Latitude, migration, and the prevalence of multiple sclerosis.

Prevalence of multiple sclerosis (MS) was determined in three groups of Caucasians in King and Pierce Counties (Washington) and in Los Angeles County (California): natives (born in study state), migrants from high-risk states (North), and migrants from low risk states (South). A diagnosis of definite or probable MS (excluding migrants with onset before migration) was established in 1816 of the persons found. The prevalence in Los Angeles County natives and in migrants from the South to either Los Angeles County or King-Pierce Counties were relatively low, while prevalence in King-Pierce Counties among natives and migrants from the North were high. The prevalence in migrants from the North to Los Angeles was intermediate. The results suggest that some degree of protection is provided to migrants by residence in low-prevalence areas, either early or later in life.