Rat bite fever is a bacterial infection spread by rodents that causes recurring fevers, joint pain, and a distinctive rash. Left untreated, it carries a mortality rate of about 13%, but it responds well to antibiotics when caught early. Despite the name, you don’t actually need to be bitten by a rat to get it.
How You Get It
Two different bacteria cause rat bite fever. The more common one, found worldwide, spreads through bites, scratches, or even just contact with a rodent’s saliva or urine. The second type is rarer and found primarily in Asia, especially Japan, though cases have appeared in the United States, Europe, and Australia.
The infection doesn’t always involve a dramatic rat bite. You can pick it up from handling contaminated bedding or cages, kissing a pet rat, or consuming food or drinks that rodents have contaminated. When the infection comes from contaminated food or milk rather than a bite, it’s sometimes called Haverhill fever. Pet rat owners, people who work with laboratory animals, and anyone living in rodent-infested housing face the highest risk.
Symptoms and Timeline
With the more common form, symptoms typically appear 3 to 10 days after exposure, though the incubation period can stretch beyond three weeks. The original bite or scratch usually heals quickly, which can make the connection easy to miss. The first sign is often a sudden fever with chills and rigors, sometimes accompanied by symptoms that feel like a bad cold or upper respiratory infection.
Within days, joint pain sets in. It tends to migrate from joint to joint, affecting both large and small joints in the arms and legs, with the knees and ankles most commonly involved. This migratory joint pain is the most persistent feature of the illness and can linger for months or even years in some cases. Nearly 75% of patients also develop a rash that can look like flat red spots, tiny pinpoint hemorrhages, or purple patches. Painful, blood-filled blisters sometimes appear on the hands and feet.
The rarer Asian form behaves a bit differently. The incubation period is longer, typically 7 to 21 days. The bite wound may initially seem to heal, then reopen or ulcerate as fever and other symptoms emerge.
When It Becomes Dangerous
Most people recover fully with treatment, but untreated rat bite fever can spiral into serious complications. These include infection of the heart valves (endocarditis), which carries a mortality rate as high as 50%. Other potential complications include abscesses, pneumonia, inflammation of the tissue surrounding the heart, pancreatitis, and, rarely, meningitis. The 13% overall mortality rate for untreated cases drops dramatically once antibiotics are started.
How It’s Diagnosed
Rat bite fever is rare enough that it often gets missed on the first visit. Doctors rely heavily on your history of rodent exposure, so mentioning any contact with rats, mice, or their environments is critical, even if the interaction seemed minor. Blood tests and cultures can confirm the diagnosis, though the bacteria can be difficult to grow in the lab. If you’ve had a fever, joint pain, and rash following any kind of rodent contact, make sure your doctor knows about the exposure.
Treatment and Recovery
Rat bite fever is treated with penicillin, typically given intravenously for a week or more before switching to an oral form. For people allergic to penicillin, alternative antibiotics are available. Most patients improve quickly once treatment starts, though the joint pain can take longer to fully resolve. The key is starting antibiotics promptly, since delays increase the risk of complications.
Reducing Your Risk
If you own pet rats, wash your hands thoroughly after handling them, cleaning their cages, or touching their bedding. Avoid kissing pet rats or letting them near your face. Keep food and drinks away from areas where rodents live or roam. If you’re bitten or scratched, clean the wound immediately and thoroughly with soap and water, and watch for symptoms over the following weeks. For people living in older buildings or areas with wild rodent activity, sealing entry points and keeping food stored securely reduces the chance of contact with contaminated surfaces.