Trading Pelts for Pestilence

Pepperell contacted researchers around Canada, including Wendy Wobeser of Queen’s University in Toronto, who had sent Pepperell the Ontario fingerprints, to organize a collaborative project to track the evolution of M. tuberculosis through Canadian history. Sure enough, M. tuberculosis strains from indigenous populations in Ontario, Saskatchewan, and Alberta, as well as strains from French Canadians in Quebec, all appeared to comprise a single lineage, characterized by a shared nitrogenous base deletion called DS6^Quebec. The minisatellite data suggested that the bacteria spread among the continent’s Aboriginal populations from a source population in Quebec, accompanying European fur traders as they set up trading posts further and further west. Historical records show that European immigrants and Canada’s indigenous populations socially interacted, and even intermarried—arrangements which made M. tuberculosis transmission likely. Though Pepperell admits that the idea sounded a little “crazy” at first, the more she dug into the data, the more she was convinced that the fur trade was the vehicle for widespread gene flow of a single lineage of M. tuberculosis among Canada’s indigenous populations.

“This is a very neat story,” says mathematical biologist Carlos Castillo-Chavez of Arizona State University, who was not involved in the research. “This is almost like the anthropology of disease—the impact of people’s mobility and migration patterns on disease evolution.”

“It’s really the most robust description of [tuberculosis] epidemiology in northern Canada,” adds Wobeser, a coauthor of the study, published in a recent issue of PNAS (108:6526-31, 2011). Though she suspected the Canadian TB strains were related from the start, she says she wouldn’t have predicted that the link dated as far back as the fur trade. “But now that I know it, it makes sense.”

The data also suggest that M. tuberculosis can persist in populations at very low levels for long periods of time, exploding into epidemics as soon as host conditions are favorable. Indeed, though the initial spread of the bacteria likely started as early as 1710, tuberculosis was not commonly seen in indigenous populations until outbreaks in the 19th and 20th centuries.

One possibility for this delay, Castillo-Chavez says, is that the continued colonization of Canada by European settlers disrupted the Aboriginal lifestyle. “This is just a suspicion,” he says, but “my perspective is that TB activation requires some weaknesses in the immune system, and I think that those weaknesses may stem from problems of nutrition”—such as may have occurred as a result of a shift in “the balance of powers” during colonization.

Regardless of the reason, the finding that M. tuberculosis can persist at such low levels in the population may explain why the bacterium is so difficult to eradicate, Pepperell says. “Probably the most interesting part is the fact that we can learn so much about the evolution of the bacteria and the evolution of epidemics by integrating this sophisticated genetic analysis with historical research,” she says.