Chapter 281. Rat-Bite Fever

Rat-bite fever is an acute febrile illness that occurs as a result of a rodent bite, usually a rat. Two distinct microorganisms, Streptobacillus moniliformis and Spirillum minus, the agent of Soduka in Asia,1 cause this infection. S moniliformis, the main etiologic agent of rat-bite fever, is a fastidious, gram-negative, pleomorphic, and often filamentous and beaded facultative anaerobic bacillus.1,2

In addition to rat-bite fever, S moniliformis causes an overlapping syndrome, Haverhill fever, also known as erythema arthriticum epidemicum.1

Sodoku, a disease reported in Asia that is currently rare in the United States, is the name given to rat-bite fever caused by S minus.

Epidemiology

S moniliformis is a normal inhabitant of the upper respiratory tract of rodents and may be excreted in rat urine. Humans are infected by the bite of a rat (or mouse, squirrel, cat, or weasel) or, less commonly, by a scratch from a rat, by handling a dead animal, or by contact with rat-eating carnivores.1,2 Approximately 50% of cases reported are in children. Infection may also be acquired by ingestion of milk or water contaminated with rat excreta, as occurred in epidemic form in 1916 in Haverhill, Massachusetts, resulting in Haverhill fever.1,3

Clinical Features

Seven to 10 days (range 2–21 days) after a rat bite, there is an abrupt onset of fever accompanied by chills, headache, vomiting, muscle pain, and often, asymmetric polyarthritis4 that may reflect sterile effusions or septic arthritis with S moniliformis present in the joint fluid.5 Several days later there is a maculopapular and sometimes petechial rash, which is most prominent on the extremities, including the palms and soles.4 The bite wound has usually healed and the site exhibits no or minimal inflammation. Generalized adenopathy commonly occurs. Young children often have diarrhea and weight loss. Many of the clinical features are similar to Rocky Mountain spotted fever. Left untreated, the infection follows a relapsing course lasting a mean of 3 weeks, but may have a fatal outcome1,6 or result in arthritis persistent for several months. Other reported manifestations include septic arthritis without a rash,5 amnionitis, brain abscess, disseminated fatal infection in infants, endocarditis, hepatitis, meningitis, myocarditis, nephritis, and pneumonia.1,6Patients with Haverhill fever exhibit fever, rash, and arthritis; vomiting and pharyngitis are more prominent than in patients with rat-bite fever.

Sodoku disease, due to S minus, has an incubation period of 1 to 4 weeks. There is fever, ulceration at the previously healed bite site, and regional lymphadenopathy. The fever may be intermittent and associated with rash, with afebrile intervening days. The infection responds rapidly to therapy with penicillin.1

Diagnostic Evaluation

The diagnosis is established by recovering S moniliformis from cultures of blood or joint fluid, but the organism is fastidious and slow growing.1 Broth enriched with blood, serum, or ascitic fluid, or blood or chocolate agar incubated in a CO2-supplemented environment should be used.1 Sodium polyanethol sulfonate can inhibit growth of S moniliformis, but it has been recovered using Bactec Peds-Plus and Bactec Plus media that contain this anticoagulant.7,8 Twenty-five percent of infected patients have a false-positive nontreponemal serologic test for syphilis.1

S minus is a spirillum-like organism that cannot be grown in vitro and is identified by darkfield microscopic examination of material from an ulcer or blood smear.

Treatment

Penicillin given for 10 to 14 days is the treatment of choice for rat-bite fever,1 although penicillin-resistant strains have been reported rarely. The organism is susceptible to many antibiotics in addition to penicillin, including ampicillin, cefuroxime, cefotaxime, and tetracycline.9 Isolates are resistant to sulfonamides. For therapy of endocarditis, the addition of streptomycin to high-dose penicillin should be considered.1 Disease caused by S minus also rapidly responds to therapy with penicillin.

Prevention and Control

Because the attack rate of rat-bite fever after a rat bite is approximately 10%, individuals sustaining rat bites should be observed closely. Penicillin prophylaxis should be considered, although its efficacy is unknown.