Detailed characterisation of the tuberculosis epidemic in Western Sydney: a descriptive epidemiological study

Discussion

Although Australia maintains a TB incidence of <10 per 100 000 population, which is recognised by WHO as being in the pre-elimination phase, pockets of immigration-driven disease persist within large cosmopolitan cities. This is the reality in all countries with high rates of migration from TB-endemic settings, but it is rarely analysed in sufficient detail to guide tailored public health responses. In Western Sydney most TB cases were born in India, China or the Philippines. “country of birth specific TB case rates” are influenced by multiple factors, including the TB prevalence in migrants' home country, the prevalence within their particular “home community” and the size and vulnerability of the migrant population, the time since migration [13], as well as the frequency of travel to and from their home country [14].

Premigration screening for active TB [2, 15] as well better living standards and reduced TB exposure in Australia, probably explain the lower TB incidence seen in Western Sydney among migrants compared to their home country. However, this does not hold true for people born in Nepal. TB case detection is suboptimal and notifications are generally under-reported in Nepal [16], but WHO TB incidence estimates aim to correct for these factors [11]. Apart from underestimation at the national level, regional differences in TB incidence within Nepal and the regions from which the Western Sydney Nepalese population have immigrated, as well as sex balance [17] may also influence the observed rates. An additional consideration is the possibility that certain protective elements of living at altitude in Nepal, which is a more hypoxic environment and has been shown to enhance antimicrobial cellular immunity [18], may be lost in Australian migrants who live at sea level. In addition, many Nepalese TB patients are international students, and the high cost of living in Sydney may lead to stress and financial hardship that could increase TB vulnerability.

The geographical topography of TB patients in Western Sydney identified hotspots in areas with the highest socioeconomic disadvantage, similar to findings in London, where a prominent east–west gradient in TB incidence has been associated with areas of migrant settlement and socioeconomic disadvantage [19, 20]. Poverty is recognised as a major global driver of the TB epidemic and even in high-income countries low socioeconomic status remains a major TB risk factor [21, 22]. Poverty is associated with both increased TB exposure risk and increased risk of disease progression following TB infection [23]. Unlike the experience in other jurisdictions [2] and parts of NSW [24], Aboriginal and Torres Strait Islander populations were not over-represented among TB cases in Western Sydney.

Very high TB incidence rates, >100 per 100 000 population, were noted among the Nepal-, Afghanistan- and India-born populations in Auburn, the Afghanistan-born population in Holroyd and the Philippines- and India-born populations in Parramatta. Providing better contextualised public health interventions in these at-risk communities should be a priority. Immigrants with language and cultural backgrounds that differ from their recipient country are more likely to experience problems with healthcare access and health communication [25]. TB-associated stigma is also pronounced in certain cultures [26]. Studies in migrant populations in Melbourne and Canada indicated TB knowledge gaps, TB-associated stigma and limited healthcare access in particular groups, all of which impacted negatively on TB diagnosis and treatment [27, 28]. A Sydney study identified communication problems when doctors discussed treatment of latent TB infection (LTBI) with migrants from culturally and linguistically diverse backgrounds, even with the use of interpreters [29].

Therefore, considering the best and most culturally appropriate methods to engage with at-risk communities require careful consideration, especially if targeted community screening and LTBI treatment is considered as a potential preventive strategy [30, 31]. Such a strategy should include the use of well-informed bilingual community health workers to increase awareness and knowledge about TB and the available services by using their cultural links within the respective migrant communities [32]. Similarly, the use of bilingual TB healthcare professionals within local TB services would assist to bridge the cultural gap between various ethnic communities and the healthcare system. The benefits and risks of community-based LTBI screening and treatment deserves formal evaluation, considering acceptability, potential stigmatisation and the high mobility of immigrant populations that might expose them to re-infection. High-risk groups in whom LTBI treatment may be most feasible include international students and healthcare workers.

