreak as naturally occurring in livestock, causing 96 cases of anthrax in humans. Of these cases, 79 were described as gastrointestinal anthrax, and 17 of them were cutaneous anthrax. Soviet officials reported that 64 of these 96 people died from gastrointestinal anthrax.

Internationally, there was a great debate about the data presented from this outbreak and its accuracy. Some speculated that the outbreak was not naturally occurring among livestock, but that it resulted from activities banned by the Biological Weapons Convention of 1972 (Convention on the Prohibition of the Development, Production, and Stockpiling of Biological and Toxin Weapons and on Their Destruction). All of the cases occurred within 4 kilometers (about 2½ miles) downwind from a Soviet military microbiology facility, and it was suspected that the cases were from the accidental airborne release of anthrax spores. Years later, Western analysts were permitted to review the outbreak to address the speculation. These analysts used data to determine that the anthrax outbreak did occur from the microbiology facility and was the largest outbreak of inhalation anthrax in history. Despite these findings, the Soviet Union maintained that the outbreak was from meat contaminated with anthrax spores. In 1992, then-president of Russia, Boris Yeltsin, admitted that the outbreak was exactly what Western analysts had determined. He stated that the air filters at the biologic facility had not been properly installed the morning of the release, allowing anthrax spores to spew out of the facility.

Before 2001, the last case of inhalation anthrax reported in the United States was in 1976. After the September 11 attacks on the World Trade Center and Pentagon, letters filled with a white powder containing anthrax spores were mailed to two U.S. Senators’ offices and news media agencies along the East Coast. Authorities recovered four letters, postmarked September 18, 2001, and October 9, 2001. The powder form allowed the anthrax to float in the air and for it to be breathed in. The powder from these letters contaminated the postal facilities they were processed through as well as the buildings where they were opened.

Until the first few people became ill with anthrax, Americans were unaware of this attack. The first case of inhalation anthrax was diagnosed on October 4, 2001. During October and November of 2001, there were a total of 11 confirmed cases of inhalation anthrax and 11 confirmed cases of cutaneous anthrax Of the 11 cases of inhalation anthrax seven of the cases were postal workers who handled the letters or worked in a postal facility where the letters were processed. Two cases were from the AMI Publishing Company, where a photo editor received a contaminated letter. The last two cases were the hardest in which to determine exposure: a 94-year old Connecticut woman and a New York City hospital employee. Investigators thought that the Connecticut women’s mail may have been cross-contaminated in a mail facility; however, no anthrax spores were ever found in her home. The exposure source of the New York City hospital employee is still unknown.

Of the 22 people who got sick with anthrax in 2001, five of them died. All of the people who died had inhalation anthrax, the most serious form of the disease. In all, 43 people tested positive for exposure to anthrax, and 10,000 more people were considered at risk of possible exposure to anthrax.

Before this event, there had never been an intentional release of anthrax in the United States. The FBI conducted an intense 7-year investigation into who may have sent the contaminated letters. Many years after the attacks, advancements in genetic testing allowed the FBI to conduct more complex testing of the spores used in the attack. Once the spores were analyzed, it was determined they came from a strain called the Ames strain and from a single spore batch known as RMR-1029, from a specific research lab. The attack and the subsequent investigation came to be known as Amerithrax. The FBI officially concluded the Amerithrax investigation on February 19, 2010.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

The genetic material of Bacillus anthracis is coded within 1 chromosome and 2 plasmids, which are fundamental for its toxicity. The spores of B. anthracis are the infectious form and can remain dormant in the environment for decades. The disease may be transmitted through the skin, gastrointestinal or respiratory systems. The bacterium causes disease through 2 mechanisms: toxemia and bacterial infection.^[1] B. anthracis begins to produce toxins within hours of germination.^[2] Protective antigen (PA) and edema factor (EF) combine to form edema toxin (ET), and PA and lethal factor (LF) combine to form lethal toxin (LT), the active toxins. Bacterial toxins have a direct cytotoxic effect by interfering with cellular pathways, being also responsible for weakening the immune system, so the initial systemic infection may occur. Anthrax lesions at any site are characterized by lymphadenopathy, extensive edema, necrosis and confluent exudate containing macrophages and neutrophils. If not stopped, the infection may affect different organs, causing septicemia and potentially death.

The genetic component of Bacillus anthracis includes 1 chromosome and 2 plasmids. These plasmids (pXO1 and pXO2) are fundamental for its toxicity:^[1]

• pXO1 – encodes 3 components of the anthrax exotoxins:

• Protective Antigen (PA)
• Lethal Factor (LF)
• Edema Factor (EF)

• pXO2 – encodes proteins involved in the glutamic acid capsule structure which is responsible for conferring resistance against phagocytosis.

The route of transmission of anthrax allows for its classification into the following:^[3]

• Cutaneous anthrax – commonly requires a prior skin lesion as a prerequisite for infection
• Gastrointestinal anthrax – contracted following ingestion of contaminated food, primarily meat from an animal that died of the disease, or conceivably from ingestion of contaminated water
• Inhalational anthrax – from breathing in airborne anthrax spores
• Injection anthrax – from the injection of a drug containing or contaminated with Bacillus anthracis

B. anthracis, the causative agent of anthrax, is a spore-forming bacterium. The spores of B. anthracis, which can remain dormant in the environment for decades, are the infectious form, but this vegetative form of B. anthracis rarely causes disease.^[4] The bacterium causes disease through 2 mechanisms: toxemia and bacterial infection.^[1] Spores introduced through the skin lead to cutaneous or injection anthrax; those introduced through the gastrointestinal tract lead to gastrointestinal anthrax; and those introduced through the lungs lead to inhalation anthrax. After entering a human or animal, B. anthracis spores are believed to germinate locally or be phagocytosed by dendritic cells and macrophages. These will then carry the spores to the lymph nodes, where they germinate.^[5][1] B. anthracis begins to produce toxins within hours of germination.^[2] Protective antigen (PA) and edema factor (EF) combine to form edema toxin (ET), and PA and lethal factor (LF) combine to form lethal toxin (LT). After binding to surface receptors, the PA portion of the complexes facilitates translocation of the toxins to the cytosol, in which EF and LF exert their toxic effects.^[6] Bacillus anthracis disseminate to multiple organs including spleen, liver, intestines, kidneys, adrenal glands, and meninges, affecting their normal functions and leading to systemic infection with a potentially fatal outcome.^[7][8][3]

