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The decisive conclusions to be drawn from the geography and prevalence of MS are: (1) a north-south (as well as west-east in the United States) gradient exists independent from genetic/racial factors; (2) major differences in prevalence occur in the absence of difference in latitude; (3) individuals from the same ethnic derivation have either the similar prevalence or have very different prevalence rates in widely separated geographical areas, and (4) specific resistance isolates are shown to exist regardless of latitude. Existing prevalence information leads to the almost inescapable conclusion that the geography of MS cannot be explained by any single known environmental or genetic factor(s) in isolation. A combination of a heterogeneous distribution of both genetic and environmental factors appears to be required to explain the available data on MS and geography.
Ebers et al. (Thu,) studied this question.