In June last year, the Ministry of Health Singapore initiated a large-scale on-site voluntary tuberculosis (TB) screening for the residents of a 10-storey apartment block after a cluster of 6 multidrug-resistant TB (MDR-TB) cases in the Block were diagnosed over the preceding 4 years. The results were released in an academic paper published in the International Journal of Infectious Diseases 3 days ago – you can read the accepted manuscript free here (disclaimer: I had a bit role in this work and am one of the authors on the paper).

There were several interesting findings, some of which had not been released on mainstream media (because too technical).

First, even with a high resolution tool such as whole genome sequencing coupled with traditional “shoeleather” epidemiology, it was not possible to work out the chain of transmission in the block. The index case for the block (A – incidentally also the first of the LAN gaming cafe TB cases in 2012) no doubt passed the isolate to his friend B. A had stayed at the apartment B shared with his mother C. After that, it was not clear at all if A had transmitted TB to the rest, or whether there was some combination of spread by both A and the others.

None of them knew each other (beyond A, B and C), and the only potential point of convergence was the lifts, which had good ventilation (and in a lift, the time of exposure is very short).

The MOH team managed to screen 241 current and 18 past residents of the apartment block, finding 2 potential active TB cases (culture-negative) and 46 (19.9%) latent TB infections. 19.9% latent TB rate is high but not unusually so (a recent unpublished study defined a population rate of 13.9% in Singapore, with 95% confidence interval of 6.7% to 22.6%).

Was the entire screening exercise useful and cost-effective? There is just no possible objective answer to that at this stage, as was reflected in the paper. However, there will be more MDR-TB cases and local clusters appearing in the future, and greater effort and attention is necessary to prevent the spread of these difficult-to-treat strains of Mycobacterium tuberculosis.