A distinctive tuberculosis was unintentionally left by 18th-century Voyageurs in remote aboriginal communities along their trade routes and still plagues those communities today, new research shows.

Remote First Nations communities in Ontario, Saskatchewan and Alberta share the same dominant strain of the infectious lung disease because they all got it from French Canadian fur traders in the 1700s — a century before any TB epidemics were reported in those communities, says a study published this week in Proceedings of the National Academy of Sciences.

The research shows that tuberculosis spread by a brief historical interaction can stay dormant in a community for 100 years or more before emerging as an epidemic, said Caitlin Pepperell, the Canadian researcher at Stanford University who led the study.

"It points out why it's so difficult to eradicate TB," she added in an interview Wednesday.

Pepperell, who is originally from Toronto, has been studying TB epidemics in aboriginal populations for about five years.

During an earlier study, she noticed that DNA fingerprints from remote Ontario and Saskatchewan communities were very similar to each other, "which I thought was odd, because they're very distant populations," she said.

'To have a single strain...that's so dominant is really unusual.'— David Alexander

It was also peculiar because TB has thousands of strains across Canada, said David Alexander, a molecular microbiologist with the Ontario Agency for Health Protection and Promotion and assistant professor at the University of Toronto. He was one of 12 Canadian and U.S. researchers who co-authored the study.

In urban areas like Toronto, every infection usually belongs to a different strain, Alexander said: "To have a single strain or a single variant that's so dominant is really unusual."

Pepperell suspected that the Ontario and Saskatchewan communities' location along historical fur trade routes was the common link between them.

With help from researchers in Ontario, Alberta and Quebec, Pepperell analyzed genes from the bacterium that causes tuberculosis, Mycobacterium tuberculosis or M. tb, from the French Canadian population of Quebec and aboriginal populations outside urban areas in Ontario, Manitoba, Saskatchewan and Alberta over the past 20 years. However, the data from Manitoba was too limited to do the type of analysis done for the other provinces. Pepperell then compared the genetic data to trade routes used by the voyageurs.

Brief, ancient contact

Only a few thousand French Canadian voyageurs were involved in trading furs with aboriginal populations between 1710 and 1870, resulting in close trade relationships and intermarriages. After that, French Canadians had almost no contact with those distant, isolated aboriginal communities.

In Canada, there are thousands of strains of the infectious lung disease tuberculosis. But one particular strain dominates in remote First Nations communities in Ontario, Saskatchewan and Alberta. (Associated Press)

And yet, genetic data in Pepperell's study showed that the French Canadian population of Quebec was the source of the dominant tuberculosis strain found in aboriginal populations along former fur trade routes today. It also showed that the original transmission took place in the 1700s.

"It's surprising that the signals of these 18th century historical events are so clear in the genetic data," Pepperell said.

No large-scale TB epidemics were reported in aboriginal communities until the late 1800s and early 1900s — about 100 years after the disease was first transmitted to them.

The researchers suggested that means tuberculosis can be spread widely by a small number of people in the absence of epidemics. Epidemics could arise later when the environment changed in a way that allows tuberculosis to thrive — for example, if conditions become crowded or social, housing or health conditions deteriorate.

That's because TB requires very close contact to spread and most healthy people infected with TB do not become sick, Alexander said. However, the disease can emerge later if an infected person is put under stress.

Pepperell said epidemic TB likely emerged as a result of First Nations communities being pushed to the brink of starvation and crowded onto reserves and into residential schools.

"The French Canadian voyageurs were certainly not responsible for those changes," she said in an email Thursday. "If we could somehow rewrite history and remove those terrible living conditions I suspect that the 'voyageur lineage' of TB would either have grumbled along at low levels or petered out."

Pepperell said the study helps researchers understand how TB works and moves between populations, and may be useful to policy makers interested in curbing the spread of TB.

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