Rat-bite fever

Did You Know?

• Diagnosis is made by detecting bacteria in skin, blood, joint fluid, or lymph nodes, or through blood antibody tests.
• Half of all reported cases are children under 12 living in inner-city conditions with overcrowding and poor sanitation.

Symptoms and signs

Symptoms differ between the two forms of rat-bite fever, though many are shared. Streptobacillary RBF, most common in the United States, typically includes chills, fever, vomiting, headaches, muscle aches, and painfully swollen joints, often in the back. A red or purple rash may spread around the bite, and skin ulcers or inflammation can develop on the hands and feet. Wounds heal slowly, and symptoms may come and go over months. Spirillary RBF, more often diagnosed in Africa, involves swollen lymph nodes in the neck, groin, or underarm. Symptoms appear within two to ten days of exposure, beginning with fever and progressing to a rash on the hands and feet within two to four days; the rash covers the body but rarely causes joint pain.

Causes

Two types of Gram-negative, facultatively anaerobic bacteria cause rat-bite fever. Spirillum minus, a Gram-negative coiled rod, is rarer and found mostly in Asia, where the disease is called sodoku. Its incubation period is two to four weeks, and the entry wound shows slow healing and marked inflammation; fever lasts longer and may recur for months. Streptobacillus moniliformis causes the streptobacillary form, also known as Haverhill fever or epidemic arthritic erythema. After an incubation of 2–10 days, it begins with high fever, rigors, headache, and polyarthralgia, followed by a widespread rash and arthritis of large joints. Untreated, this form can be fatal in 20% of cases due to endocarditis, meningoencephalitis, or septic shock.

Treatment

Treatment begins with assessment and irrigation of the bite wound. Medical attention is needed for possible tetanus or rabies prophylaxis, though rats rarely carry rabies. Antibiotic therapy should start immediately, as laboratory confirmation may take days; therapy can later be adjusted based on antimicrobial susceptibility. Intravenous penicillin is the traditional treatment for uncomplicated cases; intravenous ceftriaxone may also be used. After 5–7 days of improvement, patients may switch to oral penicillin, ampicillin, or amoxicillin for a total of 14 days. For penicillin-allergic patients, cephalosporins such as cephalexin or doxycycline may be used. In children, penicillin G procaine intramuscularly or intravenous penicillin G for 7–10 days is typical, with a switch to oral penicillin after 5 days if uncomplicated. For endocarditis, high-dose intravenous penicillin G is required for at least 4 weeks; streptomycin or gentamicin may be added for severe infections.

Epidemiology

Rat-bite fever is a zoonotic disease transmitted directly by rats, gerbils, and mice through bites or scratches, or indirectly through contaminated food or water. The causative agent, Streptobacillus moniliformis, is found in the upper respiratory tract of rats, and nearly all domestic and wild rats carry it asymptomatically. The Black rat (Rattus rattus) and Norwegian rat (Rattus norvegicus) are recognized reservoirs due to their use as laboratory animals or pets. About 1 in 10 rat bites results in RBF. The disease is not contagious between humans. Understanding its prevalence is challenging because RBF is not a reportable disease to the U.S. Centers for Disease Control, though it is a notifiable condition at the state level. Most documented cases are caused by Streptobacillus moniliformis, primarily in the United States, where approximately 200 cases have been reported. Cases have been reported worldwide, but Spirillum minus is more common in Asia, where it is called sodoku. Increasing population density and human contact with animals have led to more frequent cases, especially in densely populated urban areas.