The virulence factors of Bacillus anthracis are:

• Antiphagocytic capsule
• Toxins:

• PA
• LF
• EF

In order to infect the body, Bacillus anthracis must produce toxins. These toxins have 3 main toxic effects: edema, hemorrhage, and necrosis. Besides their direct toxic effects responsible for tissue damage, anthrax toxins are also responsible for interfering with cellular pathways, in such way that defense functions of the host’s immune system are affected. This will ultimately allow initial systemic infection by interfering with the immune system.^[1]

When isolated, the 3 structural elements of the anthrax exotoxins are non-toxic. However, when combined, they form virulent exotoxins:^[1]

• LF + PA = LT (Lethal Toxin)
• EF + PA = ET (Edema Toxin)

The PA is responsible for attaching the toxin to the cell, while the LF and the EF are responsible for the toxicity.^[1]

After germinating, B. anthracis produces and releases into the blood stream PA, LF, and EF toxins separately. However, PA is secreted in its inactivated form (PA). In order to form the exotoxin complexes with LF and EF, it must first be activated by host-cellular receptors:^[1]

• CMG2 – Capillary Morphogenesis Protein 2 (predominant toxin receptorin vivo)
• TEM8 – Tumor Endothelium Marker 8 (minor role)

CMG2 and TEM8 cleave PA into PA20 and PA63. PA63 (a C-terminal fragment) is the activated form of PA, responsible for combining with EF and LF, thereby creating the toxin oligomer PA63 oligomer receptor complex. This complex will be internalized via receptor mediated endocytosis within an endosome.^[1]

The acidic environment within the endosomes leads to the formation of a channel called PA63 oligomer channel, on the endosomal membrane. LF and EF are then released in the cytosol of the host cell, to then exert their toxic effects.^[1]

After experiments in mice, edema toxin was noted to be the major virulence factor since it caused death of mice in much lesser dosages than lethal toxin.

• Edema toxin is a calmodulin-dependent adenylyl cyclase, known to increase intracellular cAMP through the conversion of ATP into cAMP, thus affecting several intracellular pathways.
• Lethal toxin is a zinc-dependent metaloproteinase known to interfere with the mitogen-activated protein kinase (MEK), thereby hampering multiple intracellular mechanisms.^[1]

According to animal studies, spores that enter the skin of susceptible animals (either through a lesion or by injection) germinate and give rise, in about 2 – 4 hours, to a small edematous area containing capsulated bacilli. The following stages are noticed:^[3]

• The lesion increases in size and becomes defined by a zone of altered capillaries through dilatation, congestion, and swelling of the endothelial cells and through diapedesis of neutrophils.
• Macrophages and fibrin deposits appear, and the efferent lymphatics become dilated.
• Continued multiplication of the bacilli in the lesion leads to fragmentation of the connective tissue, as the edema spreads extensively away from the infection site.
• Necrosis and hemorrhage occur. If initial inoculation has happened with minimal number of spores, the primary lesion may be insignificant compared with that of the draining lymph nodes.
• The bacilli migrate from the primary lesion, by means of the lymph vessels, to the local lymph nodes where multiplication occurs, releasing a continuous stream of organisms and toxins into the efferent lymph vessels and to the spleen and other lymphoid tissues where multiplication continues.
• In animals who died following subcutaneous challenge, the local lymph nodes became haemorrhagic and edematous. Necrosis of lymphatic elements and of blood vessel walls occur and phagocytosis of the bacilli is apparent.
• Septicemia often courses with splenic enlargement

Injection anthrax will have similar pathogenesis to cutaneous anthrax, but since it is injected, it can spread throughout the body faster and it becomes harder to recognize and treat than the cutaneous form.^[9]

In inhalation anthrax, the inhaled spores will be deposited in the alveoli first. From there, they will be transported, within phagocytic cells, through the lymphatic vessels to the mediastinal lymph nodes, where they will grow and cause hemorrhagic lymphadenitis. Bacteria escape from the damaged lymph nodes and invade the blood stream via the thoracic duct. Vegetative Bacillus then travel through the bloodstream and lymph vessels, potentially causing septicemia. At the same time toxins are released, causing tissue damage and hampering the immune system to facilitate bacterial spread.^[10][11][12]

Once the bacteremia and associated toxemia reach a critical level, the severe symptoms that are characteristic of the acute phase of illness are manifested. During the acute phase, damage of the lung tissue becomes apparent on the X-ray. This damage results from the action of anthrax toxin on the endothelium of the lung’s capillary bed. Primary damage of the lung is not normally a feature of the initial phase of illness and primary pulmonary infection is an uncommon presentation.^[13][11][12]

Studies in rhesus monkeys revealed that after spore inhalation, its germination might take up to 60 days. This is the reason why antibiotic prophylaxis is recommended for 60 days.^[11]

In animal studies, the intestinal lesions caused by ingested anthrax spores range from focal to diffuse hemorrhagic necrotic enteritis of the small intestine. The tendency for localized lesions to develop in Peyer’s patches suggests a possible role of the M cell in the uptake of the anthrax bacillus.^[3]

Cutaneous infection typically produces ulcerated lesions which are covered by a scab and often contain numerous microorganisms. Anthrax eschars are generally seen on exposed unprotected regions of the body, mostly on the face, neck, hands and wrists. Generally cutaneous lesions are single, but sometimes two or more lesions are present.^[14][15]

The lesions produced by injection anthrax will be similar to the ones of the cutaneous form. The difference will reside on the fact that injection anthrax can spread throughout the body faster and be harder to recognize and treat than cutaneous anthrax.^[9]

