Worldwide, it is estimated that up to 3 million people have multiple sclerosis (MS). Generally speaking, it is least common in countries near the equator. MS is five times more likely to occur among people who live in temperate climates than among those who live in tropical regions. MS is found more often in people who live between the latitudes of 40 and 60 degrees, both north and south of the equator. In these temperate climates, 1 person in 1,000 has MS. By comparison, in warmer climates only 1 person in 10,000 has MS. The highest incidence of MS are found in the northern United States, New Zealand, southeastern Australia, the Soviet Union, and northern Europe, particularly Scandinavia and Scotland.

About 350,000 Americans have MS. Almost 200 new cases are diagnosed each week. More of these people live above the 37th parallel than below it. The 37th parallel runs east to west from Newport News, Virginia, along the northern border of North Carolina and across the northern border of Arizona to Santa Cruz, California. Above this parallel, there are 110-140 cases of MS for every 100,000 people. Below the 37th parallel, there are only 57-78 cases for every 100,000 people.

Studies have shown that the risk of developing MS is related to where a person lives before the age of 15. If a person younger than 15 moves from a high-risk region to a low risk region, or vice versa, he or she assumes the risk of the new location. If the move occurs after the age of 15, he or she retains the risk of the original location. Researchers are not sure why this is true. It is believed that some environmental agent acting prior to puberty may affect susceptibility to MS.

MS is not an inherited disease. Most people with MS do not have a close family member with the disease. Genetic factors, however, do play a role in predisposing certain people to MS. While people in the general population have about a 1 in 1,000 chance of getting MS, children and siblings of patients with MS are at greater risk -- between 1 in 100 and 1 in 50. Being related to someone with MS is not considered a major risk factor, however. Studies with identical twins show that the twin of an MS patient has a 1 in 3 chance of getting the disease, but fraternal twins have the same risk as any other siblings. These findings show that genetics is involved, but that it is obviously not the only factor. MS researchers think that people with MS have probably inherited a particular combination of genes. They suspect that people with this genetic predisposition get MS in response to some environmental trigger, such as a virus.

Racial ancestry is significant. Most patients are Caucasians descended from northern European ancestors, particularly the Scandinavians. Whites are twice as likely to get MS as blacks, but American blacks are more likely to get it than African or Caribbean blacks. Orientals have a very low occurrence of MS, and Eskimos and Gypsies are apparently immune to the disease.

Other significant factors include gender and age. Among those who are younger at onset, women outnumber men two to one. Among those who develop the disease later, however, the ratio of men to women is closer. Patients with primary progressive MS show an equal sex ratio. Most cases of MS develop between the ages of 15 and 50 (90 percent). The average age of onset is around 28 to 30. Multiple sclerosis rarely strikes children under the age of 10 or adults over the age of 60.

References

First published October 1, 1999
Last updated May 2, 2003
Copyright © 1999 Accordant Health Services, Inc. All Rights Reserved.

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