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