Rat Bite Fever

Adapted from Weese, JC NAVC Clinician’s Brief, July 2012 pg 13-17

Pet rats (North America, Europe) may carry an organism called Streptobacillus moniliformis. The rat shows no symptoms of having this bacteria. However, in rare occasions, a bite from such can cause disease in humans.

Rat bite fever is a rare but serious infection predominantly associated with pet rats. In Asia a different bacteria has been associated with disease in humans (Spirillum minus). S.moniliformis is a common oral commensal bacteria that can be found in most if not all pet rats. Transmission is mainlly through biting, although contact of rat saliva with skin lesions and other close contact (kissing, sharing food) are also a concern. Some cases have been reported in the absence of any known high-risk contact. Rat bite fever in humans has been associated with other species including various rodents and dog.

Children under 12 years of age are most often affected, likely because of close rat contact, and the greater likelihood of being bitten. Typically, the first symptoms occur 2-3 days after exposure although greater than 3 weeks has been reported. Signs include an abrupt onset of a high fever with headache, chills, vomiting, severe arthralgia and myalgia. Fever may be relapsing and a rash typically develops, but there is usually no obvious abnormality at the site of the bite. Polyarthritis is common and may be relapsin. The most serious consequence is endocarditis, which is uncommon but potentially life-threatening. Other complications include meningitis, myocarditis, pericarditis, pneumonia, and distant abscessation. Sever vomiting and pharyngitis (Haverhill fever) are most common after oral exposure.

Response to appropriate treatment (typically with a peniciliin) is usually good unless endocarditis is found, then the prognosis may be poorer. Overall mortality rate is 7-10% if untreated.

Because the organism is found in almost all healthy rats, preventive screening is not warranted. It is a difficult organism to culture as well. Treatment of every rat is also not recommended as the organism is ubiquitous.

A key consideration is whether the problem is the rat, management, a high-risk household, or the inherent and unavoidable potential for zoonotic disease exposure by pet owners.

Take Home Messages

Although the incidence of zoonotic infection is low in humans, zoonotic disease is an ever-present risk for pet owners.

All rats should be assumed to be colonized with S.moniliformis. Rat owners should be aware of the risk, and use good practices to reduce likelihood of bites, properly clean any bite wounds, and avoid contact of rat saliva with broken skin or mucous membranes.

Physicians need to be aware of pet exposure to facilitate accurate and prompt diagnosis of zoonoses.

Rat bite fever should be considered in any person who develops a fever shortly after a rat bite or close contact with rat saliva.

Although rare, rat bite fever can be severe, so the risk for infection to rat owners should not be ignored.

Measures to Reduce the Risk for Rat Bite Fever

Good handling to reduce the risk for bites

Proper traning of children on how to handle rats

Closely supervising children while they handle rats

Restricting handling by children who cannot (or will not) properly handle rats

Regularly inspecting cages and fixing any sharp edges that might lead ot injuries that could inoculate S.moniliformis

If skin lesions are present, avoid contact with rats or wearing gloves when handling rats; at a minimum, good attention should be paid to hand hygiene

Avoid kissing or sharing food with rats.


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