Streptobacillus moniliformis and Spirillum minus are the naturalpathogens of rodents. Both these agents cause a same condi-tion called rat-bite fever.
S. moniliformis is a pleomorphic Gram-negative bacillus(0.1–0.5 3 1–5 mm) that stains poorly with Gram staining. The bacteria may appear as a string of beads containing granules. The bacterium may lose its cell wall and readily develop into L forms. S. moniliformis is an aerobe and facultative anaerobe and highly fastidious bacterium. It is catalase, oxi-dase, indole, urease, and nitrate negative. It ferments glucose with production of acid but no gas. The bacterium is a slow grower. It grows on enriched media supplemented with 15% blood, 20% horse or calf serum, or 5% ascitic fluid. It produces discrete, granular or smooth, gray colonies after 3 days of incu-bation at 35–36°C in a humid aerobic condition. S. moniliformis causes rat-bite fever, a condition characterized by relapsing fever, rash, and arthralgia. Incubation period varies from 2 to 10 days. The condition manifests with sudden onset of fever, headache, myalgias followed by a petechial rash and arthritis. In untreated cases, relapses are common.
Rat-bite fever is usually caused by the bite of a rat or occasionally by the bite of a mouse, cat, squirrel, or dog. The disease also occurs without rat bite and is believed to occur due to ingestion of raw meat or water contaminated by excreta of the rat. This condition is known as Haverhill’s fever or erythemaarthricum epidemicum.
Penicillin is the antibiotic of choice for treatment of cases of rat-bite fever. Cephalosporins, erythromycin, clindamycin, and tetracycline are also used to treat the condition.