Demographic predictors of active tuberculosis in people migrating to British Columbia, Canada: a retrospective cohort study

Abstract

A s tuberculosis (TB) transmission declines in regions of low TB incidence, TB is becoming concentrated in migrant populations.1 In Canada, a country with low TB incidence, most cases of TB (about 70%) are diagnosed in people born outside of Canada, with many arriving from countries with a high TB incidence. 2Preimmigration medical screening is a cornerstone of TB prevention in Canada. Prospective permanent residents and temporary residents who will be staying in Canada for more than 6 months undergo a preimmigration medical examination that includes a history, physical examination and chest radiograph, and when indicated by radiograph abnormalities, sputum samples for smear and culture. Applicants with a diagnosis of active TB must receive treatment before they can obtain residency status.2 Postimmigration surveillance of select individuals is also conducted, whereby people considered to be at high risk for TB (i.e., evidence of prior TB or abnormalities present on chest radiographs) are highlighted for follow-up surveillance by public health authorities in Canada.2 However, a recent study found that less than 3% of active cases of TB in people migrating to Canada were detected by postimmigration surveillance.3 Currently, no routine pre-or postimmigration screening and treatment programs for latent TB infection (LTBI) for people migrating to Canada are available. Scale-up of LTBI screening and treatment in high-risk populations postimmigration is a component strategy for achieving World Health Organization (WHO) targets for TB elimination in Canada.4,5 However, with about 250 000 new permanent residents migrating to Canada annually, the large population of people who may qualify for LTBI screening and treatment is a substantial barrier to implementation.6 National guidelines recommend targeting screening for LTBI in people migrating from high-incidence countries, particularly in adult refugees up to 50 years of age and those with other risk factors for increased TB reactivation.2,4 However, many of these recommendations are based on weak evidence.

Estimating Long-term Tuberculosis Reactivation Rates in Australian Migrants

“…In our simulated cohort of Australian migrants with LTBI, TB reactivation rates appeared to be dependent on both time from migration and age, with lower rates seen with increasing time from migration and possible increases in rates in those aged 70 years and during youth. Although these trends are consistent with existing observations [9,[21][22][23][24][25], to our knowledge, this is the first time all these phenomena have been demonstrated in a single study, and the first study to use estimated LTBI prevalence among migrant populations to provide insights into the natural history of TB. While…”

“…In addition to time, there were also indications in our results that the risk of progression may differ along life course, with higher reactivation rates in elderly cohorts and youth when compared to younger cohorts that had migrated at a similar age. These observations have been made before [9,[25][26][27][28][29][30][31], and the pattern of pulmonary TB reactivation rates we observed by age post-migration resembles that of a study in Ontario, Canada that compared TST-survey data from 1958 to 1960 to pulmonary TB cases across the same region in 1962 ( Figure S10) [21]. Further evidence that reactivation rates may increase into old age can also be observed in other studies [32,33], including birth cohort studies [30,31] and a recent study in Canadian migrants [9], and plausible reasons may include weakened immune status and increasing prevalence of comorbidities associated with old age [34][35][36].…”

“…These observations have been made before [9,[25][26][27][28][29][30][31], and the pattern of pulmonary TB reactivation rates we observed by age post-migration resembles that of a study in Ontario, Canada that compared TST-survey data from 1958 to 1960 to pulmonary TB cases across the same region in 1962 ( Figure S10) [21]. Further evidence that reactivation rates may increase into old age can also be observed in other studies [32,33], including birth cohort studies [30,31] and a recent study in Canadian migrants [9], and plausible reasons may include weakened immune status and increasing prevalence of comorbidities associated with old age [34][35][36]. Previous studies have also observed a period of increased reactivation risk during youth [25][26][27][28][29], but whether these increases are due to reactivation of quiescent infections, or an increased risk of reinfection is debateable.…”

“…Therefore, observing reactivation rates by time since migration in migrant cohorts provides an opportunity to study how TB rates change with time from infection. Studies that have reported TB rates among migrants over time since migration to low-incidence settings have often found the greatest risk of disease in the first years after migration [6][7][8][9]. While some studies have shown decreasing TB rates beyond this period [6,8], in other cohorts rates did not decrease uniformly over time [7,10,11].…”

Tuberculosis Specific Interferon-Gamma Production in a Current Refugee Cohort in Western Europe

“…Based on the recent increase in refugee TB cases, the awareness for this disease grows in Western Europe. In this regard, the identification of migrants at particular risk for TB and their further diagnostic, evaluation, and treatment is a matter of interest [ 6 , 7 ]. Data on the frequency of the TB status of migrants entering Europe is scarce and consequently, diagnostic regimens in migrants have not been harmonized.…”