MULTIPLE sclerosis tends to be common in temperate latitudes and rare in regions near the equator.1-4 This geographical distribution suggests that some factor in temperate environments causes the disease, or, conversely, a protective factor is present in tropical regions. If an environmental factor accounts for the difference in frequency between temperate and tropical regions, then migration from one climatic zone to another could affect the risk of developing the disease. Individuals migrating from temperate to tropical areas would be expected to have a decreased risk of acquiring multiple sclerosis whereas those migrating in the opposite direction might increase their risk. An altered risk would be especially evident if migration occurred at a young age before there was adequate opportunity for exposure to the etiological (or protective) factor. Moreover, if risk changed sharply with migration at a particular age, it would suggest that the disease was usually acquired before
ALTER M, LEIBOWITZ U, SPEER J. Risk of Multiple Sclerosis Related to Age at Immigration to Israel. Arch Neurol. 1966;15(3):234–237. doi:10.1001/archneur.1966.00470150012002
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