Rat-bite fever

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 sodoku¹) 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.¹Human infection is usually acquired through rat bites, scratches or handling (contact with urine, excreta or secretions from mouth, eye or nose).¹However, 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).¹It is thought to be caused by ingestion of milk or water contaminated with the bacteria via rat urine.²Signs 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.¹The rash can be slow to resolve.

• An asymmetrical migratory polyarthralgia (large and small joints) occurs in around 50% of cases.¹Arthritis 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 diagnosis¹

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.¹⁴

Complications¹

• 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.