Reactive arthritis, classified as a seronegative spondyloarthropathy, typically follows an infection. Common causes include food poisoning or another infection of the intestine (called gastrointestinal reactive arthritis). Chlamydia, a sexually transmitted disease, is another common cause of reactive arthritis. Venereal infections of the bladder, urethra, or vagina can also cause reactive arthritis in many patients (called genitourinary or urogenital reactive arthritis). Reactive arthritis itself is not transmittable from one person to another.

Symptoms may include:

• Painful, swollen, stiff joints that may also be red and warm

• Stiffness that may be aggravated in the morning

• Lower-back pain

• Heel pain (such as Achilles heel)

• Eyes sensitive to light

• Mouth sores

• Sores on the genitals

Symptoms typically appear one to three weeks following an intestinal infection. Salmonella, shigella, campylobacter, and yersinia are common causative bacteria associated with intestinal infection. With reactive arthritis, one joint is usually involved rather than multiple joints; it's usually knees, ankles, or toes that are involved. The sacroiliac joints are also commonly involved. Symptoms of reactive arthritis may also affect body parts other than the joints, including tendons, skin, and eyes.

About 50 to 75 percent of patients with reactive arthritis are positive for HLA-B27. The genetic marker points to a genetic predisposition for the disease. Cases of reactive arthritis can resolve in days or weeks, while some take nearly four to twelve months to resolve. Recurring bouts of reactive arthritis are possible. Aside from medications used to treat the infection, anti-inflammatory medications are also used to treat arthritis and joint symptoms.

Reactive arthritis usually affects people between twenty and fifty years old. There is equal prevalence of reactive arthritis among males and females as a result of intestinal infections. However, men are nine times more likely than women to get reactive arthritis following venereal infections.

Also, according to NIAMS (the National Institute of Arthritis and Musculoskeletal and Skin Diseases), researchers are trying to ascertain more about the causal relationship between infection and reactive arthritis. It's still not known why infection triggers arthritis, and more specifically, why not everyone with an infection develops reactive arthritis. People with HLA-B27 are more at risk for developing reactive arthritis than those who lack the genetic factor, so the answers may, in part, have a genetic basis. Combination treatments, such as antibiotics and TNF blockers or other immunosuppressant medications, are also being studied for treatment of reactive arthritis.