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For further information, see CMDT Part 34-09: Rat-Bite Fever

Key Features

  • Rat-bite fever is an uncommon acute infectious disease caused by Spirillum minus

  • 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

  • Rat bite heals promptly, but 1 to several weeks later the site

    • Becomes swollen, indurated, and painful

    • Assumes a dusky purplish hue

    • May ulcerate

  • Regional lymphangitis and lymphadenitis

  • Fever, chills, malaise, and headache

  • Myalgia, arthralgia, arthritis

  • Splenomegaly

  • A sparse, dusky-red maculopapular rash on the trunk and extremities

  • After a few days, symptoms subside, only to reappear several days later

  • Relapsing fever for 3–4 days alternating with afebrile periods lasting 3–9 days; may persist for weeks

  • Endocarditis is rare complication

  • Differential diagnosis

    • Streptobacillary fever

    • Tularemia

    • Rickettsial disease (eg, Rocky Mountain spotted fever, epidemic typhus)

    • Pasteurella multocida infection

    • Relapsing fever


  • Leukocytosis

  • Nontreponemal test for syphilis often falsely positive

  • Organism may be identified in darkfield examination of the ulcer exudate or an aspirated lymph node


  • In acute illness,

    • Intravenous penicillin 1–2 million units every 4–6 hours is given initially

    • Ceftriaxone 1g intravenously daily is another option

  • Once improvement has occurred, therapy may be switched to

    • Oral penicillin V 500 mg four times daily for 10–14 days or

    • Amoxicillin 500 mg three times daily for 10–14 days

  • For the penicillin-allergic patient, tetracycline 500 mg orally four times daily or doxycycline 100 mg twice a day can be used

  • Prompt diagnosis and antimicrobial treatment can markedly reduce the mortality rate (usually about 10%)

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