STANFORD (US) — Stealthy and patient, tuberculosis is able to lie dormant for decades, waiting for the right time to break out in epidemic proportions.
A study tracing a particular strain back to the fur trade, when French Canadian explorers unwittingly spread it to indigenous peoples is offering clues as to how it spreads and more importantly, how to control it
At the time, the disease was most probably carried repeatedly, beginning in 1710, but didn’t erupt until the fur trade was all but over more than 150 years later.
“We found there was this widespread, low-level dispersal of tuberculosis that did not become obvious until environmental changes occurred that created conditions conducive to epidemics,” says Caitlin Pepperell, an infectious diseases specialist at Stanford University.
“Tuberculosis epidemics are the outcomes of a process that has effectively been occurring underground,” she says, unlike smallpox, which quickly escalates into epidemics. This helps explain why it has been so extraordinarily difficult to eradicate TB.”
The conditions that finally triggered epidemics in Canada in the late 1800s likely resulted from the relocation of native peoples onto reservations, where health conditions were often abysmal.
Malnutrition was probably the biggest contributing factor. The buffalo, once a dietary mainstay of indigenous peoples on the prairie, had been virtually exterminated by then and other principal food sources, including other animals, wild plants, and fish, were also severely depleted. Attempts to provide famine relief fell woefully short.
Housing was also of low quality, with too many people living in close quarters, often lacking adequate ventilation or protection against the elements.
“With tuberculosis, even such simple things as having windows that open can actually make quite a large difference in terms of the risk of transmission,” Pepperell says. “Any situation where there is poor ventilation, it is dark, and it is crowded is perfect for transmitting TB.”
The study is published online in Proceedings of the National Academy of Sciences.
“Up to now, it has been relatively hard to define the locations of origin of a strain of tuberculosis more precisely than a continent,” says Marcus Feldman, professor of biology and a co-author of the paper.
Researchers worked with archived M. tuberculosis bacterial samples from indigenous communities across Alberta, Saskatchewan, and Ontario that had been previously collected by provincial health authorities. All the analyses were done on bacteria, not people. The scientists found a single strain of tuberculosis, characterized by a specific mutation, dominated in every community.
Earlier research had established that the same strain was also dominant among French Canadian residents of Quebec. “The tuberculosis strains from Quebec are missing a specific piece of DNA; that’s how you can track them back to Quebec,” Feldman says.
Tuberculosis can only be transmitted by someone with an active infection who is in close contact with others. The only time when French Canadians and the native peoples of Canada had substantial contact of that kind was during the fur trade era, so that had to be when the strain spread from the traders to the natives.
It was common during that era for traders to live in close contact with the native peoples, sometimes for months or years at a time. Their unions gave rise to the people known as the Métis, who are of mixed indigenous and French Canadian ancestry.
“There were handfuls of cases of TB stretched out over this long time period, where clearly TB was being transmitted, but the number of cases was such that it was not obvious to anyone at the time. TB was just kind of rumbling along at this very low level and probably would have continued that way, or even petered out completely, if it wasn’t for the deplorable living conditions on the reserves.”
By 1870 the era of the fur trade in its traditional, canoe-based form was over. There was no longer any need for the voyageurs, as furs were now transported by train. Most of the indigenous tribes had been relocated to reservations, and forays by French Canadians largely ceased.
At the same time, the influx of settlers to western Canada skyrocketed. The population of western Canada grew from 110,000 in 1871 to 750,000 in 1911.
How useful the new findings will be in current TB research is unknown.
“This approach of looking at the genetics in concert with epidemiology is fairly new in the world of bacterial research,” Pepperell says. “TB research has had a very long latent period, and now we are kind of getting into an expansion phase, which is a really good thing.”
The research was supported by the National Institutes of Health.
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