Background: In a previous study we found a north-south gradient for Crohn's disease (CD) incidence in France. The aim of the present study was to determine if socioeconomic factors may influence the geographic distribution of CD and ulcerative colitis (UC) in France.
Methods: Using the national health insurance databases, incidence rates of CD and UC were estimated for each of 341 metropolitan "job areas" in 2000-2002. Relationships between incidence rates and relevant contextual variables from the 1999 French census were tested for significance using a Poisson regression. Mapping of smoothed relative risks (sRR) for CD and UC at the scale of job areas, using a Bayesian approach and adjusting for significant contextual variables, was carried out in order to search for geographic variations.
Results: CD incidence rates were negatively related to the percentage of farmers and to the percentage of housing with bathroom and toilets and positively related to the unemployment rate and to the percentage of households below the poverty threshold. Mapping of sRR for CD showed a clear north-south gradient, which was slightly improved after including the percentage of farmers and the percentage of housing with toilets. In UC we found no significant correlation between either incidence and socioeconomic variables or incidence and house equipment variables, and there was no north-south gradient. However, there was a positive and significant correlation between CD and UC incidence.
Conclusions: The present study shows that geographic risk factors of CD in France are northern latitude, nonrural areas, and areas with poor sanitary house equipment. Among these factors the most important is northern latitude.