Clues to the cause based upon the epidemiology of multiple sclerosis

Abstract

The primary goal of epidemiological research in multiple sclerosis (MS) is to determine the environmental and host factors which are related to the cause of this disease. These factors are sought by analysing MS frequency in different populations and in different parts of the world. Over the last half-century, a large literature has accumulated documenting differences in frequency of MS among peoples and places I -3 and, if reasons for these differences were known, then the list of possible environmental and host factors related to the cause of MS could at least be narrowed. Therefore, the observed differences in MS frequency can provide important clues to aetiology if their meaning were unravelled.One consistent fmding to emerge from epidemiological research in MS is that this disease has an une qual geographical distribution. Since the mid-1950S, it has been known that MS was more common in temperate than in tropical areas of the world4• 5. More recent studies of MS distribution introduced methodological refinements but the general characteristics of the geographical distribution have not been appreciably altered. Today, it is generally accepted that MS increases in frequency with geographicallatitude both in the northern 6 and southern hemisphere7• ReviewsI. 2.8 of the geographical distribution ofMS have been published and Figure 3.1 summarizes some of this information.According to one interpretation9, the geographical distribution of MS resembles a parabolic gradient (Figure 3.2) which increases sharply with latitude. An alternative interpretation IO of these same data is that zones of 35 E. J. Field (ed.), Multiple Sclerosis

Occurrence of multiple sclerosis in the north and south of New Zealand.

“…In many parts of the 138 world, studies of migrants have shown that the latitude effect is at least partly due to some acquired, exogenous influence. 28 The multitude of possible environmental factors have been reviewed elsewhere.34 29 Further work is needed to examine such hypotheses in the light of our findings in New Zealand, but some observations can already be made. The lifestyles of people living in the north and south of New Zealand are more closely similar than in many of the regions compared in the Northern Hemisphere.…”

Comparative epidemiology of multiple sclerosis and dental caries.

“…Parallels are found in MS migration studies.Asian immigrants to Britain were found to have a reduced risk of MS compared with European British (Dean et al, 1977). A more detailed study in Israel showed that Afro-Asians who immigrated to Israel after the age of five had a lower risk of MS than European immigrants of comparable ages, but Afro-Asians who immigrated before the age offive acquired a risk as high as, or higher than, the Europeans (Alter, 1977). The migration studies seem to suggest that similar racial and environmental factors influence both MS and dental caries.…”

“…The possible relationship of MS to animal fat consumption, first suggested by Swank (1950), has recently been elaborated by Dick (1976) and Alter (1977). It is suggested by these authors that dietary excesses of certain fats may cause errors in the development of myelin, possibly very early in life, and these may render the myelin unstable or susceptible to attack.…”

“…At the other extreme, MS is virtually unknown in the tropics, and numerous studies and reviews have documented that living in sunny climates, especially during the very early years of life Visscher et al, 1977), offers one protection against acquiring MS (Limburg, 1950;Acheson et al, 1960;Kurland and Reed, 1964;Goldberg, 1974). Certain populations, however, like the Eskimos, appear to be protected against acquiring MS in spite of their extremely low sunlight exposures; the Japanese also are protected without the benefit of tropical levels of sunlight (Alter, 1977).…”

“…One explanation offered states that both very high and very low levels of sunlight may protect against MS, but that there is an 'optimally detrimental' amount of sunshine which is responsible for MS (Alter, 1977). A simpler explanation can be found by considering other diseases with geographical variations similar to those of MS. One example is rickets, which has a peculiar geographical distribution explained more than 50 years ago (Mellanby, 1921).…”