Population projections suggest a large increase in the Western Sydney population over the next 20 years [3], with most new arrivals coming from high TB incidence countries. This indicates the importance of supporting efforts to improve regional and global TB control. Given likely ongoing TB importation, the most important outcome from a public health perspective is to prevent local TB transmission, which can now be tracked as a key programme performance indicator. NSW were among the first jurisdictions in the world to implement routine whole-genome sequencing of all culture-confirmed TB cases (since October 2016), assisting rapid identification of drug resistant strains, tracking of local transmission dynamics and guiding targeted public health responses. Initial findings demonstrate limited local transmission [33, 34], but a commitment to “zero TB transmission” in line with the global TB elimination agenda in low-incidence settings [35], requires ongoing vigilance and enhanced national coordination.

Current offshore premigration screening aims to identify people with active TB who may pose a transmission risk to Australian citizens. It detects around 400–500 TB cases among long-term visa applicants every year [2], who are required to demonstrate treatment completion and clinical cure before a visa is granted. However, little consideration is given to the risk of future disease development among those who stay in Australia for a prolonged period of time, apart from linking those with suspicious chest radiographs to onshore surveillance systems [15, 36]. A Canadian study found that 79% of migrants who developed TB were not identified to be at risk during initial radiographic screening [37]. These cases generally presented with more advanced disease and were associated with more secondary TB cases (local transmission) compared to those identified to be at risk and placed on post-migration surveillance [37]. Similarly, 77% of foreign-born TB cases in Western Sydney were never placed on a TBU, with a further 10% developing TB after the period of surveillance. However, a recent NSW study showed that targeted post-migration surveillance was partially effective; detecting 63% of TB cases during follow-up [38]. These data indicate potential benefit from targeted LTBI screening and treatment, as well as active surveillance of high-risk cases. Given low but stagnant TB incidence rates in NSW over the past ∼20 years [39], it is clear that more proactive strategies are required for TB elimination.

Despite general consensus that LTBI treatment is a valuable tool [40], implementation remains limited in the absence of detailed cost–benefit analyses [30, 31]. A UK modelling study found that a single interferon-γ release assay, combined with LTBI treatment, offered good value for money in immigrants from countries with a TB incidence >150 per 100 000 population [41]. These findings influenced the current National Institute for Health and Care Excellence (NICE) guidelines [42], which acknowledge the need for population-tailored approaches that are context specific. A complicating factor that is rarely considered in modelling studies is the increased mobility and home country contact that immigrant families have in a modern globalised world, as suggested by the cross-border travel data that we assessed. This implies a risk of re-infection that may reduce the protective effect of LTBI treatment.

The TB re-infection risk is difficult to quantify in the absence of a reliable test, but children of immigrant families with recent travel to a high TB incidence country were found to be five times more likely to have a positive tuberculin skin test (TST) than those without travel [43], which provides an estimate of infection/re-infection risk. Among Australians with travel-related infections, a large proportion (81%) of cases reported recent travel to their country of birth. Travel to visit friends and relatives posed an increased “all infection” risk, due to longer travel duration and increased likelihood of disease exposure [44]. Immigrant families in Western Sydney regularly travel to high TB incidence countries to visit friends and relatives, which place young children at risk of TB exposure, infection and disease, especially if they are not Bacille Calmette–Guérin (BCG) vaccinated [45]. Ideally, BCG vaccination should be offered at birth to immigrant families or be readily available as a travel vaccine in these vulnerable young children [45, 46].

Study limitations include the fact that we used routine notification data to identify TB cases and HIV test results were not recorded for a large proportion. However, NSW TB programme data are otherwise comprehensive and provide an unbiased overview of the total TB disease burden, while HIV testing rates have improved. In addition, we used a variety of complementary data sources to characterise the TB epidemic and local population dynamics in as much detail as possible. The contribution of TB importation is likely to be underestimated if we only consider non-Australian born cases, given that many Australia-born cases have parents from high TB incidence countries and/or have travelled to these countries for extended periods of time. Ethnicity of Australian-born cases could not be established, since this is not captured within the NCIMS database. The accuracy of future projections is highly dependent on government policy, but helps to plan for likely future scenarios and highlights the need for comprehensive data collection and dynamic strategies that are responsive to changing migration patterns.