Rat-bite fever is a rare zoonosis caused by the Gram negative bacterium Streptobacillus moniliformis. The organism has a worldwide distribution and is carried in the upper respiratory tract of healthy asymptomatic rats. Rats are the main reservoir of the organism. Other animals including cats, dogs, ferrets, and weasles, are susceptible to infection with S. moniliformis (which they presumably acquire via contact with rodents), and have occasionally been responsible for disease transmission.
Human infection is usually acquired either from a rat bite/scratch, handling infected rats, or, in the case of the form of disease known as Haverhill fever, ingestion of milk or water contaminated with the organism (via rat urine). A large outbreak occurred following this route of transmission in the UK in 1983 (McEvoy 1987). At a boarding school in Essex, 304 pupils (of 700, 43%) had a febrile illness with arthralgia and a rash. Four cases were blood culture positive for S. moniliformis. Raw milk initially appeared to be the vehicle of infection, but detailed epidemiological investigations and analysis showed that the vehicle was more likely to have been water. Opportunity for contamination of drinking water by rats existed, although contamination was not confirmed by the isolation of S. moniliformis from either rats or the water supply.
Rat bite fever is rarely diagnosed in the England and Wales - around one to two cases per year.
The bite itself usually heals without any signs of infection. Rat-bite fever usually develops within 7 days of the bite, and begins abruptly with fever and chills, followed by severe myalgia and joint pain, headache and nausea. Many patients develop a rash, most often on the extremities. About half of all patients develop a non-suppurative arthritis, and a minority of cases have a form of non-bacteraemic disease with septic arthritis. Person-to-person transmission does not occur. The infection can be successfully treated with antibiotics; penicillin is the treatment of choice.
Streptobacillus moniliformis is an extremely fastidious organism and microbiological diagnosis is not straightforward.
Blood culture media WITHOUT sodium polyanethol sulfonate (SPS) should be used because of the inhibitory nature of this commonly used anticoagulant. Anaerobic blood cutlure bottles have proved effective, as have resin-containing media and tryptic soy broth. The organisms are slow growing and bottles may become positive as late as after 7days incubation. In broth the growth may appear as 'puffballs' or as flocculated sediment. On solid media colonies are smooth, greyish, glistening, and may develop a 'fried-egg' appearance on prolonged incubation. Synovial fluid samples should be inoculated onto blood agar plates (as well as into SPS-free broth), and incubated aerobically in CO2 and anaerobically for up to 5 days.
Another form of rat-bite fever is caused by the Gram negative spiral bacterium Spirillum minus. This is also rare and has been resported only in Asia.
Other zoonoses transmitted by rats
Elliott SP. Rat bite fever and Streptobacillus moniliformis. Clin Microbiol Rev 2007, 20:13-22.
McEvoy M, Noah N, Pilsworth R. Outbreak of fever caused by Streptobcacillus moniliformis. The Lancet 1987;ii: 1361-3
CDC. Fatal rat-bite fever, Florida and Washington, 2003. MMWR 2005; 53:1198-1202. Available from http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5351a2.htm
Wang TKF, Wong SSY.Streptobacillus moniliformis septic arthritis: a clinical entity distinct from rat-bite fever? BMC Infectious Diseases 2007,7:56 doi:10.1186/1471-2334-7-56. Available from http://www.biomedcentral.com/1471-2334/7/56