Discussion
The performance of TB contact tracing activities is not included in routine WHO reports and infrequently monitored, even in low-incidence settings with adequate resources. However, with a renewed emphasis on TB elimination in low-incidence settings, expectations are changing. In Parramatta Chest Clinic, the elicitation of close TB contacts and screening of these contacts for TB and LTBI were generally well done. Although assessment rates were similar to those reported in studies from the USA (88%)18 and UK (86%),19 they still fall short of the CDC target of 93%.17 A major implementation gap compared with the CDC targets was the fact that LTBI treatment was initiated in <20% of those with documented LTBI. At the time, New South Wales guidance only encouraged LTBI treatment in those considered to be at increased risk of TB disease progression, such as young children and people with significant immune compromise, or those with documented recent TB infection (TST conversion).8 11
Few young children (<5 years of age) had documented LTBI, but 100% of those with a TST reading ≥10 mm received preventive therapy and completed their full course of treatment. A single child with an indeterminate (6 mm) TST was placed on CXR surveillance. Although none of the children whose first TST was negative demonstrated TST conversion on repeat testing, ‘window prophylaxis’ was not offered. ‘Window prophylaxis’ requires consideration in young children who are at high risk of rapid disease progression following primary Mycobacterium
tuberculosis infection,20 even if the initial TST or interferon-γ release assay (IGRA) result performed during the ‘conversion window’ (first 6–8 weeks after the end of TB exposure) is negative. International guidance encourages the provision of preventive therapy in this vulnerable population irrespective of the TST result, particularly in settings where LTBI testing is problematic.3 14 21–23 The most recent WHO LTBI treatment guidance encourages the use of water-dispersible isoniazid and rifampicin fixed-dose combination tablets for a period of 3 months.21 These ‘child-friendly’ medications are now available through the Global Drug Facility,24 also to high-income countries like Australia, but given incountry restrictions on the registration of therapeutic drugs the use of special access schemes is required to make these drugs available to children.
A frequently encountered dilemma in close contacts who are TST-positive at baseline is the distinction between recent and past infections, especially in people with previous TB exposure. Although this distinction can be problematic, new global guidance encourages LTBI treatment irrespective of the likely timing of primary infection,21 in order to reduce the reservoir of LTBI from which future cases may arise. Both historic and recent studies suggest that this provides a highly effective and cost-efficient strategy to prevent secondary cases and reduce TB case numbers over time,3 25–27 although it remains difficult to consider the effect of possible future reinfection in highly mobile populations.28
Guidelines in many countries used to discourage the use of isoniazid preventive therapy in people older than 35 years of age given hepatotoxicity concerns. However, in the absence of excessive alcohol use or known liver disease, the risk of isoniazid-induced hepatotoxicity is mainly elevated in people older than 60 years of age, and the most recent National Institute for Health and Care Excellence guideline (2016) recommends preventive therapy up to the age of 65 years.29 30 Newer treatment regimens such as weekly isoniazid and rifapentine for 3 months (12 weekly doses), which is now the standard of care in the USA, are generally well tolerated across a wide age range,31 but rifapentine access outside the USA is limited, and it is currently not registered for use in Australia. Previous studies reported poor treatment completion rates in healthy adult contacts provided with 6–9 months of isoniazid preventive therapy,32 but our study found excellent treatment completion rates when LTBI treatment was advised by the local clinician. Adherence rates are expected to be even higher if shorter LTBI treatment regimens become more widely available. Our findings demonstrate that the option of CXR follow-up is hardly satisfactory, given low completion rates.
Apart from the opportunity for disease prevention, TB contact tracing also has important active case-finding value,28 33 as demonstrated by the four secondary cases identified during this study. In fact, household contact tracing has been identified as one of the most efficient strategies for active case-finding, both in high and low TB incidence settings.33 34 LTBI treatment complements active TB case-finding (detection of prevalent case) to break TB transmission chains, and at a population level the combined effect of active TB case-finding and effective LTBI treatment of close contacts is expected to have a major impact on TB incidence and transmission over the long term.35 However, clinical variability in the decision to institute LTBI treatment persists among TB physicians in Australia,36 including uncertainty about the risk:benefit ratio of LTBI treatment in cases considered to be at low risk of future disease progression.37
Study limitations include the relatively small and geographically focused study population, but we believe it provides a good snapshot of the overall management of TB contacts in Western Sydney, which is highly representative of cosmopolitan urban Australia. BCG vaccination may have confounded some TST results, but BCG vaccination or the presence of a BCG scar was not consistently documented in the clinical notes and could not be analysed. During the study period IGRAs were not available for routine LTBI evaluation, but these have since been endorsed for this purpose and widespread use should improve the specificity of LTBI detection in BCG-vaccinated individuals.21 38 The HIV status of TB contacts was not recorded, which may significantly increase an individual’s risk of developing TB if LTBI is detected.39 However, HIV testing of TB contacts is not routinely performed given that the prevalence of HIV infection in Western Sydney is very low, even among patients with TB and their families.
In conclusion, we documented good TB contact elicitation and evaluation rates in Western Sydney, falling just short of the CDC targets. However, few contacts with documented LTBI received preventive therapy, except for children <5 years of age. This highlights the challenges of wide-scale LTBI treatment in contacts considered to be at low risk of future disease progression, as well as the need to provide clinicians with feasible and minimal risk treatment options, while policy makers require convincing cost:benefit analyses to motivate wide-scale implementation.