A nationwide survey of the prevalence of multiple sclerosis in immigrant populations of Sweden

Background: In 2008, immigrants constituted 14% of the population of Sweden, a high-risk area for multiple sclerosis (MS). We investigated the largest Swedish immigrant populations for the prevalence of MS. Method: Data on foreign-born MS patients were retrieved from Swedish national health and population registers. We calculated observed versus expected numbers of MS patients and gender-and age-specific prevalence ratios (PR) between immigrant populations and the general population of Sweden and, where possible, of the countries of birth. Results: The 19 largest immigrant populations included 1327 MS patients. The global variation in MS prevalence was reflected in Sweden. The prevalence in immigrant populations who had moved to Sweden from countries with a lower MS risk was however higher than in their countries of birth. Notably, the MS prevalence in the population born in Iran was at least as high as in the general population of Sweden (men: PR = 1.10, 95% CI 0.81-1.46, p = 0.537, women: PR = 1.18, 95% CI 0.97-1.44, p = 0.855) and more than twice as high as in Isfahan, Iran (men: PR = 3.06 (95% CI 2.26-4.06), p < 0.001, women: PR = 2.21 (95% CI 1.81-2.68), p < 0.001). Conclusions: The MS prevalence increased in migrants who moved to Sweden from countries with a lower MS risk. In the Iranian immigrant population the prevalence exceeded that in the general population of Sweden. This indicates that Iranians carry genetic factors that contribute to a higher MS risk when environmental-lifestyle MS risk factors change.