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.
(See also Overview of Bacteria.)
There are two types of 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 How Bacteria Shape Up). 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 How Bacteria Shape Up). 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.
Both types of rat-bite fever cause many of the same symptoms, but there are some differences .
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 (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.
Streptobacillus may infect the heart, causing endocarditis. Abscesses may form in the brain or other tissues. These problems and infectious arthritis are rare but serious.
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.
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.
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