High incidence of inflammatory bowel disease in The Netherlands

PURPOSE: To gain recent epidemiologic information about inflammatory bowel disease in The Netherlands, a prospective study over four years (1991–1995) was performed. METHODS: The incidence of inflammatory bowel disease and its subgroups was examined using standardized reports of newly diagnosed patients. A separate study compared the Inflammatory Bowel Disease Registration and computerized diagnostic files of a subgroup of general practitioners with the aim of estimating completeness of case ascertainment. RESULTS: The following mean incidence rates (per 100,000 inhabitants and year) were found: 6.9 (95 percent confidence interval, 5.9–7.9) for Crohn's disease, 10 (95 percent confidence interval, 8.7–11.2) for ulcerative colitis (23 percent of these with ulcerative proctitis), and 1.1 (95 percent confidence interval, 0.7–1.5) for indeterminate colitis. In the age category 20 to 29 years, the incidence rate of Crohn's disease with small-bowel involvement was higher in females than in males. In extended ulcerative colitis, a male preponderance was observed in the older age groups. Estimated case ascertainment was 78 percent. CONCLUSIONS: Compared with recent studies in neighboring countries, the observed age and gender standardized incidence rates are high in the south of The Netherlands. Completeness of case ascertainment might have contributed to this observation; however, case ascertainment was low in ulcerative proctitis. In the study area, differences in age and gender standardized incidence rates and in disease localizations could be compatible with an influence of environmental risk factors.