The number of women being diagnosed with multiple sclerosis in Canada is growing faster than the incidence among men, say researchers who suspect an environmental link.

Among people born in the 1930s, the ratio of women to men contracting the disease was 2:1, the researchers report in the November issue of The Lancet Neurology.

The myelin coating on nerves is damaged in people with MS.
(CBC)

For people born around 1980, the numbers rose, with more than three women with MS for every man.

"This rapid change must have environmental origins even if it is associated with a gene–environment interaction, and implies that a large proportion of multiple sclerosis cases may be preventable," conclude the authors of the study.

It was led by George Ebers, a professor of neurology at the University of Oxford in the United Kingdom who is formerly from London, Ont.

True rise in incidence

Prof. George Ebers suspects something in the environment may be contributing to the increased incidence of MS in women.
(CBC)

Multiple sclerosis is a neurodegenerative disease that attacks the brain and spinal cord. It can lead to paralysis and sometimes blindness.

Canada has one of the highest rates of MS in the world, with about 1,000 Canadians diagnosed each year and more than 75,000 living with the disease. Those between ages 15 and 40 are most at risk.

Suspicion on the higher number of women compared to men with the disease naturally falls to estrogens, the birth control pill and X chromosomal inheritance. But none seem likely explanations for the increasing rates among women, Ebers said.

"I think one of the things one thinks of here is either that it's going to be something in the environment or it is going to be an environmental interaction with genes," Ebers told CBC Newsworld on Tuesday.

The fact that the increase is among females only suggests it is a true increase in incidence, although an increase in neurologists, MRI scanners and public awareness could also play a role, he said.

Similar sex differences have been found among women in Denmark, Scotland and southern Australia, as well as in Japan, where MS is rare, Ebers said.

Environmental exposures

By looking for places where the incidence is not disproportionately higher among females, such as southern Europe, the researchers hope to find out what environmental factors might be behind the effect.

The incidence of MS is higher in northern latitudes, leading other researchers to hypothesize that exposure to sunshine, and the higher levels of vitamin D it brings, may somehow be protective. 

Speculation also includes hygiene, since children in developed countries are not exposed to as many germs as they used to be.

Whatever the environmental factor is, it likely interacts with a genetic predisposition, leading to an immunological reaction within the brain. The process then recurs throughout the life of the patient, said Dr. Jock Murray, a neurologist at Dalhousie's MS research unit in Halifax.

Ebers and his colleagues are studying a database of more than 27,000 people with MS in Canada. They plan to look at the age at which people immigrate to Canada to figure out when MS starts.