Epidemiologic contributions to multiple sclerosis

Geographically, multiple sclerosis (MS) seems to be distributed into three zones of high, medium, and low frequency. High-frequency areas, with prevalence rates over 30 per 100,000 population, include Europe between 65° and 45° north latitude, southern Canada and the northern United States, and New Zealand and southern Australia. These regions are bounded by areas of medium frequency with prevalence rates of 5 to 25 per 100,000, which include southern Europe, the southern United States, and most of Australia. Known areas of Asia and Africa (save for one white group in South Africa) are all low, with prevalence rates under 5 per 100,000 population. All high- and medium-risk areas are among predominantly white populations: In America, blacks and Orientals have much lower rates of MS than whites but still demonstrate the geographic gradients found for whites. Migration studies indicate that on the whole, migrants retain much of the risk of their birthplace. However, this risk is clearly not defined at birth: MS death rates for migrants born in one risk area and dying in another are intermediate between the rates characteristic of their birthplaces. Prevalence studies for migrants from high- to low-risk areas indicate the age of adolescence to be critical for risk retention; those migrating under age 15 years acquire the lower risk of their new residence. Furthermore, several low-to-high studies show that those migrating in childhood or adolescence increase the risk of MS. The migrant data, plus the geographic distributions, serve to define MS as an acquired, exogenous (environmental) disease whose acquisition in ordinary circumstances takes place years before clinical onset. The data fit best the “simple” or “prevalence” hypothesis: that the cause of MS will be found where the clinical disease is common. Further evidence for this viewpoint is provided by the occurrence of two epidemics of MS: one (definite) in the Faroe Islands, the other (probable) in Iceland. Both followed the occupation of those lands by British forces during World War II. If this relation is causal, MS is not only acquired but also transmittable.