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Rat-bite fever is an uncommon acute infectious disease caused by the treponeme Spirillum minus (Asia), or the bacteria Streptobacillus moniliformis (North America). It is transmitted to humans by the bite of a rat. Inhabitants of rat-infested dwellings, owners of pet rats, and laboratory workers are at greatest risk.

CLINICAL FINDINGS

A. Symptoms and Signs

In Spirillum infections, the original rat bite, unless secondarily infected, heals promptly, but 1 to several weeks later the site becomes swollen, indurated, and painful; assumes a dusky purplish hue; and may ulcerate. Regional lymphangitis and lymphadenitis, fever, chills, malaise, myalgia, arthralgia, and headache are present. Splenomegaly may occur. A sparse, dusky-red maculopapular rash appears on the trunk and extremities in many cases, and there may be frank arthritis.

After a few days, both the local and systemic symptoms subside, only to reappear several days later. This relapsing pattern of fever for 3–4 days alternating with afebrile periods lasting 3–9 days may persist for weeks. The other features, however, usually recur only during the first few relapses. Endocarditis is a rare complication of infection.

B. Laboratory Findings

Leukocytosis is often present, and the nontreponemal test for syphilis is often falsely positive. The organism may be identified in darkfield examination of the ulcer exudate or aspirated lymph node material; more commonly, it is observed after inoculation of a laboratory animal with the patient’s exudate or blood. It has not been cultured in artificial media.

DIFFERENTIAL DIAGNOSIS

Rat-bite fever must be distinguished from the rat-bite–induced lymphadenitis and rash of streptobacillary fever. Clinically, the severe arthritis and myalgias seen in streptobacillary disease are rarely seen in disease caused by S minus. Reliable differentiation requires an increasing titer of agglutinins against S moniliformis or isolation of the causative organism. Other diseases in the differential include tularemia, rickettsial disease, Pasteurella multocida infections, and relapsing fever.

TREATMENT

In acute illness, intravenous penicillin, 1–2 million units every 4–6 hours is given initially; ceftriaxone 1 g intravenously daily is another option. Once improvement has occurred, therapy may be switched to oral penicillin V 500 mg four times daily, or amoxicillin 500 mg three times daily, to complete 10–14 days of therapy. For the penicillin-allergic patient, tetracycline 500 mg orally four times daily or doxycycline 100 mg twice a day can be used.

PROGNOSIS

The reported mortality rate of about 10% should be markedly reduced by prompt diagnosis and antimicrobial treatment.

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Rahman  S  et al. Fever, petechiae, and joint pain. J Fam Pract. 2017 May;66(5):323–5.
[PubMed: 28459894]

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