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Rat-Bite Fever


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Reviewed/Revised Nov 2022 | Modified Sep 2023
Topic Resources

Rat-bite fever is an infection caused by either Streptobacillus moniliformis or Spirillum minus bacteria.

  • People usually get the infection when they are bitten by an infected rat.

  • Streptobacillary rat-bite fever causes joint pain.

  • Spirillary rat-bite fever causes swollen lymph nodes.

  • Both types cause a fever that comes and goes and a rash.

  • Doctors diagnose rat-bite fever based on the symptoms and may send a sample of blood or infected tissue to a laboratory to be cultured (streptobacillary rat-bite fever), or they may examine the sample under a microscope (spirillary rat-bite fever).

  • Rat-bite fever is effectively treated with antibiotics.

There are two types of rat-bite fever:

  • Streptobacillary rat-bite fever

  • Spirillary rat-bite fever

Streptobacillary rat-bite fever is caused by Streptobacillus moniliformis, which lives in the mouth and throat of healthy rats, mice, and gerbils. It is a rod-shaped bacteria called a bacillus (see figure ). People usually get the infection when they are bitten or scratched by a wild or pet rat or mouse. Other rodents and weasels can also spread the infection. Sometimes people become infected when they drink unpasteurized milk that contains the bacteria. The infection that results from consuming the bacteria is called Haverhill fever. Streptobacillary rat-bite fever typically occurs in the United States.

Spirillary rat-bite fever (sodoku) is caused by Spirillum minus. It is a bacteria that has a spiral shape similar to spirochetes (see figure ). People usually get the infection when they are bitten by a rat or occasionally a mouse. Consuming Spirillum bacteria does not cause the infection. Spirillary rat-bite fever occurs primarily in Asia.

Symptoms of Rat-Bite Fever

Both types of rat-bite fever cause many of the same symptoms, but there are some differences.

Streptobacillary rat-bite fever

In streptobacillary rat-bite fever, the rat bite, if present, usually heals quickly. Within a day to about 3 weeks after the bite heals, symptoms develop abruptly. They include chills, fever, vomiting, headache, and back and joint pains. A few days later, a rash of small flat, red bumps usually appears on the hands and feet. Without treatment, joint pains and infectious arthritis Infectious Arthritis Infectious arthritis is infection in the fluid and tissues of a joint usually caused by bacteria but occasionally by viruses or fungi. Bacteria, viruses, or fungi may spread through the bloodstream... read more (infection in the fluid and tissues of a joint) may last for several days or months. Fever may come and go for weeks to months.

Haverhill fever causes similar symptoms, but vomiting is more severe, and the throat may be sore.

Spirillary rat-bite fever

In spirillary rat-bite fever, the bite site usually heals quickly. However, a few days to about 4 weeks later the bite becomes red and swollen (inflamed), and a fever develops, then comes and goes. Lymph nodes are swollen. A rash sometimes develops, but it is less pronounced than the streptobacillary rash. Joints rarely become painful.

Without treatment, fevers typically come and go for up to 8 weeks.

Diagnosis of Rat-Bite Fever

  • For streptobacillary rat-bite fever, culture of a sample of blood or joint fluid

  • For spirillary rat-bite fever, examination of a sample of blood or infected tissue

Doctors can often diagnose the type of rat-bite fever by looking for key differences in symptoms. For example, doctors look to see whether people have joint pain, which occurs in streptobacillary rat-bite fever, or look to see whether people have swollen lymph nodes, which occur in spirillary rat-bite fever.

To confirm the diagnosis of streptobacillary rat-bite fever, doctors take a sample of blood or joint fluid and send it to a laboratory where bacteria, if present, can be grown (cultured) and analyzed. Occasionally, the sample is stained using special stains and examined under a microscope. Blood tests are sometimes helpful.

To confirm the diagnosis of spirillary rat-bite fever, doctors take a sample of blood or tissue around the bite or from an infected lymph node. Spirillary bacteria can be identified when a sample is examined under microscope. These bacteria cannot be cultured.

Prognosis for Rat-Bite Fever

Without treatment, about 10% of people with rat-bite fever die.

Treatment of Rat-Bite Fever

  • Antibiotics

Both types of rat-bite fever are treated with antibiotics such as penicillin and ceftriaxone given by vein and then switched to an antibiotic such as ampicillin, amoxicillin, or penicillin taken by mouth. Doxycycline is given to people who are allergic to penicillin.

If Streptobacillus moniliformis causes endocarditis, high doses of penicillin plus another antibiotic are used.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Centers for Disease Control and Prevention (CDC): Rat-Bite Fever: Information about rat-bite fever, including links about risk, treatment, and infection in animals

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