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Information for Health Care Workers

Etiologic Agent

In North America, rat-bite fever (RBF) is caused primarily by Streptobacillus moniliformis, a fastidious, non-motile, gram-negative microaerophilic bacillus. Spirillum minus, a short, thick, motile spirochete, has been reported to cause RBF predominately in Asia.


S. moniliformis and S. minus are part of the normal respiratory flora of rodents. Either organism may be transmitted to humans through bites or scratches. Infection can also result from handling an infected rodent (even with no reported bite or scratch), or ingestion of food or drink contaminated with these bacteria (Haverhill fever). Rats are considered the natural reservoir of RBF, but the bacterium has also been found in other rodent species such as, mice and gerbils. Person-to-person transmission has not been reported.

Clinical Features

Initial symptoms of S. moniliformis are non-specific and include fever, chills, myalgia, headache, and vomiting. Patients may develop a maculopapular rash on the extremities 2-4 days after fever onset, followed by polyarthritis in approximately 50% of patients. The incubation period typically ranges from 3-10 days. The symptoms and signs of Haverhill fever differ slightly from those of RBF acquired through bites and/or scratches. Haverhill fever can be associated with more severe nausea/vomiting and pharyngitis.

Symptoms due to Spirillum minus usually occur 7-21 days after exposure to an infected animal and the patient is likely to have a history of travel outside of the U.S. Following partial healing of the rat bite, common symptoms and signs include fever, ulceration at the site, lymphangitis, lymphadenopathy, and a distinct rash of purple or red plaques.

If not appropriately treated, infection may result in as endocarditis, myocarditis, meningitis, pneumonia or sepsis. The mortality rate for untreated RBF is between 7%-13%.


RBF should be suspected in people with rash, fever, and arthritis and a known or suspected history of rodent exposure. S. moniliformis is difficult to grow in culture and requires specific media and incubation conditions. RBF is diagnosed by isolating S. moniliformis from blood, synovial fluid, or other body fluids. In the absence of a positive culture, identification of pleomorphic gram-negative bacilli in appropriate specimens supports a preliminary diagnosis. S. minus does not grow in artificial media. For this reason, diagnosis is made by identifying characteristic spirochetes in appropriate specimens using darkfield microscopy or differential stains. If RBF is suspected in a severe illness or death, but a diagnosis has not been made, physicians can consider requesting diagnosis assistance from their state public health laboratories.


RBF is rare in the United States. Accurate data about incidence rates are unavailable because the disease may not be reportable to state health departments. Since RBF is not a nationally notifiable disease, trends in disease incidence in the U.S. are not available. Recent case reports have highlighted the potential risk for RBF among persons having contact with rodents at home or in the workplace.