Reactive arthritis is an autoimmune condition, named for the fact that it normally occurs as a reaction to an infection somewhere in the body. It is an uncommon but painful form of inflammatory arthritis that affects the heels, toes, fingers, lower back, and joints, such as in the knees or ankles. It is also associated with inflammation of the urethra, the eyes, and sometimes skin and mucous membranes.
There are two types of bacteria that most commonly cause reactive arthritis– bacteria associated with intestinal infections and bacteria associated genital infections. Chlamydia trachomatis, the bacterium that causes chlamydia, is known to be a trigger of reactive arthritis. But it can be triggered by other sexually transmitted diseases as well as certain intestinal infections. Common intestinal pathogens include Campylobacter, Shigella, Salmonella, and Yersinia, often the cause of food contamination.
Reactive arthritis is uncommon and not everyone who gets one of these infections will develop reactive arthritis. Risk factors for reactive arthritis include gender and genetic predisposition. The disorder is most commonly seen in men between the ages of 20 and 50, though it can occur at any age. To a lesser extent, women can also have reactive arthritis.
Some people are at increased risk because they are positive for HLA-B27, a specific protein (termed a human leukocyte antigen or HLA) that is found on cell surfaces. The gene that codes for the HLA-B27 is estimated to be present in about 65% to 96% of individuals with reactive arthritis, while HLA-B27 is found in only about 6% of the general U.S. population. Although having HLA-B27 is a risk factor for reactive arthritis, it is still possible for people who are HLA-B27 negative to develop reactive arthritis, and it may be that other genetic factors are involved.