What Causes MS?

While the cause (etiology) of MS is still not known, scientists believe that the interaction of several different factors may be involved. To answer this important question, studies are ongoing in the areas of immunology (the science of the body’s immune system), epidemiology (the study of patterns of disease in the population) and genetics. Scientists are also studying infectious agents that may play a role. Understanding what causes MS will speed the process of finding more effective ways to treat it and — ultimately — cure it, or even prevent it from occurring in the first place.

In MS, an abnormal immune-mediated response attacks the myelin coating around nerve fibers in the central nervous system, as well as the nerve fibers themselves. In recent years, researchers have been able to identify which immune cells are mounting the attack, some of the factors that cause them to attack, and some of the sites (receptors) on the attacking cells that appear to be attracted to the myelin to begin the destructive process. Ongoing efforts to learn more about the immune-mediated process in MS — what sets it in motion, how it works, and how to slow or stop it — are bringing us closer to understanding the cause of MS.

MS is known to occur more frequently in areas that are farther from the equator. Epidemiologists — scientists who study disease patterns — are looking at variations in geography, demographics (age, gender and ethnic background), genetics, infectious causes and migration patterns in an effort to understand why.

Studies have shown that people born in an area with a high risk of MS who then move — or migrate — to an area with a lower risk before the age of 15 assume the risk of their new area. Such data suggest that exposure to some environmental agent before puberty may predispose a person to develop MS later on.

Growing evidence suggests that vitamin D plays an important role. People who live closer to the equator are exposed to greater amounts of sunlight year-round. As a result, they tend to have higher levels of naturally-produced vitamin D, which is thought to support immune function and may help protect against immune-mediated diseases like MS. The possible relationship between MS and sunlight exposure is currently being looked at in a Society-funded epidemiological study in Australia.

The evidence is also growing that smoking plays an important role in MS. Studies have shown that smoking increases a person’s risk of developing MS and is associated with more severe disease and more rapid disease progression. Fortunately, the evidence also suggests that stopping smoking — whether before or after the onset of MS — is associated with a slower progression of disability.

MS “clusters” — the perception that very high numbers of cases of MS have occurred in a specific time period or location — may provide clues to environmental or genetic risk for the disease. So far, cluster studies in MS have not produced clear evidence for the existence of any causative or triggering factor or factors in MS.

Since initial exposure to numerous viruses, bacteria and other microbes occurs during childhood, and since viruses are well-recognized as causes of demyelination and inflammation, it is possible that a virus or other infectious agent is the triggering factor in MS. More than a dozen viruses and bacteria — including measles, canine distemper, human herpes virus-6, Epstein-Barr, and Chlamydia pneumonia — have been or are being investigated to determine if they are involved in the development of MS, but none have been definitively proven to trigger MS.

While MS is not hereditary, having a first-degree relative such as a parent or sibling with MS does significantly increase an individual's risk of developing the disease. Studies have shown that there is a higher prevalence of certain genes in populations with higher rates of MS. Common genetic factors have also been found in some families where there is more than one person with MS. Some researchers theorize that MS develops because a person is born with a genetic predisposition to react to some environmental agent that, upon exposure, triggers an immune-mediated response. Sophisticated new techniques for identifying genes are helping to answer questions about the role of genes in the development of MS.