Gross pathologic lesions observed in non-human primates used in aerosol challenge models of inhalation anthrax include edema, congestion, hemorrhage, and necrosis in the lungs and mediastinum. Splenitis and necrotizing or hemorrhagic lymphadenitis involving the mediastinal, tracheobronchial, and other lymph nodes are common.^[16] Primary pulmonary lesions, including those of pneumonia, are occasionally observed. Meningeal involvement ranging from edema, congestion, hemorrhage, and necrosis to suppurative or hemorrhagic meningitis, usually secondary to hematogenous spread from other types of anthrax, occurs in ≤77% of animals studied.^[17] Autopsy findings from persons who died from inhalation anthrax in Sverdlovsk and in the United States^[18] are consistent with findings from the non-human primates studies. Persons who died had extensive amounts of serosanguinous fluid in pleural cavities, edema, and hemorrhage of the mediastinum and surrounding soft tissues. 48% had cerebral edema, 21% had ascites, 17% had pericardial effusions, and 14% had petechial rash. Mediastinal lymph nodes and spleen also showed hemorrhage and necrosis.^[16][19]

On gastrointestinal infection the typical eschar may occur on different locations, including:^[11]

• Oropharynx
• Stomach
• Duodenum
• Ileum
• Cecum

According to the location of the eschar, gastrointestinal anthrax may be divided in 2 categories: oropharyngeal and abdominal.^[11]

As the eschar progresses, symptoms will appear as a result of the necrosis of the lesion, coupled with severe intestinal and mesenteric edema and lymph node enlargement in the mesentery.^[11]

Anthrax lesions at any site are characterized by extensive necrosis and confluent exudate, containing macrophages and neutrophils. In histopathological specimens or culture media, the presence of large boxcar-shaped Gram-positive bacilli in chains suggests the diagnosis.

Histologic examination of skin lesions caused by cutaneous anthrax reveals:^[20]

• Tissue necrosis
• Edema
• Lymphocytic infiltrates
• No liquefaction
• No abscess formation (lesion is not suppurative)
• Focal points of hemorrhage
• Thrombosis
• Bacilli in subcutaneous tissue, revealed by Gram staining

Histologic evaluation of affected tissues reveals:

• Numerous necrotic foci of hemorrhage in the mediastinum or hilar and peribronchial lymph nodes
• Pulmonary infection may also result in interstitial pneumonia associated with:

• Vasculitis
• Edema
• Infiltrating macrophages and neutrophils

• Bacilli are most likely to be observed in the alveolar capillaries and venules and, to a lesser degree, within the alveolar space and draining lymph nodes

Histologic evaluation of affected tissues revealed:^[20]

• Edema
• Mucosal necrosis
• Inflammatory infiltrates
• Peritoneal fluid may reveal gram-positive bacilli
• Occasionally mediastinal widening

• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on sheep blood agar (SBA) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on sheep blood agar (SBA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 5X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on sheep blood agar (SBA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on polymyxin B – lysozyme – EDTA – thallous acetate agar (PLET) medium, for a 24 hour time period, at a temperature of 37°C”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 24 hour time period, at a temperature of 37°.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 5X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on mannitol, egg yolk, polymyxin agar (MEP) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 5X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on mannitol, egg yolk, polymyxin agar (MEP) medium, for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on colistin-naladixic acid agar (CNA) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on colistin-naladixic acid agar (CNA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on mannitol, egg yolk, polymyxin agar (MEP) medium, for a 48 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on polymyxin B – lysozyme – EDTA – thallous acetate agar (PLET) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on sheep blood agar (SBA) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on sheep blood agar (SBA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 5X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on sheep blood agar (SBA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on polymyxin B – lysozyme – EDTA – thallous acetate agar (PLET) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on polymyxin B – lysozyme – EDTA – thallous acetate agar (PLET) medium, for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 48 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 48 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on phenylethyl alcohol agar (PEA) medium, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on mannitol, egg yolk, polymyxin agar (MEP) medium, for a 48 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on colistin-naladixic acid agar (CNA) medium, for a 48 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on colistin-naladixic acid agar (CNA) medium, for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Low-power magnification of 10X of a digital Keyence scope, this photograph depicts the colonial growth displayed by Pasteur strain members of the Gram-positive bacterium, Bacillus anthracis, which were cultured on colistin-naladixic acid agar (CNA) medium, for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 10X, this image depicts a number of Gram-positive Bacillus anthracis bacterial colonies of the Sterne-strain, which had been cultivated on a blood agar plate (BAP), and incubated for a 24 hour time period at a temperature of 35 °C. Of particular note was the classic “ground glass” texture exhibited by the colonies, resembling a pile of glass shards. As they continued to grow, and their edges touched, you can see that these colonies had coalesced, destroying their originally well-demarcated borders.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 10X, this image depicts a number of Gram-positive Bacillus anthracis bacterial colonies of the Sterne-strain, which had been cultivated on a blood agar plate (BAP), and incubated for a 24 hour time period at a temperature of 35 °C. Of particular note was the classic “ground glass” texture exhibited by the colonies, resembling a pile of glass shards. As they continued to grow, and their edges touched, you can see that these colonies had coalesced, destroying their originally well-demarcated borders.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 30X, this image depicts a single Gram-positive Bacillus anthracis bacterial colony of the Sterne-strain, which had been cultivated on a blood agar plate (BAP), and incubated for a 24 hour time period at a temperature of 35 °C. Of particular note was the classic “ground glass” texture exhibited by the colony, resembling a pile of glass shards. In the background, you’ll note that as these colonies continued to grow, and their edges touched, they coalesced, destroying their originally well-demarcated borders.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 20X, this image depicts a number of Gram-positive Bacillus anthracis bacterial colonies of the Sterne-strain, which had been cultivated on a blood agar plate (BAP), and incubated for a 24 hour time period at a temperature of 35 °C. Of particular note was the classic “ground glass” texture exhibited by the colonies, resembling a pile of glass shards. As they continued to grow, and their edges touched, you can see that these colonies had coalesced, destroying their originally well-demarcated borders.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Hematoxylin-eosin-stained (H&E) photomicrograph of a tissue specimen harvested from the bronchus of a deceased 60 year-old man, revealed cytoarchitectural changes associated with a case of inhalation anthrax, due to Gram-positive Bacillus anthracis bacteria.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Colonial morphology displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of phenylethyl alcohol agar (PEA), for a 24 hour time period, at a temperature of 37°C”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Colonial morphology displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of phenylethyl alcohol agar (PEA), for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Light colonial growth displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of MacConkey agar, for a 24 hour time period, at a temperature of 37°C. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Light colonial growth displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of MacConkey agar, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Light colonial growth displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of CNA agar, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Light colonial growth displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of CNA agar, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Colonial morphology displayed by Gram-positive, Pasteur strain, Bacillus anthracis bacteria, which was grown on a medium of chocolate agar, for a 24 hour time period, at a temperature of 37°C.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Bacillus anthracis bacterial colonies, which had been allowed to grow on sheep’s blood agar (SBA) for a 24 hour period. In this particular view you’ll note the appearance of what is termed a “plaque” (arrowhead), which represents an area where the bacteria had been lysed, or destroyed by the application of a localized amount of gamma phage-containing solution. Highly specific toB. anthracis, these gamma phage viruses, i.e., bacteriophages, attacked the B. anthracis bacteria, subsequently leaving this circular plaque devoid of bacterial organisms. The specificity of these gamma phages to B. anthracis makes this a positive test for the presence of this bacterium.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Bacillus anthracis bacterial colonies, which had been allowed to grow on sheep’s blood agar (SBA) for a 24 hour period. Note the classical appearance exhibited in the colonial morphology including a ground-glass, non-pigmented texture with accompanying “comma” projections from some of the individual rough-edged colonies. In this particular view, you’ll note that a “tenacity test” had been performed using an iinoculating loop, which proved positive for B. anthracis, causing the “colony to ‘stand up’ like beaten egg white”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Bacillus anthracis bacterial colonies, which have been allowed to grow on sheep’s blood agar (SBA) for a 24 hour period. Note the classical appearance exhibited in the colonial morphology including a ground-glass, non-pigmented texture with accompanying “comma” projections from some of the individual rough-edged colonies. In this particular view, you’ll note that a “tenacity test” had been performed using an iinoculating loop, which proved positive for B. anthracis, causing the “colony to ‘stand up’ like beaten egg white”.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Bacillus anthracis bacterial colonies, which have been allowed to grow on sheep’s blood agar (SBA) for a 24 hour period. Note the classical appearance exhibited in the colonial morphology including a ground-glass, non-pigmented texture with accompanying “comma” projections from some of the individual rough-edged colonies. In this particular view, you’ll note that a “tenacity test” had been performed using an iinoculating loop, which proved positive for B. anthracis, causing the “colony to ‘stand up’ like beaten egg white”.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Bacillus anthracis bacterial colonies, which have been allowed to grow on sheep’s blood agar (SBA) for a 24 hour period. Note the classical appearance exhibited in the colonial morphology including a ground-glass, non-pigmented texture with accompanying “comma” projections from some of the individual rough-edged colonies.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Bacillus anthracis bacterial colonies, which have been allowed to grow on sheep’s blood agar (SBA) for a 24 hour period. Note the classical appearance exhibited in the colonial morphology including a ground-glass, non-pigmented texture with accompanying “comma” projections from some of the individual rough-edged colonies. See PHIL 11748 for a higher magnification of these colonies.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 6,408X, this scanning electron micrograph (SEM) depicted spores from the Aimes strain of Bacillus anthracis bacteria. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 31,207X, this scanning electron micrograph (SEM) depicted spores from the Sterne strain of Bacillus anthracis bacteria. