Investigation of a cluster of multi-drug resistant tuberculosis in a high-rise apartment block in Singapore

Objective: Between February 2012 and May 2016, six residents of an 11-storey apartment block were diagnosed with MDR-TB. Based on initial tests, all isolates had similar genotypic profiles, although there were no identifiable epidemiological transmission patterns between three cases. We present findings from the cluster investigation and results of a mass screening exercise. Design: Free voluntary TB screening was offered to past and current residents of the apartment block, comprising an interview, Chest X-Ray, and Interferon Gamma Release Assay or Tuberculin skin test. Expected latent TB proportions were calculated using a reference population, and whole genome sequencing (WGS) was performed. Results: The index case was involved in a separate gaming centre outbreak involving five patrons. 241 current (67.9% of 355 residents) and 18 past residents were screened. The latent TB proportion was 19.9%, which was at the higher end of the expected range. WGS confirmed relatedness of cases’ MDR-TB isolates– eight of 10 isolates were genetically identical, while the remaining two were one Single Nucleotide Polymorphism apart. Conclusion: With WGS, TB clusters not apparent through regular activity-based contact tracing may be detected. Mass screening may help inform the extent of transmission, but is limited by participation and difficulties in interpretation.