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Infectious Arthritis

(Septic Arthritis)

by Steven Schmitt, MD

Infectious arthritis is infection in the fluid and tissues of a joint usually by bacteria but also by viruses or fungi.

  • Bacteria, viruses, or fungi may spread through the bloodstream or from a nearby infection into a joint, causing infection.

  • Pain, swelling, and fever usually develop within hours or a couple of days.

  • Joint fluid is withdrawn in a needle and tested.

  • Antibiotics are begun immediately.

There are two types of infectious arthritis: acute and chronic.

Acute infectious arthritis is generally caused by bacteria and viruses. This type of infectious arthritis begins quickly. It accounts for 95% of infectious arthritis. It can affect healthy people as well as people at high risk. Cartilage within the joint, which is essential for normal joint function, can be destroyed or damaged within hours or days. Sometimes, arthritis develops in people who have infections that do not involve the bones or joints, such as infections of the genital organs or digestive organs. This type of arthritis is a reaction to that infection and so is called reactive arthritis (see Reactive Arthritis). In reactive arthritis, the joint is inflamed but not actually infected.

Chronic infectious arthritis is usually caused by Mycobacterium tuberculosis (the main cause of tuberculosis), fungi, or bacteria. This type of infectious arthritis begins gradually over several weeks. It accounts for 5% of infectious arthritis and most often affects people who are at high risk.

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.

There are many risk factors for infectious arthritis. People at greater risk of acute infectious arthritis include those who have a past history of joint infection, use needles to inject drugs, or have abnormal joints because of arthritis (including rheumatoid arthritis, osteoarthritis, or arthritis caused by injury) and 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. Other risk factors include chronic illnesses (such as diabetes, lupus, and chronic lung or liver disorders), older age, alcoholism, and behaviors that increase risk of sexually transmitted diseases (such as sex with multiple partners and without use of condoms or a diaphragm). Acute infectious arthritis may occur in children who have no risk factors. About 50% of children with joint infection are under 3 years of age. However, routine childhood vaccination for Haemophilus influenzae and Streptococcus pneumoniae is lowering the incidence in this age group.

People at greater risk of chronic infectious arthritis include those who have rheumatoid arthritis, human immunodeficiency virus (HIV) infection, or a suppressed immune system (due to cancer or use of drugs that suppress the immune system). Most children who develop infectious arthritis do not have identified risk factors.

People who have an artificial joint are at higher risk of acute and chronic infectious arthritis.

Infecting organisms, mainly bacteria, usually spread to the joint from a nearby infection (such as osteomyelitis or an infected wound) or through the bloodstream. A joint can be infected directly if it is contaminated during surgery or by an injection or an injury (such as a bite wound from a human, dog, cat, or rat). Different bacteria can infect a joint, but the bacteria most likely to cause infection depend on a person’s age. Staphylococci, streptococci, and bacteria known as gram-negative bacilli most often infect infants and young children. 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 HIV, parvoviruses, and those that cause rubella, mumps, and hepatitis B—can infect joints in people of any age.

Did You Know...

  • People who have chronic arthritis, such as rheumatoid arthritis, and suddenly develop pain and swelling in a single joint should contact their doctor promptly because they may have an infection even if they have no fever.


In acute infectious arthritis, symptoms usually begin over hours to a few days. The infected joint usually becomes severely painful and sometimes red and warm. Moving or touching it is very painful. Fluid collects in the infected joint, causing it to swell and stiffen. Symptoms sometimes also include fever and chills.

Gonococcal arthritis usually causes more mild symptoms. People may have skin blisters, bumps, sores, rashes, or sores on the mouth or genitals. Pain may move from one joint to another before a joint becomes swollen and tender. Tendons may become inflamed.

Infants and children too young to talk tend not to move the infected joint, are irritable, may refuse to eat, and have a high, low-grade, or no fever. Young children with knee or hip infections may refuse to walk.

In chronic infectious arthritis, symptoms are usually gradual swelling, mild warmth, minimal or no redness of the joint area, and aching pain that may be mild and less severe than in acute infectious arthritis. Usually a single joint is involved. An infection that lasts a long time and that does not go away after use of conventional antibiotics may be caused by mycobacteria or fungi.

People may have other symptoms depending on the cause of infectious arthritis, such as symptoms of Lyme disease, or swollen lymph nodes if the cause is an infected bite wound.


Doctors typically suspect the diagnosis in people who have severe or unexplained arthritis and in people who have other combinations of symptoms that are known to occur in people who have infectious arthritis.

Usually, a sample of joint fluid is removed with a needle (called joint aspiration or arthrocentesis) as soon as possible. It is examined for an increased number of white blood cells and tested for bacteria and other organisms. The laboratory can usually grow and identify the infecting bacteria from the joint fluid (called a culture), 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 does 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. If doctors suspect the infectious arthritis is caused by gonococci, samples are also taken from the urethra, cervix, rectum, and throat. Tests for chlamydia of the genitals are also done.

Doctors may do magnetic resonance imaging (MRI) if the joint cannot be easily examined or aspirated. MRI and ultrasonography are also done to identify accumulations of fluid or collections of pus (abscesses).


Infectious arthritis that is caused by nongonococcal bacteria can permanently destroy joint cartilage within hours or days. Infectious arthritis that is caused by gonococcal bacteria does not usually damage joints permanently. People with rheumatoid arthritis usually do not regain total use of the infected joint and the rate of death is increased.


It is important to start antibiotics as soon as an infection is suspected, even before the laboratory has identified the infecting organism. Antibiotics that kill the bacteria that are most likely causing the infection 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. Intravenous antibiotics are continued for 2 to 4 weeks. Then, antibiotics are given by mouth at high doses for another 2 to 6 weeks.

The doctor often removes pus with a needle (aspiration) 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 see Tests for Musculoskeletal Disorders : Arthroscopy) or surgery may be needed to drain the joint. Aspiration is often done more than once. Sometimes a tube is left in place to drain the pus. Splinting of the joint (to keep it from moving) is done for the first few days to help ease pain, but physical therapy is also needed to strengthen muscles and 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 fungi and mycobacteria require long-term treatment. Infections caused by viruses usually get better without antibiotic treatment. Nonsteroidal anti-inflammatory drugs (NSAIDs) can help reduce pain, inflammation, and fever.

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