Is there a fundamental flaw in Canada's post-arrival immigrant surveillance system for tuberculosis?

Abstract

Background: New immigrants to Canada with a history of tuberculosis or evidence of old healed tuberculosis on chest radiograph are referred to public health authorities for medical surveillance. This ostensible public health protection measure identifies a subgroup of patients (referrals) who are at very low risk (compared to non-referrals) of transmission.

Methods: To assess whether earlier diagnosis or a different phenotypic expression of disease explains this difference, we systematically reconstructed the immigration and transmission histories from a well-defined cohort of recently-arrived referral and non-referral pulmonary tuberculosis cases in Canada. Incident case chest radiographs in all cases and sequential past radiographs in referrals were re-read by three experts. Change in disease severity from pre-immigration radiograph to incident radiograph was the primary, and transmission of tuberculosis, the secondary, outcome.

Results: There were 174 cohort cases; 61 (35.1%) referrals and 113 (64.9%) non-referrals. Compared to non-referrals, referrals were less likely to be symptomatic (26% vs. 80%), smear-positive (15% vs. 50%), or to have cavitation (0% vs. 35%) or extensive disease (15% vs. 59%) on chest radiograph. After adjustment for referral status, time between films, country-of-birth, age and co-morbidities, referrals were less likely to have substantial changes on chest radiograph; OR 0.058 (95% CI 0.018-0.199). All secondary cases and 82% of tuberculin skin test conversions occurred in contacts of non-referrals.

Conclusions: Phenotypically different disease, and not earlier diagnosis, explains the difference in transmission risk between referrals and non-referrals. Screening, and treating high-risk non-referrals for latent tuberculosis is necessary to eliminate tuberculosis in Canada.

Fig 1. Canada’s pre- and post-arrival immigrant surveillance system for tuberculosis.

Fig 2. Cohort of recently arrived foreign-born pulmonary tuberculosis cases.

Abbreviations: CXR chest x-ray; IME immigration medical examination. * Of the 98 “overseas IME or no IME” non-referrals, 83 had an overseas IME and 15 had neither an overseas nor an in-Canada IME. Of the 83 non-referrals with an overseas IME, 80 had a normal overseas CXR and 3 had an abnormal overseas CXR. † By past CXRs we mean >3 months before their incident case film.

Fig 3. Stable chest radiograph histories in 47 referral cases.

Abbreviations: CXR chest x-ray. Each column represents an individual referral; horizontal lines on the columns indicate when CXRs were performed relative to the date of diagnosis. For example, patient #1 had three CXRs at 51, 116 and 189 weeks prior to diagnosis. One referral did not undergo an incident case CXR and is not included.

Fig 4. Unstable chest radiograph histories in 13 referral cases.

Abbreviations: CXR chest x-ray. Each column represents an individual referral; horizontal lines on the columns indicate when CXRs were performed relative to the date of diagnosis. For example, patient #2 had 5 CXRs at 2, 48, 56, 81 and 94 weeks prior to diagnosis. On the 2^nd (at week 81), 3^rd (at week 56) and 4^th (at week 48) there was no change from the first, but on the next film (at week 2) the independent readers judged there was a minimal change from the 4^th, and on the next film after that (the incident case film at week 0) there was subtle change from the 5^th. In these 13 referrals the extent of disease remained unchanged over time in nine (8 remained minimal, 1 remained moderately-advanced). In four, patients #1, 3, 6 and 13, the extent of disease changed from minimal to moderately-advanced.