Preventing tuberculosis in the foreign-born population of Canada: a mathematical modelling study.

BACKGROUND: Foreign-born persons in Canada contribute 67% of all tuberculosis (TB) cases annually, but represent only 21% of the total population. Molecular epidemiological studies suggest that most foreign-born TB cases result from the reactivation of latent tuberculous infection (LTBI) acquired before immigration.

OBJECTIVE: To estimate the effect on incidence of a prevention strategy that would screen selected immigrants at arrival for LTBI and offer preventive treatment to those who test positive.

DESIGN: A deterministic model was developed to quantify the incidence of active TB in immigrants to Canada and validated with national immigration and TB case data.

RESULTS: Model simulations suggested that it would be optimal to screen and treat LTBI in new immigrants from countries of birth with an estimated TB incidence rate in excess of 50 per 100 000 person-years. If this strategy had been implemented in 1986, the national TB incidence rate would have fallen by 18.5%, from 5.4 to 4.4 cases per 100 000 population by 2002.

CONCLUSION: This study suggests that screening and treating LTBI in foreign-born persons from high TB incidence countries is the most effective strategy in terms of total persons screened and treated and percentage reduction in national incidence.