Tuberculosis transmission in the Indigenous peoples of the Canadian prairies

Abstract

Setting: The prairie provinces of Canada.

Objective: To characterize tuberculosis (TB) transmission among the Indigenous and non-Indigenous Canadian-born peoples of the prairie provinces of Canada.

Design: A prospective epidemiologic study of consecutively diagnosed adult (age ≥ 14 years) Canadian-born culture-positive pulmonary TB cases on the prairies, hereafter termed "potential transmitters," and the transmission events generated by them. "Transmission events" included new positive tuberculin skin tests (TSTs), TST conversions, and secondary cases among contacts.

Results: In the years 2007 and 2008, 222 potential transmitters were diagnosed on the prairies. Of these, the vast majority (198; 89.2%) were Indigenous peoples who resided in either an Indigenous community (135; 68.2%) or a major metropolitan area (44; 22.2%). Over the 4.5-year period between July 1st, 2006 and December 31st 2010, 1085 transmission events occurred in connection with these potential transmitters. Most of these transmission events were attributable to potential transmitters who identified as Indigenous (94.5%). With a few notable exceptions most transmitters and their infected contacts resided in the same community type. In multivariate models positive smear status and a higher number of close contacts were associated with increased transmission; adjusted odds ratios (ORs) and 95% confidence intervals (CIs), 4.30 [1.88, 9.84] and 2.88 [1.31, 6.34], respectively. Among infected contacts, being Indigenous was associated with disease progression; OR and 95% CI, 3.59 [1.27, 10.14] and 6.89 [2.04, 23.25] depending upon Indigenous group, while being an infected casual contact was less likely than being a close contact to be associated with disease progression, 0.66 [0.44, 1.00].

Conclusion: In the prairie provinces of Canada and among Canadian-born persons, Indigenous peoples account for the vast majority of cases with the potential to transmit as well as the vast majority of infected contacts. Active case finding and preventative therapy measures need to focus on high-incidence Indigenous communities.

Fig 1. Map of Canada highlighting the three prairie provinces, Alberta, Saskatchewan and Manitoba.

For perspective, the country of France is about one-third the size of the three prairie provinces combined (0.643M km²) and in 2006 had a population that was 12x as large (63.62M)–see http://data.worldbank.org/indicator/SP.POP.TOTL?location=FR. Abbreviations: NL Newfoundland; NB New Brunswick; PEI Prince Edward Island.

Fig 2. Tuberculosis cases in the prairie provinces of Canada between July 1, 2006 and December 31, 2010.

Population group information was missing for two of the 1069 “other” culture-positive cases; of the 26 cases meeting the original study inclusion criteria and designated a secondary case, 25 were Indigenous and 1 was non-Indigenous.