Rat-bite Fever

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PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

Synonyms: streptobacillary rat-bite fever, streptobacillosis, Haverhill fever, epidemic arthritic erythema, spirillary fever, sodoku

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] 

  • 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] 

A high index of suspicion is needed. Ask about exposure to rats or similar animals.

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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.
  • 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] 

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.
  • Penicillin is the first-line therapy.[4] 
  • Tetracycline is an alternative in those who are penicillin-allergic.[1][4] 
  • 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]
  • Avoiding contact with rats or rat-contaminated dwellings.
  • Prophylactic antibiotics after a rat bite.

Further reading & references

  1. Elliott SP; Rat bite fever and Streptobacillus moniliformis. Clin Microbiol Rev. 2007 Jan;20(1):13-22.
  2. Rat-bite fever; Public Health England
  3. 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 Mar;56(2):531-3. doi: 10.1111/j.1556-4029.2010.01675.x. Epub 2011 Feb 9.
  4. McKee G, Pewarchuk J; Rat-bite fever. CMAJ. 2013 Oct 15;185(15):1346. doi: 10.1503/cmaj.121704. Epub 2013 Mar 25.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Colin Tidy
Current Version:
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
2697 (v25)
Last Checked:
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