In the U.S., rat bite fever is primarily caused by transmission of S. moniliformis from the bite of a rat.^[1] However, approximately 30% of patients diagnosed with rat bite fever do not recall being scratched or bitten by an infected animal.^[2][3] Transmission of the bacterium is also known to occur via consumption of infected water, close contact with, or handling of rats.^[1][4] Haverhill fever, named after the 1926 outbreak of the disease in Haverhill, Massachusetts, is a form of rat bite fever that can result from ingesting food contaminated with S. moniliformis.^[5] In 1986 at a boarding school in the United Kingdom, another outbreak of Haverhill fever was reported. Some 304 people were reported to have been afflicted.^[6] Infection was suspected to have resulted from the consumption of either unpasteurized milk or water contaminated with rat feces.^[7] Infected individuals described symptoms including a sudden development of vomiting, severe headache, and cold sweats with a high fever.^[7] Parker and Hudson first isolated the cause of this outbreak, which they named Haverhilia multiformis.^[3] This organism was later matched to S. moniliformis after further research.^[8]

Symptoms of rate bite fever include the abrupt onset of fever ranging from 38.0 °C to 41 °C.^[5] Approximately 75% of infected individuals develop a rash in addition to hemorrhaging vesicles.^[1] Both the rash and vesicles are usually located on the hands and feet, although the rash has been known to spread to other parts of the body.^[9]

The microaerophilic nature of S. moniliformis makes identification difficult.^[5] PCR testing is being utilized more for its identification.^[8] However, there is still a 13% mortality rate for untreated cases.^[4] Immunocompromised individuals, such as HIV-positive individuals, are more at risk of death from this disease.^[10] Lab personnel and pet store workers, who work closely with animals on a daily basis, also have an increased risk of infection.^[5]

Although S. moniliformis is believed to be part of the commensal bacteria of the respiratory tract of rats,^[11] rats have occasionally shown signs of the disease.^[5] Antibiotics used to treat infection may cause the formation of the L-form, which persists in the body although this form is not pathogenic.^[5]