Objectives: The risk of inflammatory bowel disease (IBD) contributed by the environment can be elucidated by assessing the risk in migrants from low prevalence to Western countries. The incidence of IBD in immigrants to Canada and their Canadian-born children was compared with nonimmigrants.
Methods: A population-based cohort of IBD patients derived from health administrative data was linked to immigration data to determine the standardized incidence of IBD in immigrants to Ontario, Canada, by region of birth between 1994 and 2010. The hazard contributed by younger age at immigration was determined. Incidence for Ontario-born children of immigrant mothers was compared with the children of nonimmigrants.
Results: In 2,144,660 immigrants, incidence of IBD was 7.3/100,000 person-years compared with 23.9/100,000 in 12,036,921 nonimmigrants (incidence rate ratio (IRR) 0.34, 95% CI 0.26-0.44). Incidence was lowest risk in East Asians (IRR 0.14, 95% CI 0.11-0.18) and highest in Western Europeans/North Americans (IRR 0.59, 95% CI 0.46-0.75). Increased age at immigration was associated with decreased risk of IBD (HR 0.986, 95% CI 0.982-0.990), a 14% increased risk per younger decade of life at immigration. Children of immigrants from the Middle East/North Africa, South Asia, Sub-Saharan Africa, and North America/Western Europe had similar risk of IBD as children of nonimmigrants; however, the incidence remained lower among children of immigrants from other regions.
Conclusions: Younger age at arrival to Canada increased the risk of IBD in immigrants. Canadian-born children of immigrants from some regions assumed the high Canadian incidence of IBD, indicating that the underlying risk is activated with earlier life exposure to the Canadian environment in certain groups.