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Rat-bite fever is caused by either Streptobacillus moniliformis or Spirillum minus. Symptoms of the streptobacillary form include fever, rash, and arthralgias. The spirillary form causes relapsing fever, rash, and regional lymphadenitis. Diagnosis is clinical and confirmed by culture and sometimes rising antibody titers. Treatment is with oxacillin-clavulanate or doxycycline.

Rat-bite fever is transmitted to humans in up to 10% of rat bites. However, there may be no history of rat bite. Both the streptobacillary and spirillary forms affect mainly urban dwellers living in crowded conditions and biomedical laboratory personnel. The streptobacillary form is more common.

Streptobacillary rat-bite fever: This form is caused by the pleomorphic gram-negative bacillus Streptobacillus moniliformis, an organism present in the oropharynx of healthy rats. Epidemics have been associated with ingestion of unpasteurized S. moniliformis–contaminated milk (Haverhill fever), but infection is usually a consequence of a bite by a wild rat or mouse. Other rodents and weasels have also been implicated.

The primary wound usually heals promptly, but after an incubation period of 1 to 22 (usually < 10) days, a viral-like syndrome develops abruptly, causing chills, fever, vomiting, headache, and back and joint pains. Most patients develop a morbilliform, petechial rash on the hands and feet about 3 days later. Polyarthralgia or arthritis, usually affecting the large joints asymmetrically, develops in many patients within 1 wk and may persist for several days or months if untreated. Bacterial endocarditis and abscesses in the brain or other tissues are rare but serious. Some patients have infected pericardial effusion and infected amniotic fluid.

Diagnosis is confirmed by culturing the organism from blood or joint fluid. Measurable agglutinins develop during the 2nd or 3rd wk and are diagnostically important if the titer increases. The WBC count ranges between 6,000 and 30,000/μL. The streptobacillary form usually can be differentiated clinically from the spirillary form.

Treatment includes amoxicillin-clavulanate Some Trade Names

875/125 mg po bid, procaine penicillin G Some Trade Names
BICILLIN
WYCILLIN
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600,000 units IM bid, or penicillin V 500 mg po qid for 7 to 10 days. Erythromycin Some Trade Names
ERY-TAB
ERYTHROCIN
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500 mg po qid may be used for patients allergic to penicillin. Doxycycline Some Trade Names
PERIOSTAT
VIBRAMYCIN
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100 mg bid for 14 days is an alternative.

Spirillary rat-bite fever (sodoku): Spirillum minus infection is acquired through a rat or, occasionally a mouse, bite. The wound usually heals promptly, but inflammation recurs at the site after 4 to 28 (usually > 10) days, accompanied by a relapsing fever and regional lymphadenitis. A roseolar-urticarial rash sometimes develops but is less prominent than the streptobacillary rash. Systemic symptoms commonly accompany fever, but arthritis is rare. In untreated patients, 2- to 4-day cycles of fever usually recur for 4 to 8 wk, but febrile episodes rarely recur for > 1 yr.

Diagnosis is by direct visualization or culture of Spirillum from blood smears or tissue from lesions or lymph nodes, or by Giemsa stain or darkfield examination of blood from inoculated mice. The WBC count ranges between 5,000 and 30,000/μL. The Venereal Disease Research Laboratory (VDRL) results are false-positive in ¹⁄₂ the patients. The disease may easily be confused with malaria, meningococcemia, or Borrelia recurrentis infection, all of which are characterized by relapsing fever.

Treatment is the same as for the streptobacillary form.

Last full review/revision November 2005

Content last modified November 2005