Onchocerciasis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Onchocerciasis is a Neglected Tropical Disease (NTD) caused by the parasitic worm Onchocerca volvulus. It is transmitted through repeated bites by black flies of the genus Simulium. The disease is called River Blindness because the black fly that transmits the infection lives and breeds near fast-flowing streams and rivers and the infection can result in blindness. In addition to visual impairment or blindness, onchocerciasis causes skin disease, including nodules under the skin or debilitating itching. Worldwide onchocerciasis is second only to trachoma as an infectious cause of blindness.

Dr. Rodolfo Robles Valverde’s study on patients with river blindness in Guatemala led to the discovery that the disease is caused by filaria of O. volvulus, and sheds light on the life cycle and transmission of the parasite.

Onchocerciasis may be classified into different types depending on the cutaneous lesions seen.

The disease spreads from person to person by the bite of a black fly. When a black fly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person’s skin enter and infect the black fly. The larvae develop over 2 weeks in the fly to a stage that is infectious to humans. An infectious black fly will typically drop larvae when biting a person. The larvae then penetrate the skin to infect the person. Because the worms reproduce only in humans but need to complete some of their development inside the black fly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual.

Onchocerca volvulus is a nematode that causes onchocerciasis mostly in Africa. Onchocerca volvulus, along with most filarial nematodes, share an endosymbiotic relationship with the bacterium Wolbachia. In the absence of Wolbachia, larval development of the O. volvulus is disrupted or ceased.

Patients with onchocerciasis may manifests with skin rash, eye disease and skin swelling. It needs to be differentiated from other diseases with similar manifestations.

Onchocercal infections are found in tropical climates. About 99% of onchocerciasis cases occur in Africa. Also, the parasite is found in limited areas in the America and in Yemen in the Middle East.

Those most at risk are people who live in areas where the parasite is spread followed by adventure travelers, missionaries, and peace corps volunteers who are exposed for long periods of time (generally more than 3 months) to black fly bites in areas where the parasite exists. The disease is most intensely transmitted in remote rural African agricultural villages which are located near rapidly flowing streams.

There can be inflammation of the optic nerve resulting in vision loss, particularly peripheral vision, and eventually blindness.

Infected persons may be without symptoms. Those with symptoms will usually have one or more of the three manifestations: skin rash (usually itchy), eye disease, and nodules under the skin. The most serious manifestation consists of lesions in the eye that can lead to visual impairment andblindness.

The gold standard test for the diagnosis of onchocerciasis remains the skin snip biopsy. The biopsy is performed using a sclerocorneal biopsy punch or by elevating a small cone of skin (3 mm in diameter) with a needle and shaving it off with a scalpel. There are antibody tests that can assist in the diagnosis of onchocerciasis, though many are not available outside the research setting

The treatment of choice for onchocerciasis is ivermectin, which has been shown to reduce the occurrence of blindness and to reduce the occurrence and severity of skin symptoms. An evolving treatment is doxycycline, which has been shown in studies to kill Wolbachia, an endosymbiotic rickettsia-like bacteria that appears to be required for the survival of the O.volvulus macrofilariae and for embryogenesis. Doxycycline does not kill the microfilariae, so treatment with ivermectin would be needed to result in a more rapid decrease of symptoms.

There are no vaccines or medications available to prevent becoming infected with O. volvulus. The best prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeves and long pants during the day when blackflies bite, and wearing permethrin– treated clothing.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Dr. Rodolfo Robles Valverde’s study on patients with river blindness in Guatemala led to the discovery that the disease is caused by filaria of O. volvulus, and sheds light on the life cycle and transmission of the parasite.

In 1915, Dr. Rodolfo Robles Valverde’s study on patients with river blindness in Guatemala led to the discovery that the disease is caused by filaria of O. volvulus, and sheds light on the life cycle and transmission of the parasite.^[1] Using case studies of coffee plantation workers in Guatemala, Robles hypothesized the vector of the disease is a day-biting insect, and more specifically, two anthropophilic species of Simulium flies found to be endemic to the areas. He published his findings on a new disease from Guatemala associated with subcutaneous nodules, anterior ocular (eye) lesions, dermatitis, and microfilariae in 1917.^[2]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Onchocerciasis may be classified into different types depending on the cutaneous lesions seen.

Onchocerciasis may be divided into the following types:^[1]

An acute phase, it is characterized by swelling of the face, with erythema and itching.^[1] Onchocerciasis causes different kinds of skin changes, which vary in different geographic regions. This skin change, erisípela de la costa, of acute onchocerciasis is most commonly seen among victims in Central and South America.

