A population-based cohort study of Mycobacterium tuberculosis Beijing strains: an emerging public health threat in an immigrant-receiving country?

Abstract

Introduction: Mycobacterium tuberculosis Beijing strains are frequently associated with tuberculosis outbreaks and drug resistance. However, contradictory evidence and limited study generalizability make it difficult to foresee if the emergence of Beijing strains in high-income immigrant-receiving countries poses an increased public health threat. The purpose of this study was to determine if Beijing strains are associated with high risk disease presentations relative to other strains within Canada.

Methods: This was a retrospective population-based study of culture-confirmed active TB cases in a major immigrant-receiving province of Canada in 1991 through 2007. Of 1,852 eligible cases, 1,826 (99%) were successfully genotyped. Demographic, clinical, and mycobacteriologic surveillance data were combined with molecular diagnostic data. The main outcome measures were site of disease, lung cavitation, sputum smear positivity, bacillary load, and first-line antituberculosis drug resistance.

Results: A total of 350 (19%) patients had Beijing strains; 298 (85%) of these were born in the Western Pacific. Compared to non-Beijing strains, Beijing strains were significantly more likely to be associated with polyresistance (aOR 1.8; 95% CI 1.0-3.3; p = 0.046) and multidrug-resistance (aOR 3.4; 1.0-11.3; p = 0.049). Conversely, Beijing strains were no more likely than non-Beijing strains to be associated with respiratory disease (aOR 1.3; 1.0-1.8; p = 0.053), high bacillary load (aOR 1.2; 0.6-2.7), lung cavitation (aOR 1.0; 0.7-1.5), immediately life-threatening forms of tuberculosis (aOR 0.8; 0.5-1.6), and monoresistance (aOR 0.9; 0.6-1.3). In subgroup analyses, Beijing strains only had a significant association with multidrug-resistant tuberculosis (aOR 6.1; 1.2-30.4), and an association of borderline significance with polyresistant tuberculosis (aOR 1.8; 1.0-3.5; p = 0.062), among individuals born in the Western Pacific.

Conclusion: Other than an increased risk of polyresistant or multidrug-resistant tuberculosis, Beijing strains appear to pose no more of a public health threat than non-Beijing strains within a high-income immigrant-receiving country.

Figure 1. Annual incidence rates of Beijing and non-Beijing lineage strains by population group.

In each panel: rates are per 100,000 person-years (y-axis); time corresponds to the year of diagnosis (x-axis); bars with dark grey shading are cases with Beijing strains; and bars with light grey shading are cases with non-Beijing strains. The panels represent total cases (panel A) as well as cases within the foreign-born Western Pacific (panel B), foreign-born ‘other’ (panel C) and Canadian-born (panel D) populations.