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An assessment of Canada’s foreign-born tuberculosis surveillance strategy and insights gained into TB transmission
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- Author / Creator
- Asadi, Leyla
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This thesis, comprising three published research papers, examines both Canada's tuberculosis (TB) surveillance strategies among foreign-born individuals and broader TB transmission dynamics.
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.
The first two studies address Canada'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.
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 ("referrals") and those not required to undergo post-arrival TB surveillance ("non-referrals"). 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.
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.
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.
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. -
- Graduation date
- Spring 2024
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.