Small area variation in the incidence of childhood insulin-dependent diabetes mellitus in Yorkshire, UK: Links with overcrowding and population density

Background. The incidence of insulin-dependent diabetes mellitus (IDDM) incidence varies between and within countries. The origins of this variation are disputed, but they involve both genetic and non-genetic influences. To explore the role of environmental factors in the aetiology of IDDM we have examined the incidence in small geographical areas and related it to variables derived from national censuses. Methods. This is an ecological analysis of incidence data from a register of children with IDDM covering the counties of West Yorkshire, North Yorkshire and Humberside in the north of England. All children aged less than or equal to 16, diagnosed with IDDM between 1978 and 1990 were eligible for inclusion. Spatial variation in incidence between electoral wards was investigated using Poisson regression, in relation to socioeconomic status, population density, urban-rural status and measures of geographical isolation. Ward child populations varied in size from 84 to 7197 (mean = 1545). Results. Rates were significantly lower in wards of high population density and with many overcrowded houses. The rate ratio for areas in the upper half of the childhood density distribution was 0.88 (95% confidence interval (CI) : 0.78-0.99) and for the two upper tertiles of household overcrowding the rate ratios were 0.84 (95% CI : 0.74-0.95) and 0.68 (95% CI : 0.58-0.79) respectively. Conclusions. The incidence of childhood IDDM was associated with environmental factors including population density and overcrowded homes. A possible inference from these data is that patterns of infection are involved in the occurrence of IDDM. Analytical epidemiological studies will be needed to investigate these ideas further.