Migration and multiple sclerosis: the French West Indies experience.

The French West Indies (FWI), i.e., the islands of Martinique and Guadeloupe, have recently experienced the emergence of multiple sclerosis (MS). This epidemiological upheaval followed a return migration of the FWI population that had previously migrated to continental France. The prevalence MS was 14.8/10(5) (95% CI: 11.9-17.7) on Dec. 31, 1999 and its mean annual incidence was 1.4/10(5) (95% CI: 1.0-1.8) for the period July 1997 to June 2002. The prevalence of MS in Martinique, that received more return migration, is higher than that of Guadeloupe (21.0/10(5) vs. 8.5/10(5)). This emergence of MS has been accompanied also by an inversion of its clinical spectrum, with recurrent neuromyelitis optica accounting for only 17.8% of cases. The standardized ratio of the incidence of MS among migrants is 1.71 (95% CI: 1.19-2.38; P<0.01) and if migration to continental France occurred before the age of 15 it is 4.05 (95% CI: 2.17-6.83; P<0.0001). According to recent data, a drastic reduction in exposure to sunlight and to intestinal parasites during childhood, found preferentially among migrants, are possible environmental factors responsible for this emergence.