It has long been thought that multiple sclerosis isn't an equal-opportunity disease when it comes to afflicting women and men.
In recent decades, far more women have been getting the ailment, with most experts believing that multiple sclerosis in Canada afflicts about twice as many females as males.
But a team of researchers combing over Canadian data on those with multiple sclerosis has made a startling discovery: Women with the disease have inexplicably started to outnumber men by a ratio of more than 3 to 1.
And what is more, the researchers have found that the gender ratio, rather than being stable as has been commonly thought, has been rising in an almost uninterrupted fashion for at least 50 years, jumping from about 1.9 women for every man for those born in the 1930s, to 3.2 women for every man for those born around 1980.
The rise over this long period has been so pronounced that it has made multiple sclerosis an overwhelmingly female-dominated disease, even though it wasn't originally.
Until the 1940s most doctors believed there was no difference in the incident rate based on gender, and it was only in the 1950s and 1960s that physicians began to notice that the disease was more common in women.
A paper on the new findings is appearing in the November edition of the journal Lancet Neurology and the researchers who conducted the study of the Canadian multiple sclerosis data are speculating that something new has arisen in the environment in the past half-century to make women far more likely to develop the disease, a factor that inexplicably doesn't seem to affect men.
If multiple sclerosis rates are rising because of an environmentally induced cause among women, the discovery of what this might be would be highly significant because it suggests a possible avenue for preventing the often disabling disease.
"What is going on here is something presumably that is preventable," said George Ebers, a professor in the department of clinical neurology at the University of Oxford in the United Kingdom and lead author of the study. "We just need to find out what it is in the environment because it has to be environmental. Your genes don't change over two generations, three generations."
Scientists do not know what causes multiple sclerosis, although the disease is more prevalent in wealthy countries. Canada has one of the highest rates in the world, as do many nations in Northern Europe, which has led to speculation the cause may have something to do with vitamin D. Levels of the vitamin fall off in northern countries during the winter months because sunlight isn't intense enough to allow people to make it in their skin.
For those with multiple sclerosis, the research confirms what has often been the subject of anecdotal observations.
Marla Zaichick, who was diagnosed with the illness 10 years ago and is part of an organization called Women against MS, says the gender difference is readily apparent in groups of people with the disease. "You definitely see it," said Ms. Zaichick, who lives in Mississauga. "Yes, you do see men," but women are "just so much more predominant."
For Ms. Zaichick, who has a sister with MS and a brother who doesn't have it, one of the worries about the greater odds of women getting the disease is that she has a 10-year-old daughter, Ariel, and is concerned that she may be at higher risk. The paper said "the factors causing the increasing number of women with multiple sclerosis can only be speculated on, but given the short duration over which this is occurring, genetic change can be excluded."
