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Division of Foodborne, Bacterial and Mycotic Diseases (DFBMD)

Rat-bite Fever

General Information | Technical Information | Additional Information

Clinical Features

Initial symptoms are non-specific and include fever, chills, myalgias, arthralgias, headache, vomiting. Patients may develop a maculopapular rash on the extremities or septic arthritis 2-4 days after fever onset. The incubation period typically ranges from 2-10 days. If not appropriately treated, severe manifestations may include endocarditis, myocarditis, meningitis, pneumonia and sepsis. In rare cases, death occurs.

Etiologic Agent

In the United States , Rat-bite fever (RBF) is caused primarily by infection with Streptobacillus moniliformis. S. moniliformis is a fastidious, non-motile, gram-negative microaerophilic bacillus. Spirillum minus causes RBF in Africa and Asia . S. minus is a short, thick, motile spirochete.


RBF is rare in the United States . However, accurate data about incidence rates are unavailable because the disease is not reportable in any state.


Severe complications such as endocarditis, myocarditis, pericarditis, pneumonia, meningitis, and focal organ abscesses may occur. Rapidly fatal cases have been reported. Untreated RBF is associated with a mortality of 7%-10%. With appropriate antimicrobial therapy, the clinical course may be shortened and severe complications may be prevented.


S. moniliformis and S. minus are part of the normal respiratory flora of rats. Both organisms may be transmitted to humans through rat bites or scratches. Infection can also result from handling an infected rat, with no reported bite or scratch. Infection with S. moniliformis can also occur through ingestion of food or drink contaminated with rat excrement ( Haverhill fever). Other rodents (e.g. mice, gerbils) may also be reservoirs. Person-to-person transmission has not been reported.

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Risk Groups

Persons who are at risk for RBF include those who handle rats at home or in the workplace (e.g., laboratories or pet stores). Children living in crowded urban dwellings or rural areas infested with wild rats are also at risk.


The findings of rash, fever, and arthritis in individuals with a history of rat exposure suggest the diagnosis of Rat-bite fever.

Diagnosis of S. moniliformis is typically made by isolating the organism from blood or synovial fluid. Specific media and incubation conditions should be used. In the absence of a positive culture, identification of pleomorphic gram-negative bacilli in appropriate specimens supports a preliminary diagnosis. Since the organism does not grow in artificial media, diagnosis of S. minus is made by identifying characteristic spirochetes in appropriate specimens using darkfield microscopy or differential stains.


Rat-bite fever is not a reportable disease in any state. However, unexplained deaths or critical illnesses or rare diseases of public health importance may be reportable in certain states. If RBF is suspected in a severe illness or death but a diagnosis has not been made, physicians can consider reporting the case to their state or local health department.


Since Rat-bite fever is not a reportable disease, trends in disease incidence are not available. However, recent reports have highlighted the potential risk for RBF among persons having contact with rats at home or in the workplace.


Diagnosis of S. moniliformis is difficult and requires specific media and incubation conditions. While infection is thought to be rare, severe complications and death may occur. The challenge is to increase awareness for appropriate risk-reduction activities and possible symptoms of RBF among persons who have exposure to rats. Clinicians should be aware of the possible symptoms and methods of diagnosis of RBF.


Advances in molecular methods of diagnosis, such as broad-range polymerase chain reaction (PCR) may improve the ability to diagnose S. moniliformis .

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Page last modified: March 27, 2008
Content Source: National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)

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