Transmission of Mycobacterium tuberculosis Beijing Strains, Alberta, Canada, 1991–2007

Deanne Langlois-Klassen, Ambikaipakan Senthilselvan, Linda Chui, Dennis Kunimoto, L. Duncan Saunders, Dick Menzies, and Richard Long

Author affiliations: University of Alberta, Edmonton, Alberta, Canada (D. Langlois-Klassen, A. Senthilselvan, L. Chui, D. Kunimoto, L.D. Saunders, R. Long); Provincial Laboratory for Public Health, Edmonton (L. Chui); McGill University Health Centre, Montreal, Quebec, Canada (D. Menzies); McGill University, Montreal (D. Menzies)

Abstract

Beijing strains are speculated to have a selective advantage over other Mycobacterium tuberculosis strains because of increased transmissibility and virulence. In Alberta, a province of Canada that receives a large number of immigrants, we conducted a population-based study to determine whether Beijing strains were associated with increased transmission leading to disease compared with non-Beijing strains. Beijing strains accounted for 258 (19%) of 1,379 pulmonary tuberculosis cases in 1991–2007; overall, 21% of Beijing cases and 37% of non-Beijing cases were associated with transmission clusters. Beijing index cases had significantly fewer secondary cases within 2 years than did non-Beijing cases, but this difference disappeared after adjustment for demographic characteristics, infectiousness, and M. tuberculosis lineage. In a province that has effective tuberculosis control, transmission of Beijing strains posed no more of a public health threat than did non-Beijing strains.

The Beijing lineage of Mycobacterium tuberculosis (also referred to as the East Asian lineage or lineage 2) is an emerging public health threat (1,2). The Beijing lineage accounts for 13% of M. tuberculosis strains globally (3) and 19%–27% of M. tuberculosis strains in low tuberculosis (TB) incidence immigrant-receiving countries, such as Australia, the United States, and Canada (4–6). In addition to their recent global dissemination, Beijing lineage strains raise concern because of frequent associations with drug resistance and multidrug-resistant TB in particular (1,6–8). Reports of Beijing strains that are extensively drug resistant further intensify these concerns (7).
The rapid global expansion of Beijing strains and their frequent (albeit inconsistent) association with large TB outbreaks and younger patients has led to speculation that Beijing strains have a selective advantage over other M. tuberculosis lineages as conferred through increased transmissibility and virulence (1,2,7). This hypothesis is supported by experimental evidence of the increased virulence of Beijing lineage strains relative to other M. tuberculosis strains in vitro and in animal models (9,10). Evidence also suggests that the fitness of some Beijing strains is retained after the acquisition of drug resistance (11). Nevertheless, intragenotypic variation in virulence has been described in the Beijing family (12,13) and, in a review, Coscolla and Gagneux (14) concluded that the current body of evidence is insufficient to support the increased transmissibility of these strains.
Immigration is the main determinant of TB epidemiology in low incidence settings (15,16). Consequently, the importation of potentially more pathogenic strains, such as those in the Beijing family, could have major implications for TB prevention and elimination efforts within immigrant-receiving countries. Surveillance activities that identify the sources and transmission patterns of emerging and/or expanding M. tuberculosis strains will be increasingly vital if TB prevention and care programs are to maintain their effectiveness within the context of dynamic immigration policies and highly mobile populations.
We aimed to investigate the association of Beijing and non-Beijing lineage strains with transmission in a low TB incidence immigrant-receiving province of Canada. In particular, we sought to determine whether the Beijing lineage of M. tuberculosis is a greater public health threat than other strains because of increased transmission leading to disease.