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Magnification of 12,483X, this scanning electron micrograph (SEM) depicted spores from the Sterne strain of Bacillus anthracis bacteria.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Gram-positive, endospore-forming Bacillus anthracisbacteria. B. anthracis is the pathologic microorganism responsible for the disease “anthrax”, an acute infectious disease, which most commonly occurs in wild and domestic vertebrates (cattle, sheep, goats, camels, antelopes, and other herbivores), but it can also occur in humans when they are exposed to infected animals, or tissue from infected animals.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Gross pathologic posterior oblique view of a chimpanzee’s lungs that had contracted fatal inhalation anthrax.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Micrograph revealing submucosal hemorrhage in the small intestine, in a case of fatal human anthrax; H&E stain; Mg. 240X.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Micrograph revealing submucosal hemorrhage in the small intestine, in a case of fatal human anthrax; H&E stain; Mg. 240X.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Meningeal infection due to Bacillus anthracis bacteria using an H&E stain” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Blood agar culture plate growing B. anthracis and other soil flora.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Brain section through the ventricles revealing an inter ventricular hemorrhage as consequence of virulence factors of Bacillus anthracis (edema toxin, lethal toxin, and antiphagocytic capsular antigen). The toxins are responsible for the primary clinical manifestations of hemorrhage, edema, and necrosis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Agar culture plate is growing Bacillus anthracis colonies.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Blood agar culture plate growing B. anthracis and other soil flora.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  Cutaneous anthrax lesion on the arm of a 50 year old female.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Magnification of 12,483X, this scanning electron micrograph (SEM) depicted spores from the Sterne strain of Bacillus anthracis bacteria. ”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Magnification of 12,483X, this scanning electron micrograph (SEM) depicted spores from the Sterne strain of Bacillus anthracis bacteria.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Magnification of 6,408X, this scanning electron micrograph (SEM) depicted spores from the Aimes strain of Bacillus anthracis bacteria.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Photomicrograph of Bacillus anthracis bacteria using Gram-stain technique”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis taken from the peritoneum using a Hiss capsule stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Photomicrograph of Bacillus anthracis bacteria taken from heart blood, and using Carbol Fuchsin stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Bacillus anthracis positive encapsulation test is demonstrated using two different agar media”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Photomicrograph demonstrating a positive Gram stain with Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis tenacity positive on sheep blood agar.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis tenacity positive on sheep blood agar”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis tenacity positive on sheep blood agar”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Bacillus anthracis Avirulent Pasteur Strain, Non-hemolytic on sheep blood agar” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis spores seen under phase contrast microscopy”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis spores seen under phase contrast microscopy”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis spores seen under phase contrast microscopy”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis spores seen under phase contrast microscopy”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “B. anthracis Direct Fluorescent Antibody (DFA) capsule stain”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “B. anthracis Direct Fluorescent Antibody (DFA) capsule stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “B. anthracis Direct Fluorescent Antibody (DFA) capsule stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “B. anthracis Direct Fluorescent Antibody (DFA) cell wall stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “B. anthracis Direct Fluorescent Antibody (DFA) cell wall stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Bacillus anthracis gamma phage lysis on sheep blood agar.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Bacillus anthracis Indian Ink capsule stain.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Bacillus anthracis M’Fadyean capsule stain”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Bacillus anthracis capsule production on bicarbonate agar medium”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Hemorrhagic lymph node due to inhalation anthrax”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Transmission electron micrograph of Bacillus anthracis.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Transmission electron micrograph of Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Transmission electron micrograph of Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Transmission electron micrograph of Bacillus anthracis.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Transmission electron micrographic image of Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Transmission electron micrograph of Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Confocal micrograph of Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Numerous rod-shaped Bacillus anthracis bacteria, some that had linked together to form chains, while others remained solitary”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Necrosis of lymph node due to anthrax”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis from agar culture Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Photomicrograph of meninges demonstrating the presence of Bacillus anthracis in a case of fatal inhalation anthrax.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Photomicrograph of lung tissue demonstrating pneumonia in a case of fatal human inhalation anthraxAdapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Bacillus anthracis in lung Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Meningeal hemorrhage due to anthrax”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Photomicrograph of meninges demonstrating hemorrhage due to fatal inhalation anthrax”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Severe hemorrhagic necrosis of lymph node due to anthrax”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Mild meningitis with hemorrhage due to Bacillus anthracis”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Photomicrograph of serohemorrhagic meningitis in a case of fatal human anthrax with the presence ofBacillus anthracis.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Blood agar plate culture of Bacillus anthracis, with a positive gamma phage test”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Sheep blood agar plate culture of Bacillus anthracis and Bacillus cereus”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Gross pathology of fixed, cut brain showing hemorrhagic meningitis due to inhalation anthrax.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Gross pathology of fixed, cut brain showing hemorrhagic meningitis due to inhalation anthrax.”Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Gram-stained photomicrograph revealing numerous rod-shaped Bacillus anthracis bacteria that had linked together to form long filamentous chains.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Gram-stained photomicrograph revealing numerous rod-shaped Bacillus anthracis bacteria that had linked together to form long filamentous chains.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Hematoxylin-eosin (H&E)-stained photomicrograph of a small intestinal tissue sample revealed the presence of histopathologic changes indicative of marked mucosal and submucosal hemorrhage, with accompanying arteriolar degeneration in a case of fatal human anthrax.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Hematoxylin-eosin (H&E)-stained photomicrograph of a meningeal tissue sample revealed the presence of histopathologic changes indicative of hemorrhagic meningitis in a case of fatal human anthrax.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
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  “Histopathology of mediastinal lymph node in fatal human anthrax” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Histopathology of mediastinal lymph node in fatal human anthrax” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Hematoxylin-eosin (H&E)-stained photomicrograph of a mediastinal lymph node tissue sample revealed the presence of histopathologic changes indicative of an occluded blood vessel in a case of fatal human anthrax.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Hematoxylin-eosin (H&E)-stained photomicrograph of a mediastinal lymph node tissue sample revealed the presence of histopathologic changes indicative of medullary hemorrhage and necrosis in a case of fatal human anthrax.Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Hematoxylin-eosin (H&E)-stained photomicrograph of a mediastinal lymph node tissue sample revealed the presence of histopathologic changes indicative of medullary necrosis in a case of fatal human anthrax.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]
• 
  “Under a relatively-low magnification of 96X, this hematoxylin-eosin (H&E)-stained photomicrograph of a mediastinal lymph node tissue sample revealed the presence of histopathologic changes indicative of medullary necrosis in a case of fatal human anthrax.” Adapted from Public Health Image Library (PHIL), Centers for Disease Control and Prevention.^[21]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

