Cluster or coincidence? Biostatistician weighs in on MS cluster claim in Ottawa
Since we published the CBC story of an Ottawa man looking for answers about a potential multiple sclerosis cluster, our inbox has been flooded with emails from people across the country, and around the world.
It seems 59-year-old Jacques Dutrisac’s personal quest to find out why 14 former neighbours were all diagnosed with MS as adults, as he was, has resonated with others.
Most of the emails came from people who also have the disease, including more than a dozen who say they also grew up in Elmvale Acres at the same time as Dutrisac, and were diagnosed with MS as adults.
Countless people have written to say they find it hard to believe it’s a coincidence, as some doctors have suggested.
Many asked to be put in touch with Dutrisac, in the hope of finding answers.
But we’ve also heard from several audience members who question whether this group is really that unusual, given that Canada has the highest MS rate in the world, with roughly 1 in 350 people in this country living with the disease.
As Ian Ewing, one of those commenting on the CBC article, wrote in an email to CBC News, “…we can ballpark the [Elmvale Acres] rate as being pretty close to the national average …. Mr. Dutrisac’s cohort has been diagnosed with MS at basically the exact rate you’d expect.”
While it is possible the concentration of cases from Elmvale Acres is statistically consistent with national figures, the group is still worthy of researchers’ attention, according to Dr. Rama Nair, a biostatistician and professor of epidemiology at the University of Ottawa.
“My first thought [after hearing the story] was, ‘Gee, that is concerning,’ because there’s a large number of cases in a very limited geographical area, so immediately that raises some concerns as to, ‘OK, there must be something here,'” said Nair.
“So as a statistician, one of the first things I look at is, is this unusual? Is this more likely than by chance? So sometimes you say, ‘Oh yeah that looks really funny,’ but it may be simply a random chance. It’s unlikely you’ll get hit by lightning [twice] in a row, but it can happen.”
Many variables at play
Nair said comparing the group from Elmvale Acres to the national MS rate isn’t necessarily helpful since “so many phenomena, so many things happen in the middle.”
Put simply, there are a lot of variables at play, raising the possibility the Elmvale Acres cluster claim is “totally invalid,” Nair said.
He said it would be more helpful to compare the neighbourhood to a different neighbourhood in another city that has a population with a similar average age and ethnic background to Elmvale Acres.
Researchers could then compare the number of people with MS in the two neighbourhoods to get a better idea of how many people you could reasonably expect to be diagnosed with the disease in a given area.
“Unless we have that, we can never really prove anything. That’s why even if [epidemiologists] have very strong evidence, we seldom say, ‘This is the cause,'” said Nair.
Scientists’ ‘big mistake’
Roughly 20 years ago, Nair was part of a team of epidemiologists brought in to study a potential cancer cluster in the Toronto suburb of Whitchurch-Stouffville, but no significant evidence of a cluster was ever found.
He said Dutrisac’s personal story, and others like it, “feeds into [the public’s] anxiety a little more than it should,” but he still believes the group from Elmvale Acres should be taken seriously because there are “human emotions” involved.
“I think that is the big mistake that many of us do as scientists, because we try to get at the data, and we are sort of removed from the people,” said Nair.
“We look at the data and say, ‘There is nothing in here.’ But the other side is also important … because people are people … many times these are sort of run of the mill cases, but from their perspective it is very serious because they are sick.”
So will there ever be any firm answers for the group from Elmvale Acres?
Nair said it’s impossible to know with certainty whether the group is a cluster. Even a costly and lengthy study might not result in definitive answers, said Nair.
“[In] statistics we always talk about probability, chance. Life is full of chances. Everything can happen.”