Mycobacterium tuberculosis transmission over an 11-year period in a low-incidence, urban setting

Montreal, Canada, has a mean annual tuberculosis (TB) incidence of 9 per 100,000 population, 1996-2007.

To characterise potential Mycobacterium tuberculosis transmission by patient subgroups defined by age, sex, birthplace, smear and human immunodeficiency virus status, and to estimate the proportion of cases that resulted from transmission between these patient subgroups.

Retrospective study using DNA fingerprinting techniques, with clinical and demographic information from the public health department. Among cases with matching fingerprints, a pulmonary index case was identified. The transmission index was defined as the average number of subsequent TB cases generated directly or indirectly from an index case, and was compared among subgroups, including Haitian immigrants.

Compared to non-Haitian foreign-born index cases, Canadian-born index cases were associated with 2.38 times as many (95%CI 1.24-4.58) subsequent cases, while Haitian-born index cases were associated with 3.58 times as many (95%CI 1.74-7.36). Smear-positive index cases were not independently associated with increased transmission. However, middle-aged Canadian-born index patients were associated with a disproportionate number of subsequent cases.

In Montreal, index patients from several high-risk groups are associated with subsequent transmission. This approach can be applied to other low-incidence settings to identify where targeted interventions could potentially further reduce transmission.