An assessment of Canada’s foreign-born tuberculosis surveillance strategy and insights gained into TB transmission
Creators: Asadi, Leyla
Publisher: name: University of Alberta Library
Year: 2024

<p>This thesis, comprising three published research papers, examines both Canada&#39;s tuberculosis (TB) surveillance strategies among foreign-born individuals and broader TB transmission dynamics.<br>
In Canada, a country with low TB incidence, most TB cases occur among the foreign-born population. The global epidemiology of TB and the origins of Canada’s TB migrants present significant challenges in reducing annual TB rates through current control programs.<br>
The first two studies address Canada&#39;s TB surveillance strategy among foreign-born individuals, predominantly conducted through immigration medical exams. Utilizing data from Immigration, Refugees, and Citizenship Canada, along with provincial TB registry data, we established two cohorts of migrants in Alberta.<br>
The first study, analyzing a cohort of migrants who arrived in Alberta between 2002 and 2013, compares TB incidence and transmission between those referred for post-arrival TB surveillance (&quot;referrals&quot;) and those not required to undergo post-arrival TB surveillance (&quot;non-referrals&quot;). Our findings reveal that while referrals were more likely to be diagnosed with pulmonary TB, they had less severe disease and were significantly less likely (by 80%) to transmit. This could be interpreted as evidence of the program’s effectiveness, where early identification of referrals potentially prevents progression to a more infectious disease state. However, an alternative hypothesis posits that unlike referrals, who have a history of TB disease treatment or radiographic evidence of old, healed TB, non-referrals with TB infection may not have developed an immune response as capable of controlling M. tuberculosis.<br>
The second study uses chest radiographs from pre-arrival immigration medical exams to track the development of pulmonary TB disease in migrants diagnosed within two years of arriving in Canada. We adjusted for the time difference between pre-arrival and post-arrival radiographs and determined that non-referrals progressed from TB infection to pulmonary TB disease more rapidly and aggressively than referrals. These findings imply a possible biological difference between the groups and suggest the benefit of moving beyond chest radiograph screening to latent TB infection screening and treatment among some non-referrals.<br><br>
The third study investigates transmission differences based on sputum-smear microscopy status. Analyzing all pulmonary TB cases in Alberta from 2004 to 2016, we compared secondary cases arising from sputum smear-negative versus sputum smear-positive pulmonary TB cases. Using both DNA fingerprint clustering and conventional epidemiology, as well as genome sequencing among a subgroup of temporally and geographically linked cases, we found that smear-negative cases posed a 50% lower transmission risk than previously reported. <br>
The thesis concludes by exploring potential changes to Canada’s TB surveillance strategy. It also discusses the potential and limitations of whole genome sequencing, the influence of super-spreader events in TB transmission, and the utility of bioaerosol sampling in interrupting such transmission events.</p>