Rat-bite fever (RBF) is a febrile illness resulting from infection with either Streptobacillus moniliformis or Spirillum minus. The causative agent of the more common RBF is S moniliformis, which is found worldwide. RBF due to S minus is found primarily in Asia. Alternative terms for this form of RBF include spirillary fever and the Japanese name sodoku (from so meaning rat and doku meaning poison). RBF due to S minus rarely occurs in the United States but has occurred in laboratory workers. Travelers to endemic areas are also at risk.

Spirillum minus is a gram-negative, spiral-shaped rod that cannot be grown on artificial media. It has been found in up to 25% of wild rats (in oropharynx, blood, or eye exudate). It is much less common in laboratory rats.

RBF due to S minus arises from the bite of a rat. At highest risk for RBF are persons with exposure to rats including laboratory workers, undomiciled persons, and children with rats as pets.

RBF due to S minus typically presents with the following sequence:

• The initial rat bite heals.
• One to four weeks later, the bite site becomes edematous and tender. Regional lymphangitis appears. A systemic febrile illness occurs. Arthritis is typically absent. Infants and children may develop severe diarrhea and weight loss. The systemic symptoms may be accompanied by a diffuse macular rash.
• The bite site then ulcerates and may form an eschar. Laboratory examination may show leukocytosis.

Without antimicrobial therapy, the illness may become relapsing. The pattern is typically a recurrent pattern of 3 days of fever followed by a 3- to 9-day afebrile period.

If untreated, potential complications of both forms of RBF may include meningitis, endocarditis, myocarditis, hepatitis, nephritis, and splenomegaly.