Multiple sclerosis in the Orkney and Shetland Islands VI: The effects of migration and social structure

A number of studies have now shown a markedly higher prevalence of multiple sclerosis (MS) in northern Scotland.1 The prevalence rates appear particularly high in the islands, with Orkney having the highest rate reported anywhere in the world.2 3 Our specific task in this paper is to consider the possible effect of differential migration upon prevalence. The hypothesis is that part or all of the difference in prevalence may be the result of differences in migration, and the possibility that persons with MS are less likely to migrate, and that those who do are most likely to return after onset of the disease. Bradford Hill's4 pioneering study of the effect of migration on mortality rates produced the earliest clear evidence for our migration hypothesis. Additional and more recent evidence has come from those studies which have used height as an index of health status; most of these show that migrants are invariably taller than residents.5 The best British data are those collected by Martin,6 the Scottish Council for Research in Education,7 and Illsley, Finlayson, and Thompson.8 The latter are of particular interest because Aberdeen is the main destination for migrants from Orkney and Shetland. The authors demonstrated that migrants into the city were taller and had lower prematurity and perinatal death rates than Aberdeen women, and that the superiority was most marked among the more distant in-migrants. In the absence of studies of MS which provide a direct test of the migration hypothesis, this paper proceeds on the assumption that persons with MS migrate differently from the remaining section of the population. Assuming that those with MS are less likely to migrate, and that those who do are more likely to return, can we account for the observed prevalence rates? Before examining the special situations of Orkney and Shetland, we consider a number of factors which might plausibly affect prevalence rates.