Epidemiology of juvenile chronic arthritis: Risk dependent on sibship, parental income, and housing

Objective. We studied the socioeconomic background of children with juvenil e chronic arthritis (JCA) diagnosed during the years 1988-91 in Denmark. Th e working hypothesis is that JCA may be triggered by one or several differe nt infectious agents and that the amount of exposure to infectious agents i n infancy and childhood affects the risk of JCA. Methods. In this case-control study, we investigated socioeconomic variable s prior to disease onset from national registers, primarily the Fertility D atabase of Statistics Denmark, in a. national cohort of all 220 known cases of JCA fulfilling the EULAR criteria incident during the years 1988-91, id entified from national and local diagnosis registers. There were 4 controls per case, matched for sex, age, and county of residence. Socioeconomic var iables as risk factors were quantified by odds ratios, which are equivalent to relative risks of contracting JCA if exposed to a risk factor. Results. Three socioeconomic variables were significantly and mutually inde pendently associated with the risk of developing JCA during the following y ear. An only child had a risk of JCA 1.6 rimes that of a child with sibling s. Children whose parents had a high income had a relative risk of 1.9. Chi ldren living in an urban flat had a risk 2.7 times that of children living on a farm, We found no space-time clustering of cases and no cyclical varia tions of incidence rates. Conclusion. The absence of clustering and of seasonal variation does not su pport a theory of triggering by infection. The hitherto unreported effects of the socioeconomic variables on thr risk of JCA are of the same order of magnitude as reported for certain HLA alleles. Our findings do not lend ful l support to either of the 2 mechanisms, that growing up under either hygie nic or unhygienic conditions increases the risk of JCA, and lack an obvious biological explanation.