Infectious arthritis is infection in the fluid and tissues of a joint usually caused by bacteria but occasionally by viruses or fungi.
There are two types of infectious arthritis:
Acute infectious arthritis that is caused by bacteria 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. In reactive arthritis, the joint is inflamed but not actually infected.
Chronic 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 (gonococci), which cause gonorrhea, viruses (such as hepatitis), and occasionally some other bacteria can infect a few or many joints at the same time.
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 person or dog, cat, or rat).
Different bacteria can infect a joint, but the bacteria most likely to cause infection depend on a person’s age:
There are many risk factors for infectious arthritis. Most children who develop infectious arthritis do not have identified risk factors.
Acute infectious arthritis is usually caused by bacteria and viruses.
Risk factors for acute infectious arthritis include
A past history of joint infection
An artificial joint or joint surgery
Use of needles to inject drugs
Behaviors that increase risk of sexually transmitted diseases (such as sex with multiple partners and without use of condoms)
An infection that reaches the bloodstream (bacteremia)
People being treated with dialysis
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.
Chronic infectious arthritis is usually caused by Mycobacterium tuberculosis (the main cause of tuberculosis), fungi, or bacteria.
Risk factors for chronic infectious arthritis include
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 milder symptoms. People typically have a fever for 5 to 7 days. People may have skin blisters, bumps, sores, rashes, or sores on the mouth or genitals and on the trunk, hands, or legs. 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 may have a fever, or may have 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 of infectious arthritis 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 causing 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 and determine whether the infection is anywhere else.
If doctors suspect the infectious arthritis is caused by gonococci, samples are also taken from the urethra, cervix, rectum, and throat. Tests for chlamydial infection of the genitals (another sexually transmitted disease) are also done because many people who have gonorrhea also have a chlamydial infection.
To make the bacteria easier to detect and identify, doctors may analyze the joint fluid using the polymerase chain reaction technique (a type of nucleic acid amplification test [NAAT]) to detect the DNA of gonococci and mycobacteria.
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 risk 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 and may be more difficult to cure with antibiotics. If drainage with a needle is difficult (as with a hip joint) or unsuccessful, arthroscopy (a procedure using a small scope to view the inside of the joint directly) 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 then started 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.