A contagious virus or bacterium is unlikely to be the cause of multiple sclerosis, according to a study in Annals of Neurology (November 2000), the scientific journal of the American Neurological Association. The researchers base this conclusion on the finding that spouses of people with multiple sclerosis (MS) are not at increased risk of having the disease.

On the other hand, the study found that the offspring of two people with MS have about a one in three chance of developing MS. "This suggests that the finding of increased risk for relatives of MS patients is due to genetic factors," said George Ebers, M.D, head of the department of clinical neurology at the University of Oxford in England and lead author of the report.

Multiple sclerosis (MS) is a disorder of the nerve fibers of the brain and spinal cord. In MS patients, scarring (sclerosis) replaces myelin, a substance that normally insulates the nerves and speeds electrical conduction through the fibers.

Though the cause of MS is not known, most circumstantial evidence has suggested that it is an autoimmune disorder wherein the immune system's defense mechanisms mistakenly destroy the myelin. Research has therefore been focused on finding the process that corrupts the immune system, and physicians have attempted, with limited success, to treat the disease with drugs that suppress the immune system.

On the other hand, several studies have suggested that an environmental agent, perhaps an infectious organism such as a virus, may be involved in the disease. However, attempts to isolate such an organism have not produced a clear suspect.

As part of the Canadien Collaborative Study on MS, researchers from England and Canada interviewed more than 13,000 MS patients in Canada who were living with, or had lived with, someone else in a "marital" relationship, whether official or common-law. Only 23 of the patients reported that their spouses had MS, a rate of less than 0.2 percent. This is about the rate that would be expected in the general population. "We therefore believe that there is no increased risk to spouses of MS patients," concluded Ebers. In the second portion of the study, the researchers found 6 MS patients among the 49 offspring of couples in which both parents had MS. When they adjusted this value for the varied ages of the offspring, they arrived at an estimate that about 30% of the children of conjugal MS parents would develop MS at some point in their lives. This value is very similar to the rate at which both children of a set of identical twins develop MS, pointing to a strong genetic component in the disease. Clearly, however, since the concordance is not 100% for identical twins, or for the offspring of conjugal pairs with MS, there are factors other than genes: the so-called "environmental" factors. These could include chemicals, microbes, or radiation, and could affect the human organism at any time from conception through adulthood.

Ebers also points out that chance may play a large role. He suggests that some people may have an innate susceptibility to MS, but will only do so if several factors act simultaneously to trigger the disease.

"The data fit with the paradigm of MS being a disease of both genes and environment, but the environmental factors are not operating at the micro environmental level but at a broader, population level," said Ebers.