The aim of this study was to determine the seasonality of month of birth (MOB) in children with juvenile idiopathic arthritis (JIA) as compared to the general population.
Cosinor analysis was used to analyse MOB rhythmicity in 558 children with JIA from a simple rheumatology clinic compared with the MOB pattern of the general population in Israel (n=1.040558). Statistical differences between groups were also analysed by non-parametrical tests.
Patients with JIA showed different patterns from that of the general population. A rhythmic pattern of 12 months was found in the MOB patterns of JIA patients. This rhythm with a peak between November to March and a nadir in summer was a mirror image of the rhythmic pattern observed for MOB of the healthy population. Males showed a pattern with combined rhythm of 8 and 6 months with peaks in winter, while females' MOB pattern showed no rhythmicity. Testing different JIA subtypes, only the patients with the enthesitis-related arthritis (ERA) subtype showed rhythmicity in MOB. Rhythmicity patterns were different for males and females, and differed according to several disease characteristics.
The observed pattern of MOB in JIA patients is distinctive and different from that in the healthy population supporting the hypothesis that autoimmune process may begin in utero or in the perinatal period due to seasonal environmental pathogenic agents.