The causative agent of anthrax is B. anthracis, a nonmotile, Gram-positive, aerobic or facultatively anaerobic, endospore-forming, rod-shaped bacterium. The spores of B. anthracis, which can remain dormant in the environment for decades, are the infectious form, but this vegetative B. anthracis rarely causes disease.^[1] The Bacillus may enter the body through the skin, lungs, gastrointestinal system or by injection, after which they will travel to the lymph nodes. The virulence factors will facilitate the translocation of the toxins to the cytosol. The natural reservoirs of Bacillus anthracis include humans, mammals, herbivores, reptiles, and birds.

French physician Casimir Davaine (1812-1882) demonstrated the symptoms of anthrax were invariably accompanied by the microbe B. anthracis.^[2] German physician Aloys Pollender (1799–1879) is also credited for this discovery. B. anthracis was the first bacterium conclusively demonstrated to cause disease, by Robert Koch in 1876.^[3] The species name anthracis is from the Greek anthrax (ἄνθραξ), meaning “coal” and referring to the most common form of the disease, cutaneous anthrax, in which large, black skin lesions are formed.

B. anthracis, the causative agent of anthrax, is a nonmotile, Gram-positive, aerobic or facultatively anaerobic, endospore-forming, rod-shaped bacterium approximately 4 μm by 1 μm, although under the microscope it frequently appears in chains of cells. Like other Bacillus, Bacillus anthracis is saprophyte, being able to live in vegetation, air, water and soil.^[4]

These bacterial cells may occur isolated, form groups of 2 or more cells in the body, or long chains in cultures.^[4] In blood smears, smears of tissues or lesion fluid from diagnostic specimens, these chains are two to a few cells in length. In smears made from in vitro cultures, they can appear as endless strings of cells – responsible for the characteristic tackiness of the colonies and for the flocculating nature of broth cultures. Cell cultures appear with a large, grey and curled structure, resembling a “medusa head”.^[4]

B. anthracis have a characteristic square-ended appearance, traditionally associated with its vegetative state, although this may not always be very clear. In the presence of oxygen, ideally at 32 – 35 ºC, and towards the end of the exponential phase of growth, one ellipsoidal spore (approximately 2 μm by 1 μm in size) is formed within each cell.^[5][4] Commonly the spores will be produced once the cell senses scarcity of nutrients.^[6]

The spores of B. anthracis, which can remain dormant in the environment for decades, being resistant to heat and disinfectants, are the infectious form, but vegetative B. anthracis rarely causes disease.^[7][4]

In the absence of oxygen and under a high partial pressure of Co2, in the presence of bicarbonate, the vegetative cell secretes its polypeptide capsule. This is one of the two established in vivo virulence factors of B. anthracis. The capsule is also a primary diagnostic aid.^[5] Protective antigen (PA) and edema factor (EF) combine to form edema toxin (ET) and PA and lethal factor (LF) combine to form lethal toxin (LT), the active toxins.^[8][9]

B. anthracis has a single chromosome which is a circular, 5,227,293-bp DNA molecule.^[12] It also has two circular, extrachromosomal, double-stranded DNA plasmids, pXO1 and pXO2. Both the pXO1 and pXO2 plasmids are required for full virulence and represent two distinct plasmid families.^[13]

The pXO1 plasmid (182 kb) contains the genes that encode for the anthrax toxin components: pag (protective antigen, PA), lef (lethal factor, LF), and cya (edema factor, EF). These factors are contained within a 44.8-kb pathogenicity island (PAI). The lethal toxin is a combination of PA with LF and the edema toxin is a combination of PA with EF. The PAI also contains genes which encode a transcriptional activator AtxA and the repressor PagR, both of which regulate the expression of the anthrax toxin genes.^[13]

pXO2 encodes a five-gene operon (capBCADE) which synthesizes a poly-γ-D-glutamic acid (polyglutamate) capsule. This capsule allows B. anthracis to evade the host immune system by protecting itself from phagocytosis. Expression of the capsule operon is activated by the transcriptional regulators AcpA and AcpB, located in the pXO2 pathogenicity island (35 kb). Interestingly, AcpA and AcpB expression are under the control of AtxA from pXO1.^[13]

The 89 known strains of B. anthracis include:

• Sterne strain (34F2; aka the “Weybridge strain”), used by Max Sterne in his 1930s vaccines
• Vollum strain, formerly weaponized by the US, UK, and Iraq; isolated from cow in Oxfordshire, UK, in 1935
  • Vollum M-36, virulent British research strain; passed through macaques 36 times
  • Vollum 1B, weaponized by the US and UK in the 1940s-60s
  • Vollum-14578, UK biotesting contaminated Gruinard Island, Scotland, in 1940s
  • V770-NP1-R, the avirulent, nonencapsulated strain used in the BioThrax vaccine
• Anthrax 836, highly virulent strain weaponized by the USSR; discovered in Kirov in 1953
• Ames strain, isolated from a cow in Texas in 1981; famously used in AMERITHRAX letter attacks (2001)
  • Ames Ancestor
  • Ames Florida
• H9401, isolated from human patient in Korea; used in investigational anthrax vaccines^[14]

Whole genome sequencing has made reconstruction of the B. anthracis phylogeny extremely accurate. A contributing factor to the reconstruction is B. anthracis being monomorphic, meaning it has low genetic diversity, including the absence of any measurable lateral DNA transfer since its derivation as a species. The lack of diversity is due to a short evolutionary history that has precluded mutational saturation in single nucleotide polymorphisms.^[15]

A short evolutionary time does not necessarily mean a short chronological time. When DNA is replicated, mistakes occur which become genetic mutations. The buildup of these mutations over time leads to the evolution of a species. During the B. anthracis lifecycle, it spends a significant amount of time in the soil spore reservoir stage, a stage in which DNA replication does not occur. These prolonged periods of dormancy have greatly reduced the evolutionary rate of the organism.^[15]

B. anthracis belongs to the B. cereus group consisting of the strains: B. cereus, B. anthracis, B. thuringiensis, B. weihenstephanensis, B. mycoides, and B. pseudomycoides. The first three strains are pathogenic or opportunistic to insects or mammals, while the last three are not considered pathogenic. The strains of this group are genetically and phenotypically heterogeneous overall, but some of the strains are more closely related and phylogenetically intermixed at the chromosome level. The B. cereus group generally exhibits complex genomes and most carry varying numbers of plasmids.^[13]

B. cereus is a soil-dwelling bacterium which can colonize the gut of invertebrates as a symbiont^[16] and is a frequent cause of food poisoning^[17] It produces an emetic toxin, enterotoxins, and other virulence factors.^[18] The enterotoxins and virulence factors are encoded on the chromosome, while the emetic toxin is encoded on a 270-kb plasmid, pCER270.^[13]

B. thuringiensis is an insect pathogen and is characterized by production of parasporal crystals of insecticidal toxins Cry and Cyt.^[19] The genes encoding these proteins are commonly located on plasmids which can be lost from the organism, making it indistinguishable from B. cereus.^[13]

PlcR is a global transcriptional regulator which controls most of the secreted virulence factors in B. cereus and B. thuringiensis. It is chromosomally encoded and is ubiquitous throughout the cell.^[20] In B. anthracis, however, the plcR gene contains a single base change at position 640, a nonsense mutation, which creates a dysfunctional protein. While 1% of the B. cereus group carries an inactivated plcR gene, none of them carries the specific mutation found only in B. anthracis.^[21]

The plcR gene is part of a two-gene operon with papR.^[22][23] The papR gene encodes a small protein which is secreted from the cell and the reimported as a processed heptapeptide forming a quorum-sensing system.^[23][24] The lack of PlcR in B. anthracis is a principle characteristic differentiating it from other members of the B. cereus group. While B. cereus and B. thuringiensis depend on the plcR gene for expression of their virulence factors, B. anthracis relies on the pXO1 and pXO2 plasmids for its virulence.^[13]

Components of tea, such as polyphenols, have the ability to inhibit the activity both of B. anthracis and its toxin considerably; spores, however, are not affected. The addition of milk to the tea completely inhibits its antibacterial activity against anthrax.^[25] Activity against the B. athracis in the laboratory does not prove that drinking tea affects the course of an infection, since it is unknown how these polyphenols are absorbed and distributed within the body.

