Infectious arthritis (septic arthritis) is infection in the fluid and tissues of a joint usually caused by bacteria, but sometimes caused by viruses or fungi.
People at risk of infectious arthritis include those who have abnormal joints because of arthritis (including rheumatoid arthritis, osteoarthritis, or arthritis from injury) who develop an infection that reaches the bloodstream. For example, an older person with pneumonia and sepsis (a bloodstream infection) may fall and injure a wrist. Bleeding into the injured wrist may then result in infectious arthritis.
Infecting organisms, mainly bacteria, usually reach the joint through the bloodstream, but a joint can be infected directly if it is contaminated during surgery or by an injection or an injury. Different bacteria can infect a joint, but the bacteria most likely to cause infection depend on a person's age. Staphylococci and bacteria known as gram-negative bacilli most often infect infants and young children, whereas gonococci (bacteria that cause gonorrhea), staphylococci, and streptococci most often infect older children and adults. Occasionally, spirochetes (a type of bacteria), such as those that cause Lyme disease and syphilis, can infect joints.
Viruses—such as the human immunodeficiency virus (HIV), parvoviruses, and those that cause rubella, mumps, and hepatitis B—can infect joints in people of any age. A slowly developing chronic infectious arthritis is most often caused by Mycobacterium tuberculosis (the main cause of tuberculosis) or fungi.
Infants usually have fever and pain and tend to be fussy. Generally, infants do not move the infected joint because moving or touching it is painful. Young children with knee or hip infections may refuse to walk. In older children and adults, symptoms usually begin over hours to a few days. The infected joint usually becomes red and warm, and moving or touching it is very painful. Fluid collects in the infected joint, causing it to swell and stiffen. Symptoms also include fever and chills. Less dramatic symptoms (such as less pain and a lower fever) usually occur in people with chronic infectious arthritis that is caused by mycobacteria or fungi.
The joints most commonly infected are the knee, shoulder, wrist, hip, elbow, and the joints of the fingers. Most bacterial, fungal, and mycobacterial infections affect only one joint or, occasionally, several joints. For example, the bacteria that cause Lyme disease most often infect knee joints. Gonococcal bacteria and viruses can infect a few or many joints at the same time.
Usually, a sample of joint fluid is removed with a needle as soon as possible. It is examined for white blood cells and tested for bacteria and other organisms. The laboratory can almost always grow and identify the infecting bacteria from the joint fluid, unless the person has recently taken antibiotics. However, the bacteria that cause gonorrhea, Lyme disease, and syphilis are difficult to recover from joint fluid. If bacteria do grow in culture, the laboratory then tests which antibiotics would be effective.
A doctor usually orders blood tests because bacteria from joint infections often appear in the bloodstream. Sputum, spinal fluid, and urine may also be tested for bacteria to help determine the source of infection.
Prognosis and Treatment
Because an infected joint can be destroyed within days or sometimes within hours without prompt treatment, antibiotics must be started as soon as an infection is suspected, even before the laboratory has identified the infecting organism. Antibiotics that kill the most likely bacteria are given until the infecting organism is identified, usually within 48 hours of testing the joint fluid. Antibiotics are given by vein (intravenously) at first, to ensure that enough of the drug reaches the infected joint. If the antibiotics are effective against the infecting bacteria, improvement usually occurs within 48 hours. As soon as the doctor receives the laboratory results, the antibiotic may be changed depending on the sensitivity of the particular bacteria to specific antibiotics.
The doctor often removes pus with a needle to prevent its accumulation, because accumulated pus may damage a joint. If drainage with a needle is difficult (as with a hip joint) or unsuccessful, arthroscopy (a procedure using a small scope to view the interior of the joint directly—see Diagnosis of Musculoskeletal Disorders: Arthroscopy) or surgery may be needed to drain the joint. Sometimes a tube is left in place to drain the pus. Splinting the joint (to keep it from moving) can help ease pain at first, but physical therapy is also needed to prevent stiffness and permanent loss of function.
Infections caused by fungi are treated with antifungal drugs. Infections caused by mycobacteria are treated with a combination of antibiotics. Infections caused by viruses usually get better without treatment—only acetaminophen or a nonsteroidal anti-inflammatory drug (NSAID) is needed for pain and fever.
Last full review/revision February 2008 by Steven Schmitt, MD