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

Diagnosis

• Leukocytosis

• Nontreponemal test for syphilis often falsely positive

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

• Differential diagnosis

  • Streptobacillary fever

  • Tularemia

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

  • Pasteurella multocida infection

  • Relapsing fever

Treatment

• 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%)