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

We studied the socioeconomic background of children with juvenile chronic arthritis (JCA) diagnosed during the years 1988-91 in Denmark. The working hypothesis is that JCA may be triggered by one or several different infectious agents and that the amount of exposure to infectious agents in infancy and childhood affects the risk of JCA. In this case-control study, we investigated socioeconomic variables prior to disease onset from national registers, primarily the Fertility Database of Statistics Denmark, in a national cohort of all 220 known cases of JCA fulfilling the EULAR criteria incident during the years 1988-91, identified from national and local diagnosis registers. There were 4 controls per case, matched for sex, age, and county of residence. Socioeconomic variables as risk factors were quantified by odds ratios, which are equivalent to relative risks of contracting JCA if exposed to a risk factor. Three socioeconomic variables were significantly and mutually independently associated with the risk of developing JCA during the following year. An only child had a risk of JCA 1.6 times that of a child with siblings. Children whose parents had a high income had a relative risk of 1.9. Children 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 variations of incidence rates. The absence of clustering and of seasonal variation does not support a theory of triggering by infection. The hitherto unreported effects of the socioeconomic variables on the risk of JCA are of the same order of magnitude as reported for certain HLA alleles. Our findings do not lend full support to either of the 2 mechanisms, that growing up under either hygienic or unhygienic conditions increases the risk of JCA, and lack an obvious biological explanation.