Piecing the puzzle together: foreign-born tuberculosis in an immigrant-receiving country.

In major immigrant-receiving countries, annual foreign-born tuberculosis (TB) case counts and rates are relatively constant. Why this is so, and who might be a high-yield target for screening for latent TB infection, remain open questions. Foreign-born TB in Canada during 1986-2002 was retrospectively examined using national TB and immigration data as well as census data. Case counts and rates were analysed in relation to demographics, immigration period and time since arrival. Pre-1986 immigrants (n=3,860,853) and 1986-2002 immigrants (n=3,463,283) contributed 8,662 and 9,613 TB cases, respectively. Immigrants arriving ≤ 5 yrs ago and those arriving >10 yrs ago contributed almost equally to the annual foreign-born TB case count despite a 3.5-fold difference in in-country person-yrs. Remarkably stable and relatively low TB incidence was observed among immigrants >10 yrs post-arrival. Conversely, TB incidence within 5 yrs of arrival was dynamic, demonstrating a strong inverse association with time since arrival and higher sensitivity to changes in immigration level than shifts toward higher incidence source countries. Relative constancy in foreign-born TB incidence is explained by a complex convergence of several factors. Immigrants born in high-incidence countries who arrived ≤ 2 yrs ago and were aged 15-34 yrs upon arrival constitute high-yield targets for preventive therapy.

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.

A population-based study of tuberculosis epidemiology and innovative service delivery in Canada.

OBJECTIVE: To compare and interpret tuberculosis (TB) incidence rates in a Canadian population across two decennials (1989-1998 and 1999-2008) as a benchmark for World Health Organization targets and the long-term goal of TB elimination. The population under study was served by two urban clinics in the first decennial and two urban and one provincial clinic in the second. METHODS: TB rates among Status Indians, Canadian-born 'others' and the foreign-born were estimated using provincial and national databases. Program performance was measured in on-reserve Status Indians in each decennial. RESULTS: In each decennial, the incidence rate in Status Indians and the foreign-born was greater than that in the Canadian-born 'others'; respectively 27.7 and 33.0 times in Status Indians, and 8.0 and 20.9 times in the foreign-born. Between decennials, the rate fell by 56% in Status Indians, 58% in Canadian-born 'others', and 18% in the foreign-born. On-reserve Status Indians had higher rates than off-reserve Status Indians, and the three-clinic model out-performed the two-clinic model among those on-reserve. Rates in the foreign-born varied by World Bank region, and were highest among those from Africa and Asia. CONCLUSION: Status Indians and the foreign-born are at increased risk of TB in Canada. Significant progress towards TB elimination has been made in Status Indians but not in the foreign-born.