This cutaneous condition is characterized by inflammation accompanied by hyperpigmentation.^[1]

A cutaneous condition, it is a localized type of onchocerciasis.^[1] Additionally, the various skin changes associated with onchocerciasis may be described as follows:^[1]

The spotted depigmentation of the skin that may occur with onchocerciasis^[1]

The thickening of human skin that may be associated with onchocerciasis^[1]

The thickened, wrinkled skin changes that may result with onchocerciasis^[1]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

The disease spreads from person to person by the bite of a black fly. When a black fly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person’s skin enter and infect the black fly. The larvae develop over 2 weeks in the fly to a stage that is infectious to humans. An infectious black fly will typically drop larvae when biting a person. The larvae then penetrate the skin to infect the person. Because the worms reproduce only in humans but need to complete some of their development inside the black fly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual.

During a blood meal, an infected blackfly (genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound (1). In subcutaneous tissues the larvae (2) develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues (3). Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms. Females measure 33 to 50 cm in length and 270 to 400 μm in diameter, while males measure 19 to 42 mm by 130 to 210 μm. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years. The microfilariae, measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that may reach 2 years. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues (4). A blackfly ingests the microfilariae during a blood meal (5). After ingestion, the microfilariae migrate from the blackfly’s midgut through the hemocoel to the thoracic muscles (6). There the microfilariae develop into first-stage larvae (7) and subsequently into third-stage infective larvae (8) . The third-stage infective larvae migrate to the blackfly’s proboscis (9) and can infect another human when the fly takes a blood meal (1).

The disease spreads from person to person by the bite of a black fly. Black flies bite during the day. When a black fly bites a person who has onchocerciasis, microscopic worm larvae (called microfilariae) in the infected person’s skin enter and infect the blackfly.

Shown below is an image of blackfly.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Onchocerca volvulus is a nematode that causes onchocerciasis mostly in Africa. Onchocerca volvulus, along with most filarial nematodes, share an endosymbiotic relationship with the bacterium Wolbachia. In the absence of Wolbachia, larval development of the O. volvulus is disrupted or ceased.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Patients with onchocerciasis may manifests with skin rash, eye disease and skin swelling. It needs to be differentiated from other diseases with similar manifestations.

• Glaucoma
• Herpes simplex
• Id Reaction (Autoeczematization)
• Insect bites
• Interstitial keratitis
• Keratoconjunctivitis
• Keratopathy
• Leprosy
• Lichen planus
• Morphea
• Optic neuropathy
• Pityriasis lichenoides
• Pityriasis rosea
• Sarcoidosis
• Scabies
• Syphilis
• Tinea capitis
• Tinea corporis
• Tinea cruris
• Trachoma
• Tuberculosis
• Uveitis
• Vitiligo
• Yaws

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Onchocercal infections are found in tropical climates. About 99% of onchocerciasis cases occur in Africa. Also, the parasite is found in limited areas in America and in Yemen in the Middle East.

Onchocercal infections are found in tropical climates. The main burden is in sub-Saharan Africa. The World Health Organization (WHO) estimates that 37 million people are infected with O. volvulus worldwide; of these people 270,000 are blind and 500,000 have some sort of visual impairment. Some 90 million people are at risk for becoming infected with the parasite. About 99% of onchocerciasis cases occur in Africa. As of 2008, about 18 million people were infected with this parasite; about 300,000 of those had been permanently blinded.^[1] Onchocerciasis is currently endemic in 30 African countries, Yemen, and isolated regions of South America.^[2] Over 85 million people live in endemic areas, and half of these reside in Nigeria. Another 120 million people are at risk for contracting the disease. Due to the vector’s breeding habitat, the disease is more severe along the major rivers in the northern and central areas of the continent, and severity declines in villages farther from rivers. Travelers who do not stay long in those areas have little risk of developing the disease, as it requires prolonged exposure to the fly bites and parasite introduction. According to a 2002 WHO report, onchocerciasis has not caused a single death, but its global burden is 987,000 disability adjusted life years (DALYs). The severe pruritus alone accounts for 60% of the DALYs. Infection reduces the host’s immunity and resistance to other diseases, which results in an estimated reduction in life expectancy of 13 years.^[2]

The parasite is found in limited areas in America and in Yemen in the Middle East.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Those most at risk are people who live in areas where the parasite is spread followed by adventure travelers, missionaries, and peace corps volunteers who are exposed for long periods of time (generally more than 3 months) to black fly bites in areas where the parasite exists. The disease is most intensely transmitted in remote rural African agricultural villages which are located near rapidly flowing streams.

The people most at risk for acquiring onchocerciasis are

• Those who live near streams or rivers where there are Simulium black flies. Most of the areas where the black flies are found are rural agricultural areas in sub-Saharan Africa.
• People who travel for short periods of time (generally less than 3 months) to areas where the parasite is found have a low chance of becoming infected with O. volvulus. Those travelers to at-risk areas most likely to become infected are
  • Long-term missionaries
  • Peace Corps and other long-term volunteers
  • Field researchers.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

There can be inflammation of the optic nerve resulting in vision loss, particularly peripheral vision, and eventually blindness.

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