Infectious causes of arthritis include bacterial, viral or postviral, and fungal. Osteomyelitis and reactive arthritis can also be confused with JIA.


Joint involvement in septic arthritis may be by hematogenous spread, direct extension from local tissues, or as a reactive arthritis.

Bacterial septic arthritis is usually monoarticular in children, but multiple joints can be involved. Children may present with fever, joint pain, and decreased joint mobility, especially in the knees, hips, ankles, and elbows. A child may not allow the affected joint to be touched and, sometimes, may not even allow the affected joint to be seen. An ambulatory child will refuse to bear weight on the affected extremity. Premature infants presenting with irritability, fever, and hips positioned in abduction, flexion, and external rotation should be checked for septic arthritis of the hip. Boys 3 to 10 years who present with hip or referred knee pain should be checked for transient synovitis. Ear infections are the most common source of bacteria leading to septic arthritis in children (123). Osteomyelitis or diskitis can develop in children with septic or reactive arthritis.

In all age groups, 80% of cases are caused by gram-positive aerobes (60% S. aureus; 15% beta-hemolytic streptococci; 5% Streptococcus pneumoniae), and approximately 20% of cases are caused by gram-negative anaerobes. In neonates and infants younger than 6 months, S. aureus and gram-negative anaerobes comprise the majority of infections.

Clinically affected joints require emergent aspiration and treatment. Aspiration of joint fluid is necessary for possibly identifying the agent and relieving pain.

Joint fluid reveals increased white blood cells (WBCs), protein, and low-to-normal glucose. Radiographic findings progress from soft tissue swelling to juxta-articular osteoporosis, joint space narrowing, and erosion. Treatment consists of appropriate antibiotic therapy, joint aspiration to relieve pressure and pain, and physical therapy to maintain ROM.


Reactive arthritis is different from septic arthritis in that it is an autoimmune response triggered by antigen deposit in the joint spaces; synovial fluid cultures are negative. It is set off by a preceding infection, the most common of which would be a genital infection with Chlamydia trachomatis in the United States, usually in adult males (124). Reactive arthritis after Yersinia and Campylobacter can be associated with HLA-B27. Yersinia enterocolitica infection can show persistence of the organism in joint fluid, especially the knee. The main goal of treatment is to identify and eradicate the underlying infectious source with appropriate antibiotics, if still present. Analgesics, steroids, and immunosuppressants may be needed for patients with severe reactive symptoms that do not respond to any other treatment.


Lyme disease is caused by the spirochete, Borrelia burgdorferi, with transmission to humans via the deer tick, Ixodes dammini. Lyme disease is the most common tick-borne disease in North America and Europe. The initial phase of Lyme disease (lasting about 4 weeks) consists of fever, fatigue, headache, arthralgias, myalgias, stiff neck, and erythema migrans. Erythema migrans looks like a reverse target skin lesion, as it is a large, red lesion with a central clearing area; it occurs 1 to 30 days after the tick bite. The late phase, lasting months to years, is characterized by arthritis, cardiac disease, and neurologic disease. Intermittent episodes of unilateral arthritis involve the knee most often; hip, shoulder, elbow, wrist, and ankle may also be involved. In 85% of children, the arthritis resolves before the end of the initial treatment; in 10%, a chronic inflammatory phase develops.

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