Rat-bite fever: Rat-bite fever is a rarely diagnosed infection. In the United States, the most frequent cause of rat-bite fever is Streptobacillus moniliformis, a fastidious gram-negative bacterium commonly carried by healthy rodents, including pets and laboratory animals. In Asia, most cases are due to Spirillum minus, a gram-negative, coiled rod also carried by rodents.

Infection may occur not only from bites by colonized animals but also from a scratch or from fecal-oral transmission. Incubation is typically 2-10 days, and symptoms usually begin abruptly, with fever, chills, severe migratory arthralgias and myalgias, vomiting, and headache. Young children may have diarrhea and weight loss. After 2-4 days after these symptoms appear, patients have a maculopapular, petechial, morbilliform, pustular, or vesicular rash on their extremities, including the palms and the soles. Symptoms may be most severe in the lower extremities, and fever may resolve and relapse in an irregular fashion. Evidence of the bite itself is often absent by the time the patient presents. Regional lymphadenopathy is present in only S minus infections.

Complications of rat-bite fever include focal abscesses, pneumonia, meningitis, endocarditis, myocarditis, anemia, and multiorgan failure. Death has been reported in as many as 13-25% of untreated patients. Up to one half of patients develop septic arthritis or asymmetric polyarthritis.

The diagnosis is based on clinical findings and confirmed with blood cultures. S moniliformis requires 10-20% serum medium and an environment with 5-10% carbon dioxide for optimal growth. Even under these conditions, this organism may take as long as 20 days to grow. Sodium polyanethol sulfonate, an anticoagulant, can inhibit bacterial growth if present in culture bottles. Patients typically have a marked leukocytosis with a leftward shift. As many as 25% of patients have false-positive serologic results for syphilis.

The treatment of choice for rat-bite fever is intravenous penicillin in doses of 200,000 U given every 4 hours for 7 days. This is followed by oral penicillin 500 mg given 4 times a day for an additional 7 days. In patients with penicillin allergy, doxycycline 100 mg given orally twice a day is the recommended alternative. S moniliformis is also usually sensitive to ampicillin, cefotaxime, and azithromycin. Prophylaxis for rat bites should include aggressive cleansing of the bite wound, prophylactic antibiotics taken for 3 days, immunization with tetanus toxoid if indicated, and regular wound care. No known cases of rat-transmitted rabies have been reported in the United States; therefore, prophylaxis is not recommended here.