Rat-bite Fever
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Synonyms: streptobacillary rat-bite fever, streptobacillosis, Haverhill fever, epidemic arthritic erythema, spirillary fever, sodoku
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Rat-bite fever is a zoonosis usually caused by infection with the bacterium Streptobacillus moniliformis. Another form of rat-bite fever (more common in Asia where it is known as sodoku[1]) is caused by infection with the Gram-negative bacterium Spirillum minus.[2]
Rats are the usual hosts of the bacteria, with Streptobacillus moniliformis being carried in the pharyngeal flora of 10-100% of healthy wild and laboratory rats.[1]Human infection is usually acquired through rat bites, scratches or handling (contact with urine, excreta or secretions from mouth, eye or nose).[1]However, guinea pigs, weasels, gerbils, squirrels and ferrets may occasionally be responsible for human infection.[1]
A variant of rat-bite fever is known as Haverhill fever (so called after the town in the USA in which there was an outbreak in 1926).[1]It is thought to be caused by ingestion of milk or water contaminated with the bacteria via rat urine.[2]Signs and symptoms are identical to those of rat-bite fever caused by S. moniliformis but there is no history of rat bite or exposure. Large groups of people may be affected; outbreaks can occur.
Person-to-person transmission of infection does not occur.[2]
Epidemiology
- Rat-bite fever is rare.
- Rats throughout the world may carry S. moniliformis but the disease is most often reported in Asia, Europe and North America.
- Rat-bite fever caused by S. minus is mainly reported in Asia.
- There are only 1-2 cases of rat-bite fever per year in the UK.[2]
- People who keep rats as pets, as well as laboratory technicians and pet shop employees working with rats, are at increased risk.
- Children also seem to be particularly susceptible.[3]
Presentation
A high index of suspicion is needed. Ask about exposure to rats or similar animals.
S. moniliformis infection
- The classic triad of symptoms is fever, rash and polyarthritis.
- The incubation period is usually less than seven days (but can be three days to three weeks).[1]
- Evidence of the bite may have disappeared.
- Initially there is sudden high fever (38-41°) with rigors, nausea and vomiting, severe headache, sore throat, myalgia and joint pains.[1]
- The fever usually resolves in three to five days but it can recur.[1]
- A diffuse rash occurs in around 75% of cases. It can be maculopapular, petechial or purpuric. Haemorrhagic vesicles can develop on the extremities.[1]The rash can be slow to resolve.
- An asymmetrical migratory polyarthralgia (large and small joints) occurs in around 50% of cases.[1]Arthritis with joint pain, redness and swelling can occur.The polyarthralgia can last several years in some people.[1]
S. minus infection
- Incubation period is usually longer (14-18 days).[1]
- The rat-bite often ulcerates, and there is marked local lymphadenopathy.
- The fever is relapsing and remitting.
- A typical violaceous red-brown macular rash develops in 50% of people. Plaques and urticarial lesions may also be present.[1]
- Joint involvement is rare.
Investigations
- FBC: leukocytosis.
- Bacterial culture: blood cultures, joint fluid or pus. S. minus may be isolated from the wound.
- Isolation of the organism can be difficult. The bacteria are slow-growing so it may take up to seven days for the culture to become positive. Media without sodium poly-anethol sulfonate (SPS) should be used because the anticoagulant can inhibit bacterial growth.[2]
- Discussion with the microbiology laboratory is advised in suspected cases.[4]
- PCR can also be used to identify the bacteria.[4]
Differential diagnosis[1]
These include:
- Sepsis due to streptococcal and staphylococcal bacteria.
- Lyme disease.
- Brucellosis.
- Rickettsial infections including Rocky Mountain spotted fever.
- Leptospirosis.
- Secondary syphilis.
- Viral infections including Epstein-Barr virus.
- Other causes of relapsing fever, including malaria and typhoid fever.
- Collagen vascular diseases.
- Drug reactions.
Management
- Penicillin is the first-line therapy.[4]
- Tetracycline is an alternative in those who are penicillin-allergic.[1, 4]
Complications[1]
- Parotitis
- Tenosynovitis
- Bronchitis, pneumonia
- Metastatic abscesses, septicaemia
- Endocarditis, myocarditis, pericarditis
- Vasculitis and polyarteritis nodosa
- Meningitis, encephalitis
- Nephritis
- Hepatitis
Prognosis
- If treated, prognosis is excellent and symptoms usually subside in a few days.
- Untreated, the mortality rate is approximately 10%.[1]
- Arthritis may last for several months.[1]
Prevention
- Avoiding contact with rats or rat-contaminated dwellings.
- Prophylactic antibiotics after a rat bite.
Further reading and references
Elliott SP; Rat bite fever and Streptobacillus moniliformis. Clin Microbiol Rev. 2007 Jan20(1):13-22.
Rat-bite fever; Public Health England
Banerjee P, Ali Z, Fowler DR; Rat bite fever, a fatal case of Streptobacillus moniliformis infection in a 14-month-old boy. J Forensic Sci. 2011 Mar56(2):531-3. doi: 10.1111/j.1556-4029.2010.01675.x. Epub 2011 Feb 9.
McKee G, Pewarchuk J; Rat-bite fever. CMAJ. 2013 Oct 15185(15):1346. doi: 10.1503/cmaj.121704. Epub 2013 Mar 25.