New Home: New Zealand Health of Migrants and Tuberculosis Elimination

Abstract

Background

Tuberculosis (TB) disproportionately affects migrants to New Zealand. In 2016, 79% of all cases were in those born overseas. The World Health Organization (WHO) has called for low-TB incidence countries such as New Zealand to aim for TB elimination. A focus on reducing TB in risk groups, including migrants, is necessary to achieve this. Despite radiological screening, TB still occurs in migrants, and little is known about specific risk factors for TB in migrants to this country. This study aims to reduce the impact of TB in migrants to New Zealand by identifying risk factors and auditing current TB immigration screening procedures.

Objectives

1) To calculate TB incidence in migrants according to nationality, region, visa type, age and sex. 2) To assess if TB was identified at the time of immigration screening, and if not could it have been.

Methods

Cohort study: TB cases were linked to the Statistics New Zealand Integrated Data Infrastructure (IDI) migration records from 01/01/2007 to 31/12/2016. All long-term migrants to New Zealand were included in the study. Border movements for arrivals and departures were used to calculate total time in New Zealand per migrant. TB rates per 100,000 person-years were calculated for nationality, region of origin, visa category, age and sex. Multivariate analysis of incidence rate ratios was calculated for region of origin and WHO incidence in country of origin adjusting for visa type, age and sex.

Clinical audit: TB cases were linked to the Immigration New Zealand (INZ) health screening records from 01/02/2015 to 31/01/2018. Screening records, x-ray reports and specialist letters for TB cases were audited to identify whether TB was diagnosed by screening, and in cases where it was not diagnosed, possible gaps in the screening programme.

Results

Cohort Study: There were 873 TB cases in 1,597,077 migrants, with a total of 2,851,287 person-years of follow up. The highest incidence of TB was seen in those from South-Central Asia, with 104.8 cases per 100,000 person-years (95% CI: 95.7- 114.9), followed by South-East Asia with an incidence of 59.2 per 100,000 (95% CI: 51.9-67.5). Incidence by country was positively associated with incidence in the country of origin, but incidence was less in New Zealand than in the country of origin. Incidence in New Zealand was proportionate to WHO estimated incidence, with the exception of migrants from the highest WHO incidence bracket 350 per 100,000, where New Zealand incidence reduced. Migrants from countries with a WHO incidence of <40 per 100,000 had a New Zealand incidence of 2.8 per 100,000 (95% CI: 2.0-4.0). On multivariate analysis, region of origin and WHO estimated incidence were strongly associated with TB risk in New Zealand. Age <15 was associated with a reduced risk. Visa category was not associated with risk of TB once other factors were adjusted for.

Clinical Audit: There were 120 cases of TB included for audit. Immigration screening detected TB, or was likely to have detected TB in 46 (38%) cases. In 74 (62%) cases, screening did not detect TB. In those not detected by screening, 13 (18%) were notified within six months of screening, and thus were likely cases of prevalent, active TB. Six of these cases were pulmonary TB, and five of these either had old, or no x-ray submitted as part of their immigration medical. Seven were cases of extrapulmonary TB. There were 50 (68% of those not detected by screening) cases of likely reactivation of latent TB (LTBI). The remaining cases not detected by screening were likely due to inactive or early TB (8 cases) or possibly infection in New Zealand (3 cases).

Conclusion

This study is the first to describe risk factors for a large cohort of migrants to New Zealand and is the first audit of the country’s screening programme. The cohort study identified the main risk factors for TB in migrants are the world region and TB incidence in the country of origin. The clinical audit showed that there is limited scope to improve on currently used x-ray screening of migrants, with most TB not detected by screening being due to reactivation of LTBI.

This study has identified high-risk migrant groups that could benefit from enhanced TB screening. Overall findings support the idea that LTBI is the cause of most TB in migrants in New Zealand. Further study is needed to assess possible benefits and costs with additional screening, such as for LTBI in high risk groups. There is a low incidence of TB in New Zealand in migrants from low WHO incidence countries, and consideration should be given to abandoning screening in these groups. A change in screening procedures will reduce the incidence of TB in New Zealand and its considerable impact on migrant communities, and help progress New Zealand towards TB elimination.