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 variations and temporal variations of these indicators were noted. Many of these studies support the hypothesis that the effect of a transmissible factor (eg, a virus or another exogenous factor) affects the risk of the disease. This hypothesis is confirmed by three lines of evidence:

• data from population studies;
• results of studying migration;
• presence of clusters.

The study of the structure of mortality and prevalence of multiple sclerosis showed that the frequency of the disease increases with distance from the equator. South-northern (in the southern hemisphere - north-south) risk gradient of the disease allowed epidemiologists to divide the whole world into zones with high (> 30 per 100 000), medium (5-29 per 100 000) and low (? 1 5 per 100 000 ) prevalence of multiple sclerosis). Areas with a high prevalence of multiple sclerosis are located in North America and Europe above 40 parallels (in the Northern Hemisphere), as well as in Australia and New Zealand (in the Southern Hemisphere).

[1], [2], [3], [4], [5], [6], [7], [8],

Studies of the prevalence of multiple sclerosis

Although re-survey of the same regions, prevalence rates tend to increase, the relationship between the risk of multiple sclerosis and geographical latitude persists in many areas, especially in North America, Australia and New Zealand. In some European countries, due to improved diagnostic methods, prevalence rates have been revised upward. For example, in Spain, Italy, Sardinia, Cyprus, which were previously classified as low-risk areas, in recent studies, the prevalence rate was above 40 per 100,000. When examining these regions, unexplained geographical variations were also noted. For example, on Malta the prevalence of multiple sclerosis was significantly lower than in Sicily, although they are separated from each other by less than 200 km. In Israel, the country of immigrants, the prevalence of multiple sclerosis is higher than one would expect, based on the latitude on which this country is located. In some areas of the British Isles, the prevalence of multiple sclerosis reaches almost epidemic proportions, with the highest prevalence of the disease in the world recorded in Orkney and the Shetland Islands off the coast of Scotland - respectively 309 and 184 per 100 000 population. The prevalence of multiple sclerosis is also quite high in Norway, Sweden, Finland and Germany. On the contrary, multiple sclerosis is extremely rare in the indigenous population of Africa (unlike the English-speaking white inhabitants of the Republic of South Africa). The prevalence of multiple sclerosis is also very low among Japanese.

Migration Studies

Several migratory studies have also confirmed the dependence of the incidence of multiple sclerosis on geographical factors. There is a change in risk in people moving from one place to another, which indicates the dependence of the risk of the disease on various external factors. In a case-control study among World War II veterans living in the United States, it was shown that the risk in subgroups of servicemen who were recruited from regions with a different prevalence of disease depended on the place of birth, but was also influenced by the place of residence at the time of recruitment for military service. This phenomenon was also observed among black veterans, whose prevalence of multiple sclerosis was on average 2 times lower than that of whites.

A study of migrants in Israel showed that both the place of birth and age at the time of immigration influenced the incidence of the disease in various ethnic groups. Thus, the prevalence of multiple sclerosis was higher in Ashkenazi immigrants, who were from countries of Northern Europe with a high prevalence of disease than Sephardim immigrants from countries of Asia and Africa with a low prevalence of the disease. In Ashkenazi immigrants, the differences depended on the age at which migration occurred: for those who immigrated to puberty, the risk of the disease was significantly lower than those who immigrated in a later period. This indicates that the onset of multiple sclerosis is affected by some external factor, acting at the age of 15 years.

This dependence of the risk of PC on the age of immigration is also noted in the study of many generations of immigrants to London from Africa and Asia and persons who immigrated to South Africa from Europe. Whether this pattern can be attributed to the difference in genetic factors in migrant groups and indigenous populations is still debatable, although most experts are still convinced that external factors play a role.

Clustered incidence of multiple sclerosis

On the Faroe Islands, located in the northern Atlantic Ocean between Iceland and Norway, until 1943, there was no case of multiple sclerosis. But after 1945, the prevalence of multiple sclerosis increased to 10 cases per 100 000 population, and in the next few years decreased. These changes in prevalence were associated with the occupation of the islands by British troops. Kurtzke suggested that the British bore the "primary affect of multiple sclerosis" - an asymptomatic condition that could cause disease in predisposed individuals. After a certain latency period of at least 2 years, people aged 11-45 years who were predisposed to the disease developed multiple sclerosis. From 1943 to 1982, 46 cases of multiple sclerosis were recorded. Later, Kurtzke reported a second epidemic in Iceland around the same period, which also coincided with the presence of foreign troops. However, similar "epidemic" outbreaks were not observed in other geographic areas with a low incidence of multiple sclerosis, which were subjected to occupation by British or American troops.

There have also been reports of a number of other episodes of unexplained increase in cases of multiple sclerosis in other regions of the world, but mostly they were due to accidental coincidence. Thus, in Key West, Florida, 37 patients with a reliable or probable diagnosis of multiple sclerosis were identified, 34 of whom developed the disease when they lived on the island, 9 of whom worked as nurses.

[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]