Small-area variations and sociodemographic correlates for the incidence of Crohn's disease and ulcerative colitis

The objectives of this study were to describe variations in the incidence of inflammatory bowel disease (IBD) within the Canadian province of Manitoba and to analyze sociodemographic factors associated with these variations. The authors used the Manitoba Health insurance databases to measure incidence rates of Crohn's disease and ulcerative colitis for each of 52 postal areas in Manitoba, in 1987-1996. The sociodemographic characteristics of the postal areas were based on data from the 1996 Canadian census. The overall incidence rates of Crohn's disease and ulcerative colitis were identical-15.6 per 100,000. Both diseases showed substantial geographic variation, with incidence rates differing significantly from the provincial average in 15 postal areas for Crohn's disease and in 13 postal areas for ulcerative colitis. There was a significant geographic correlation in the incidence of Crohn's disease and ulcerative colitis (r = 0.49, p < 0.001). The incidence of IBD was higher in urban areas (incidence rate ratio (IRR) = 1.21, 95% confidence interval (CI): 1.00, 1.45). Aboriginal Canadians had significantly lower rates of both Crohn's disease (IRR = 0.11, 95% CI: 0.05, 0.22) and ulcerative colitis (IRR = 0.57, 95% CI: 0.42, 0.79). A higher incidence of IBD was ecologically associated with a higher average family income, a lower proportion of immigrant and Aboriginal Canadian populations, and a smaller average family size.