Advances in genotyping methods have led to improved genetic analysis for variation and relatedness. These methods include multiple-locus variable-number tandem repeat analysis (MLVA) and typing systems using canonical single-nucleotide polymorphisms. The Ames ancestor chromosome was sequenced in 2003^[12] and contributes to the identification of genes involved in the virulence of B. anthracis. Recently, B. anthracis isolate H9401 was isolated from a Korean patient suffering from gastrointestinal anthrax. The goal of the Republic of Korea is to use this strain as a challenge strain to develop a recombinant vaccine against anthrax.^[14]

The H9401 strain isolated in the Republic of Korea was sequenced using 454 GS-FLX technology and analyzed using several bioinformatics tools to align, annotate, and compare H9401 to other B. anthracis strains. The sequencing coverage level suggests a molecular ratio of pXO1:pXO2:chromosome as 3:2:1 which is identical to the Ames Florida and Ames Ancestor strains. H9401 has 99.679% sequence homology with Ames Ancestor with an amino acid sequence homology of 99.870%. H9401 has a circular chromosome (5,218,947 bp with 5,480 predicted ORFs), the pXO1 plasmid (181,700 bp with 202 predicted ORFs), and the pXO2 plasmid (94,824 bp with 110 predicted ORFs).^[14] As compared to the Ames Ancestor chromosome above, the H9401 chromosome is about 8.5 kb smaller. Due to the high pathogenecity and sequence similarity to the Ames Ancestor, H9401 will be used as a reference for testing the efficacy of candidate anthrax vaccines by the Repbulic of Korea.^[14]

As with most other pathogenic bacteria, B. anthracis must acquire iron to grow and proliferate in its host environment. The most readily available iron sources for pathogenic bacteria are the heme groups used by the host in the transport of oxygen. To scavenge heme from host hemoglobin and myoglobin, B. anthracis uses two secretory siderophore proteins, IsdX1 and IsdX2. These proteins can separate heme from hemoglobin, allowing surface proteins of B. anthracis to transport it into the cell.^[26]

Bacillus anthracis is thought to have originated in Egypt and Mesopotamia. Many scholars think that in Moses’ time, during the 10 plagues of Egypt, anthrax may have caused what was known as the fifth plague, described as a sickness affecting horses, cattle, sheep, camels and oxen.

After entering the body (through the skin, lungs, gastrointestinal tract or by injection), B. anthracis spores are believed to germinate locally or be transported by phagocytic cells to the lymphatics and regional lymph nodes, where they germinate.^[9][27] After binding to cell surface receptors, the PA portion of the complexes facilitates translocation of the toxins to the cytosol.^[28][9]

Natural reservoirs of Bacillus anthracis include:^[5][4][9]

• Humans
• Mammals
• Herbivores
• Reptiles
• Birds

Wikimedia Commons has media related to Bacillus anthracis.

• Bacillus anthracis genomes and related information at PATRIC, a Bioinformatics Resource Center funded by NIAID
• Hazards in Animal Research Database – Bacillus anthracis
• Pathema-Bacillus Resource

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

The differential diagnosis of anthrax includes a wide range of infectious and non-infectious conditions. Depending on the mode of anthrax exposure in the patient (cutaneous, ingestion, inhalation or injection), there will be different forms of the disease.^[1] A history of exposure to contaminated animal materials, occupational exposure, and living in an endemic area, is crucial when considering the diagnosis of anthrax. Additional tests to isolate Bacillus anthracis are required to differentiate anthrax from other diagnoses, thereby confirming the correct etiologic agent.

• A history of exposure to contaminated animal materials, occupational exposure, and living in an endemic area is crucial when considering a diagnosis of anthrax. A painless, pruritic papule, surrounding vesicles and edema, usually on an exposed region of the body should raise concern of cutaneous anthrax, which is confirmed by the demonstration of Gram-positive encapsulated bacilli on the lesion, and/or positive culture for Bacillus anthracis and/or positive specialized tests.

• The differential diagnosis of the anthrax eschar includes a wide range of infectious and non-infectious conditions, including:^[1]

• Generally there are other diseases and conditions lack the characteristic edema of anthrax. The absence of pus, the lack of pain, and the patient’s occupation may provide further diagnostic clues. The outbreak of Rift Valley fever, initially thought to be anthrax in livestock, also affected numerous humans.

• In the differential diagnosis of the severe forms, orbital cellulitis, dacryocystitis and deep tissue infection of the neck, should be considered in the case of severe anthrax lesions involving the face, neck and anterior chest wall. Necrotizing soft tissue infections, particularly group A streptococcal infections and gas gangrene, and severe cellulitis due to staphylococci, should also be considered in the differential diagnosis of severe forms of cutaneous anthrax. Gas and abscess formation are not observed in patients with cutaneous anthrax. Abscess formation is only seen when the lesion is infected with other bacteria, such as streptococci or staphylococci.

• The list of differential diagnosis of oropharyngeal anthrax includes:

• The list of differential diagnosis of gastrointestinal anthrax includes:

• The list of differential diagnosis of inhalation anthrax includes:

• Meningitis is a potential complication of anthrax infection. The list of differential diagnosis of anthrax meningitis includes:

• The definitive diagnosis is obtained by visualization of the capsulated bacilli in the cerebrospinal fluid and/or by culture.^[1]

• Sepsis is a potential complication of anthrax infection.

