Rat-Bite Fever

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

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

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

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%)