Background: The ratio of female to male (F:M) 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 of Canada, with onset post-migration (n=2531). We compared the (y.o.b.) was a significant predictor of sex ratio in immigrants (χ 2 = 21.4, p<0.001 correlation r= 0.61). The rate of change of sex ratio was increasing in all migrant subgroups (by a factor of 1.16 per 10y period, p<0.001), with the steepest increase observed in those from Southern Europe trends to native-born Canadians, by region of origin and age at migration.
Results: Regression analysis showed that year of birth (1.27/10y, p<0.001). The overall immigrant F:M ratio was 2.17, but varied by country of origin. It was significantly lower in migrants from Southern Europe compared to Northern Europe or USA (1.89 vs 2.14 and 2.86, p=0.023 and p=0.0003, respectively). Increasing age at immigration was associated with decreasing sex ratio (p=0.041). The sex ratio of individuals migrating <21 was significantly higher than those migrating ≥21 (2.79 vs 1.96, p=0.004).
Conclusions: 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.