• The definitive diagnosis of anthrax is made by the isolation of Bacillus anthracis from the primary lesion, from blood cultures or by detection of the toxin or DNA of B. anthracis in these specimens.^[1]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Incidence of the natural disease in humans is dependent on the level of exposure to affected animals. For any country, national incidence data for non-industrial cases reflect the livestock situation. Human case rates for anthrax are highest in Africa and Central and Southern Asia. While, statistically, in Northern Europe and countries with similar epidemiological situations, there is one human cutaneous case per every 10 livestock carcasses butchered, there can be some 10 human cutaneous and enteric cases per single carcass butchered in Africa, India, and the Southern Russian Federation.^[1]

• Human case rates for anthrax are highest in Africa, Central and Southern Asia. In locations where the disease manifests infrequently in livestock, it is rarely seen in humans. However, low sporadic incidence may result in obliviousness and disregard, leading to a surge in the number of human exposures from a case in livestock.

• Historical analysis of global epidemiological data reveals the following approximate ratios:
  • In Northern Europe and countries with similar epidemiological situations, there is one human cutaneous case, per 10 livestock carcasses butchered.
  • In Africa, India, and the Southern Russian Federation, there can be some 10 human cutaneous and enteric cases per single carcass. Rural malnutrition and poor veterinary supervision has resulted in significant numbers of human cases each year in Chad, Ethiopia, India, Zambia and Zimbabwe.

• While enteric anthrax is frequently lethal, subclinical cases, which provide subsequent immunity, also occur. Because they lead individuals to perceive that there is a lower risk of contracting lethal disease, from the consumption of meat from animals having succumbed to sudden death, subclinical cases contribute indirectly to the diseases’ persistence in indigenous populations.

• In regions where ingestion anthrax occurs commonly, oropharyngeal anthrax appears to be a relatively infrequent manifestation.

• Some caution should be exercised in making projections of potential human cases, based on fixed human to animal ratios. There are variables that may dramatically alter the situation from area to area, such as economic conditions, surveillance data quality, and dietary habits.

• In the United States and North-Western Europe, cutaneous anthrax associated with animal anthrax has been rare since the first half of the 20th century, with most cutaneous cases being associated with processing of imported goat hair, hides and other animal products. Despite the rarity of the human disease since then, many thousands of animal cases have occurred.

• Similarly, in Haiti human cutaneous anthrax is quite common, but reports of animal anthrax are essentially non-existent despite a well-documented problem with B. anthracis-contaminated goat skin products. Unlike cutaneous anthrax, ingestion anthrax is notably rare in Haiti, presumably because of the local practice of cooking all meat well before consumption.

• The value of hides and cultural demands for caretakers in at least some regions of Africa to preserve as much as possible from dead animals, to present later to the owner, exacerbate the problem of persisting contaminated animal parts.

• In other countries such as Thailand, ingestion anthrax is associated with consumption of undercooked meats.

• Intestinal anthrax was quite a common disease on the Korean peninsula prior to about 1940 and was still seen in the 1990s.

• In Sub-Saharan Africa, the value of the meat from an animal that has died unexpectedly, outweighs the perceived risks of illness that might result from eating it.^[1]

• Industrial anthrax incidence data can be inferred from the volume and weight of potentially affected materials handled or imported, taking into account the quality of prevention, such as vaccination of personnel and forced ventilation of the workplace. These relationships are essentially all that can be used for many countries where human anthrax is infrequently, erratically or incompletely reported.^[1]
• In addition, certain countries suppress anthrax reporting at the local or national levels.^[1]

• In contrast to reports of anthrax in animals, age-related bias is generally not apparent in human anthrax, and differences in incidence have been readily explained in terms of likely exposure of the different groups to the organism.
• The lack of obvious age-related differences was also noted in the records of 112 anthrax cases, occurring in 7 villages bordering the Tarangire national Park in the United Republic of Tanzania between 1986 and 1999.^[1]

• In contrast to reports of anthrax in animals, sex-related bias is generally not apparent in human anthrax. Differences in incidence have been readily explained in terms of likely exposure of the different groups to the organism.
• The lack of obvious sex-related differences was also noted in the records of 112 anthrax cases, occurring in 7 villages bordering the Tarangire national Park in the United Republic of Tanzania between 1986 and 1999.
• There is, however, a bias towards higher occupational risk of exposure to anthrax in men in many countries.^[1]

Developed countries, such as the United States and North-Western Europe, have lower incidence of anthrax.^[1]

Developing countries, such as Africa, India, Haiti, and the Southern Russian Federation have higher incidence of anthrax, particularly due to malnutrition and poor veterinary supervision.^[1] However, unreliable reporting makes it difficult to estimate the true incidence of human anthrax in these countries.^[2]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Risk factors for contracting anthrax include handling of livestock or livestock products, playing animal hide drums, working in a laboratory researching anthrax, and traveling to an endemic region such as Central and South America, Sub-Saharan Africa, Central and southwestern Asia, Southern and eastern Europe, or the Caribbean. Risk factors for anthrax in the setting of bioterrorism are: working as a mail handler, military personnel, or response worker.

People at higher risk of being infected with anthrax include:^[1]

• Veterinarians
• Laboratory professionals dealing the bacteria
• Health care workers
• Livestock producers
• People who handle animal products
• Mail handlers, military personnel, and response workers, in case of bioterrorism
• People who make or play animal hide drums
• Travelers, particularly to the follow areas:

• Central and South America
• Sub-Saharan Africa
• Central and southwestern Asia
• Southern and eastern Europe
• The Caribbean

The following link of the Department of Labor can be used to determine whether or not one is at risk for an anthrax infection.[3]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Respiratory distress

The anthrax prognosis will depend on a number of factors, including:

• The type of anthrax (see Types of Anthrax)
• How early the anthrax is diagnosed
• The strain of anthrax bacteria (Bacillus anthracis)
• The patient’s age and general health.

Early anthrax treatment for all anthrax types improves the anthrax prognosis.^[1]

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 complications and prognosis