Rat-bite fever: symptoms, diagnosis and treatment

Rat-bite fever is a zoonosis, an infection passed from animals to people. Patient.info says it is usually caused by the bacterium Streptobacillus moniliformis, which is carried in the throat flora of a large proportion of healthy wild and laboratory rats. Another form, known as sodoku and more common in Asia, is caused by Spirillum minus.

The MSD Manual adds that rat-bite fever can be caused by the bite of any rodent, or of a carnivore that preys on rodents. A variant called Haverhill fever is thought to be caused by swallowing milk or water contaminated by rat urine, with symptoms identical to the bite-acquired form but no history of a bite or exposure. Patient.info says person-to-person transmission does not occur.

Patient.info says people who keep rats as pets, laboratory technicians and pet shop employees working with rats are at increased risk, and that children seem to be particularly susceptible. The MSD Manual says both forms mainly affect urban dwellers living in crowded conditions and biomedical laboratory personnel.

Because infection can follow handling rather than a bite, a person may not recall being bitten. The MSD Manual says the infection is transmitted to humans in up to 10% of rat bites, but there may be no history of a bite at all.

For the usual (streptobacillary) form, Patient.info says the classic triad is fever, rash and polyarthritis. The incubation period is usually less than seven days but can be three days to three weeks, and evidence of the bite may have disappeared. The illness starts suddenly with high fever and rigors.

• Sudden high fever with shaking chills (rigors).

• Nausea and vomiting, severe headache, sore throat and muscle aches.

• Joint pains that move from joint to joint, and sometimes frank arthritis with redness and swelling.

• A rash in around 75% of cases, which may be flat or raised spots, tiny red or purple dots, or small blisters on the hands and feet.

• Fever that usually settles in three to five days but may recur.

The MSD Manual explains that the spirillary form (sodoku) behaves differently. After a bite that initially seems to heal, inflammation recurs at the site after 4 to 28 days with a relapsing fever and regional swollen glands. A red-brown rash may develop, and arthritis is rare. Patient.info says the incubation period is usually longer, around 14 to 18 days, and the rat-bite often ulcerates.

This form is mainly reported in Asia, so it is rarely the explanation for illness acquired in the UK, though travel history is relevant.

Patient.info stresses that a high index of suspicion is needed and that doctors should ask about exposure to rats or similar animals. The diagnosis is clinical and confirmed by culturing the organism from blood, joint fluid or pus. The bacteria are slow-growing, so it may take up to seven days for a culture to become positive, and discussion with the microbiology laboratory is advised.

The MSD Manual says PCR and antibody tests may also be helpful, and that certain syphilis blood tests can be falsely positive. Doctors also consider other causes of fever and rash, including sepsis, Lyme disease, brucellosis, leptospirosis, viral infections and drug reactions.

Patient.info says penicillin is the first-line therapy and tetracycline is an alternative for people who are allergic to penicillin. The MSD Manual lists penicillin or doxycycline. Treated, prognosis is excellent and symptoms usually subside in a few days according to Patient.info, although arthritis may last for several months.

Complications can be serious. Patient.info lists infections of the heart (endocarditis, myocarditis, pericarditis), meningitis, abscesses, septicaemia and pneumonia, and says that untreated, the mortality rate is approximately 10%. The MSD Manual describes bacterial endocarditis, septic arthritis and abscesses as rare but serious.

The NHS advises cleaning a bite wound with soap and warm water, gently squeezing it so it bleeds slightly, pressing a clean dressing on if it is bleeding heavily, then drying and covering it. It says to get help from NHS 111 if the bite is hot or swollen, you have a high temperature or feel hot, cold or shivery, fluid is leaking from it, it smells unpleasant, you were bitten on the hands, feet, face or head, or you have not had a tetanus vaccine in the last 10 years. Go to A&E if the wound is large or deep or you cannot stop the bleeding.

Patient.info lists prevention as avoiding contact with rats or rat-contaminated dwellings, and prophylactic antibiotics after a rat bite. If you develop fever, rash or joint pain in the days or weeks after a rat bite, scratch or handling of rats, tell the doctor about the exposure.

On the NHS, the first contact is usually a GP, NHS 111 or an emergency department. Because the infection is rare and culture can be slow, it helps to say clearly that you have had contact with rats so the laboratory can be told.

Private doctors can arrange the same blood tests. The MSD Manual says the microbiology laboratory should be notified so that specific culture conditions can be used, so the key step in either setting is making sure the possibility has been raised.