Geographical variations of inflammatory bowel disease in France: a study based on national health insurance data

A north-south gradient in inflammatory bowel disease (IBD) incidence has been found in Europe and the United States. Its existence is inferred from comparisons of registries that cover only small portions of territories. Several studies suggest that IBD incidence in the north has reached a plateau, whereas in the south it has risen sharply. This evolution tends to reduce the north-south gradient, and it is uncertain whether it still exists. In France, patients with IBD are fully reimbursed for their health expenses by the national health insurance system, which is a potential source of data concerning the incidence of IBD at the national level. The aim of this study was to assess the geographical distribution of Crohn's disease (CD) and ulcerative colitis (UC) in France and to test the north-south gradient hypothesis.

This study was conducted in metropolitan France and included patients to whom IBD reimbursement was newly attributed between January 1, 2000 and December 31, 2002. Data provided relate to age, sex, postcode area of residence, and IBD type. The mapping of geographical distribution of smoothed relative risks (RR) of CD and UC was carried out using a Bayesian approach, taking into account autocorrelation and population size in each département.

In the overall population, incidence rates were 8.2 for CD and 7.2 for UC per 100,000 inhabitants. A clear north-south gradient was shown for CD. Départements with the highest smoothed RR were located in the northern third of France. By contrast, the geographical distribution of smoothed RR of UC was homogeneous.

This study shows a north-south gradient in France for CD but not for UC.