Infectious arthritis is a form of arthritis caused by an infection in the joint. It is also called septic arthritis.
In a different form of arthritis, called reactive arthritis, an infection in another party of the body – usually the intestines, genitals or urinary tract – triggers an inflammatory response in the joints. Unlike septic arthritis, however, the infection itself is not present in the joint. Several types of systemic infections – including Lyme disease, infectious hepatitis, fifth disease, mumps, German measles and rheumatic fever – can also have joint symptoms or can trigger arthritis, but as with reactive arthritis the joint itself is not infected.
Most types of infectious arthritis are caused by bacteria. The most common of these is Staphylococcus aureus (staph), a bacterium that lives on even healthy skin. Infectious arthritis can also be caused by viral or fungal infections.
In most cases, infectious arthritis develops when an infection somewhere else in the body travels through the bloodstream to the joint. Less commonly the infection enters the joint directly through a puncture wound or surgery on or near the joint.
The most common symptoms of infectious arthritis are intense swelling and pain, usually in a single joint. In about half of all cases, infectious arthritis involves the knee, but hips, ankles and wrists are also commonly affected. Less commonly, infectious arthritis affects more than one joint.
Joint pain and swelling may be accompanied by other signs of infection, including fever and chills.
Diagnosis of infectious arthritis will include a complete medical history, physical exam and laboratory tests. Analyzing a sample of joint fluid can determine what organism is causing the infection and help the doctor plan treatment. X-rays and other imaging tests of the affected joint also may be ordered to assess any damage to the joint.
Treatment will depend on the type of germ causing the infection. Bacterial infections are almost always treated with antibiotics. The type of antibiotic used will depend on the specific bacterium causing the infection. Antibiotics may be taken by mouth or given by intravenous injection. Antibiotics often stop the infection in a few days to a few weeks, but in some cases, they must be given over several months.
Infectious arthritis caused by a fungus can be hard to treat, often requiring several months of antifungal medicine and sometimes surgery to remove the infected tissue. Infectious arthritis caused by a virus, on the other hand, usually goes away on its own with no specific treatment.
Treatment may also be necessary to relieve pain and inflammation of infectious arthritis. In some cases, the doctor may drain the infected joint by inserting a needle into it and withdrawing fluid.
In addition to treatment prescribed by the doctor, it is important to rest and protect inflamed joints. After the infection is gone, gentle exercise is helpful for building muscle strength to support the joint and improve range of motion.