Sibling Exposure and Risk of Juvenile Idiopathic Arthritis

Objective Susceptibility to juvenile idiopathic arthritis (JIA) is presumed to be determined by both genes and environment. However, the environmental factors remain largely unknown. The hygiene hypothesis suggests that exposure to siblings, as a marker of exposure to microbes in early life, may protect against the development of later immune disorders. Some prior evidence suggests this may also be true for JIA. The present study was undertaken to test this hypothesis in detail. Methods We conducted a comprehensive analysis of the role of sibling exposure in JIA risk within the Childhood Arthritis Risk Factor Identification Study JIA case–hospital control sample (302 cases and 676 controls) from Victoria, Australia. Results We found that, compared to being an only child, having any siblings was protective against JIA, with an adjusted odds ratio (OR) of 0.46 (95% confidence interval [95% CI] 0.28–0.74) (P = 0.001). The protective association appeared to increase with increasing number of siblings (e.g., for ≥3 siblings, adjusted OR 0.25 [95% CI 0.13–0.48], P