Infections and Immunodeficiencies That Mimic Rheumatic Disease
Infection mimicking arthritis
Septic arthritis is an emergency: fever, refusal to bear weight, and an acutely swollen joint require aspiration before antibiotics whenever feasible; Staphylococcus aureus predominates, with Kingella kingae in toddlers. Lyme arthritis causes a large, relatively painless knee effusion in endemic areas. Reactive arthritis follows enteric or genital infection and associates with HLA-B27. Acute rheumatic fever (Jones criteria) follows group A streptococcal pharyngitis with migratory arthritis, carditis, chorea, erythema marginatum, or subcutaneous nodules. Viral arthritides such as parvovirus B19 are self-limited.
Malignancy and immunodeficiency
Leukemia and neuroblastoma can present with bone pain, night pain, limp, and cytopenias - check the blood count before attributing pain to JIA. Immunodeficiency mimics include complement deficiency (SLE-like disease plus encapsulated infections), common variable immunodeficiency, chronic granulomatous disease, and Wiskott-Aldrich syndrome.
Traps
Never start corticosteroids for presumed JIA before excluding septic arthritis and malignancy. A child with lupus-like features plus recurrent pneumococcal or meningococcal infection needs complement studies. Live vaccines are contraindicated in many immunodeficiencies and during significant immunosuppression.
Rule of thumb
Acute monoarthritis is septic until aspirated; bone pain with cytopenias is leukemia until the marrow says otherwise.