Reactive Arthritis Overview

What is reactive arthritis?

Reactive arthritis, also known as Reiter’s syndrome, is an autoimmune inflammatory disorder that arises after an infection elsewhere in the body. The immune system, in attempting to eliminate the pathogen, mistakenly targets the joints, eyes, and skin, leading to inflammation. Although it can develop in anyone, it most often emerges in young adults following a bacterial or viral infection in the lower digestive or urinary tract during the recovery period.

Reactive arthritis falls within a group of diseases called spondyloarthropathies, which share features such as inflammation of the spine and sacroiliac joints. Unlike rheumatoid arthritis, it does not typically involve symmetrical joint swelling or damage to cartilage. Instead, it often targets peripheral joints like the knees, ankles, and fingers, and may also affect the eyes and skin, reflecting its systemic nature, particularly during the first few weeks.

Statistical data show that reactive arthritis affects about 1–2 per 10,000 people each year, with a higher prevalence among males in their twenties and thirties. The condition is more common in populations carrying the HLA‑B27 gene, but it can occur in individuals without this genetic marker. Age, sex, and ethnicity influence incidence, though precise global rates vary by region. Health authorities recommend monitoring for new cases in endemic areas.

How does an infection trigger reactive arthritis?

The trigger most frequently involves bacteria from the gastrointestinal tract, such as Campylobacter jejuni, Salmonella species, or Shigella dysenteriae. These organisms colonize the intestinal mucosa and can escape into the bloodstream or lymphatic system, exposing the immune system to their surface proteins. The initial infection often presents as diarrhea, abdominal pain, or fever before any joint symptoms appear. This lag time can range from days to weeks after the infection.

The immune system mounts a response that involves T‑cell activation and antibody production. In some individuals, the antibodies cross‑react with proteins found in joint cartilage, leading to inflammation. This molecular mimicry is the core mechanism that transforms a localized infection into a systemic joint disorder. The immune response can persist even after the bacteria are cleared, sustaining joint inflammation for months or longer.

Other infection sites, such as the urinary tract or respiratory system, can also trigger the disease. Chlamydia trachomatis is a common genitourinary pathogen linked to reactive arthritis, especially in sexually active adults. Respiratory infections caused by Mycoplasma pneumoniae or certain viruses are less frequent but still documented triggers. In all cases, the initial infection must be significant enough to provoke a measurable immune response.

What are the common symptoms and how do they progress?

Joint manifestations are the hallmark of reactive arthritis. Pain, stiffness, and swelling usually begin in the lower limbs, affecting knees, ankles, and sometimes the hips. The arthritis is often asymmetric, meaning only one side of a joint may be involved, and it can flare intermittently over weeks or months. Patients may also experience reduced range of motion, particularly after periods of inactivity.

Extra‑articular symptoms often accompany joint disease. Conjunctivitis, presenting as red, itchy eyes, is common, while urethritis can cause burning during urination and a discharge. Skin eruptions such as keratoderma blennorrhagicum appear as thickened, scaly patches on the soles, and a pityriasis‑like rash may develop on the trunk. These extra‑articular signs usually arise within a few weeks of the initial infection and can help clinicians differentiate reactive arthritis from other rheumatic disorders.

The severity and duration of symptoms vary widely. In some individuals, arthritis resolves within a few months; in others, it persists for years, sometimes progressing to chronic ankylosing changes. Early joint inflammation can lead to structural damage if not addressed promptly, underscoring the importance of timely evaluation. Patients experiencing prolonged pain should seek rheumatologic assessment to prevent potential joint degeneration and to discuss appropriate management options.

Why does reactive arthritis appear in some people but not others?

Genetic predisposition plays a substantial role. The HLA‑B27 allele is present in approximately 70–80 % of people with reactive arthritis, compared to about 5 % of the general population. Carriers of HLA‑B27 have a higher likelihood of mounting an exaggerated immune response to bacterial antigens, leading to joint inflammation. However, not all individuals with HLA‑B27 develop the disease, indicating that additional environmental or immunological factors are involved.

The bacterial strain itself influences disease risk. Certain serotypes of Shigella or Campylobacter possess virulence factors that more readily trigger cross‑reactivity. Moreover, the timing of antibiotic treatment—whether it is administered early or delayed—can alter the likelihood of developing reactive arthritis. Environmental exposure, such as travel to regions with high infection rates, and individual immune status, including prior infections or autoimmune conditions, also contribute to the overall risk profile.

Below is a concise comparison of the most common infectious triggers, the typical organisms involved, and the clinical features most often associated with each. This overview helps clinicians recognize patterns that may guide diagnostic testing and anticipate the potential for extra‑articular manifestations, improving patient care through targeted evaluation.