While there are no obvious environmental risk factors that cause multiple sclerosis, there are many known facts about potential triggers. Worldwide, the disease affects up to 2.5 million people—however, for reasons not fully understood, its prevalence varies widely according to geographic areas. It occurs most frequently in Caucasians of Northern European ancestry.
Areas that were settled or visited by Vikings and other northern European tribes have the most MS. Countries in Europe, North America, South Africa, certain areas of the Mediterranean, Australia and New Zealand have many people with MS, while the disease is relatively rare in Asia, Africa, and tropical countries. It clearly favors temperate regions of the world.
Migration studies have also increased our knowledge of risk factors in MS. Migrants from high to low risk areas (from temperate to tropical climates for example) retain the risk of their birthplace if they are least 15 years old when they move. If they move at a younger age there is a suggestion that these people may acquire the benefit of the protection of the low risk area.
Investigators have explored both noninfectious and infectious agents to explain the patterns of geographical variation in the cause of MS. For example, sunlight and vitamin D have been investigated. It has been found that the average annual hours of sunshine and the average December daily solar radiation at place of birth were strongly correlated with the presence of MS. There is also a theory that the stronger intensity of the sun may influence the occurrence of MS. However, none of the studies that have looked into this theory have been able to demonstrate that high-intensity sunlight causes MS. These are simply association studies that need to be interpreted in light of all the other factors that contribute to the development of MS.
The Role of Infectious Agents
Researchers have also investigated the role of infectious agents, including bacteria and viruses, in triggering MS. Studies show the risk of developing MS is about 10 times greater in people who experienced an infection from the Epstein Barr virus than those who did not. This risk increases about 20 fold in people who developed clinical mononucleosis.
While the cause of MS is unknown, genetic factors along with geographical issues and infectious agents are being investigated. Information about genetic predisposition in MS comes from a variety of studies. It has been shown that MS occurs in fraternal twins less frequently than in identical twins. Other genes have been isolated as potentially being able to explain susceptibility to the disease. These genes are currently being studied worldwide. Genes may also play a role in disease progression. For example, an individual’s ability to repair myelin and preserve their axons may be genetically determined.
Finally, gender issues are quite prominent in determining risk factors in MS. Studies have shown that women are more likely to develop MS than men. Other studies have shown that hormonal changes that occur, for example, during the menstrual cycle and after delivering a child (postpartum period) may be linked to acute relapses of the disease.
While some researchers have concluded that men do not do as well as women in the long run, a more recent study indicates that although men may progress (worsen) faster, both genders ultimately have some degree of disability at the same age.
Finally, some investigators have suggested that those with a younger age of onset may have a better prognosis and those people diagnosed later in life will not do well over time. The reasons for this are unclear and further research is warranted.