Multiple Sclerosis - Epidemiology

Epidemiology of multiple sclerosis

Since the 1920s, numerous epidemiological studies have been undertaken to determine the incidence and prevalence of multiple sclerosis. Geographic and temporal variations in these rates have been noted. Many of these studies support the hypothesis that exposure to a transmissible factor (e.g., a virus or other exogenous factor) influences the risk of developing the disease. This hypothesis is supported by three lines of evidence:

• population research data;
• results of migration studies;
• the presence of clusters.

A study of the mortality structure and prevalence of multiple sclerosis has shown that the incidence of the disease increases with distance from the equator. The south-north (north-south in the southern hemisphere) gradient of disease risk allowed epidemiologists to divide the world into zones with high (> 30 per 100,000), medium (5-29 per 100,000) and low (&1t; 5 per 100,000) prevalence of multiple sclerosis. Zones with high prevalence of multiple sclerosis are located in North America and Europe above the 40th parallel (in the Northern Hemisphere), as well as in Australia and New Zealand (in the Southern Hemisphere).

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Research on the prevalence of multiple sclerosis

Although prevalence rates tend to increase when the same areas are re-examined, the relationship between MS risk and latitude is consistent in many areas, particularly North America, Australia, and New Zealand. In some European countries, improved diagnostic techniques have resulted in higher prevalence rates. For example, Spain, Italy, Sardinia, and Cyprus, which were previously considered low-risk areas, have recently been found to have prevalence rates above 40 per 100,000. Unexplained geographic variations have also been noted in these areas. For example, the island of Malta has a significantly lower MS prevalence than Sicily, although the two are less than 200 km apart. In Israel, a country of immigrants, the MS prevalence is higher than would be expected given its latitude. In some areas of the British Isles, the prevalence of multiple sclerosis reaches almost epidemic proportions, with the highest prevalence in the world being in Orkney and the Shetland Islands off the coast of Scotland, with rates of 309 and 184 per 100,000 population, respectively. The prevalence of multiple sclerosis is also quite high in Norway, Sweden, Finland and Germany. In contrast, multiple sclerosis is extremely rare among the indigenous population of Africa (unlike the English-speaking white population of South Africa). The prevalence of multiple sclerosis is also very low among the Japanese.

Migration studies

Several migration studies have also confirmed the dependence of MS incidence on geographic factors. Risk has been found to vary among individuals moving from one location to another, suggesting that the risk of the disease is dependent on various environmental factors. A case-control study of World War II veterans living in the United States showed that risk in subgroups of servicemen recruited from regions with different disease prevalence depended on place of birth, but was also influenced by place of residence at the time of recruitment. This phenomenon was also observed among black veterans, in whom the prevalence of MS was, on average, half that of whites.

A study of migrants in Israel showed that both place of birth and age at immigration influenced the incidence of the disease in different ethnic groups. For example, the prevalence of multiple sclerosis was higher in Ashkenazi immigrants, who came from Northern European countries with a high prevalence of the disease, than in Sephardim, who immigrated from Asian and African countries with a low prevalence of the disease. Among Ashkenazi immigrants, the differences depended on the age at which migration occurred: those who immigrated before puberty had a significantly lower risk of the disease than those who immigrated later. This suggests that the occurrence of multiple sclerosis is influenced by some external factor acting before the age of 15.

A similar relationship between the risk of PC and age at immigration has also been noted in studies of many generations of immigrants to London from Africa and Asia, and of individuals who immigrated to South Africa from Europe. Whether this pattern can be explained by differences in genetic factors between migrant groups and native populations is still a matter of debate, although most experts believe that environmental factors play a role.

Cluster incidence of multiple sclerosis

The Faroe Islands, located in the North Atlantic Ocean between Iceland and Norway, had no cases of multiple sclerosis before 1943. But after 1945, the prevalence of multiple sclerosis increased to 10 cases per 100,000 population, and then decreased in the next few years. These changes in prevalence were associated with the occupation of the islands by British troops. Kurtzke suggested that the British carried within themselves a "primary affect of multiple sclerosis" - an asymptomatic condition that could cause the disease in susceptible individuals. After a certain latent period of at least 2 years, multiple sclerosis developed in individuals aged 11-45 who were predisposed to the disease. From 1943 to 1982, 46 cases of multiple sclerosis were registered. Kurtzke later reported a second epidemic in Iceland around the same time, which also coincided with the presence of foreign troops. However, similar “epidemic” outbreaks have not been observed in other geographic areas with low MS incidence that were occupied by British or American troops.

A number of other unexplained increases in MS cases have been reported in other parts of the world, but most have been attributed to coincidence. In Key West, Florida, 37 patients with definite or probable MS were identified, 34 of whom developed the disease while living on the island, nine of whom were nurses.

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