Effect of immigration on multiple sclerosis sex ratio in Canada: the Canadian Collaborative Study

Abstract

Background: The ratio of female to male (F:M) multiple sclerosis (MS) cases varies geographically, generally being greater in areas of high prevalence. In many regions, including Canada, rising MS incidence in women has been implied by the marked increase in F:M ratio. Methods: We examined the F:M ratio over time in MS patients in the Canadian Collaborative Study born outside Canada, with onset postmigration (n = 2531). We compared the trends to native-born Canadians, by region of origin and age at migration. Results: Regression analysis showed that year of birth (YOB) was a significant predictor of sex ratio in immigrants (χ² = 21.4, pConclusions: MS sex ratio in immigrants to Canada is increasing but variable by region of origin and influenced by age at migration. The findings highlight the importance of environmental effect(s) in MS risk, which are likely gender-specific.