News of this outbreak – involving 6 MDR-TB cases to date – was released to the press yesterday evening by the Singapore Ministry of Health. The Channelnewsasia report also includes video footage of our Director of Medical Services as well as Prof Sonny Wang, long-term director of the TB Control Unit. These 6 cases were shown to be linked to each other via molecular fingerprinting of their TB isolates – in this case MIRU-VNTR and spoligotyping.
TB being a slow infection, the 6 cases were diagnosed over a period of 4 years. Because no clear link (other than that they are living in the same block) between most of the cases could be made despite intensive contact investigations, the local health authorities are taking the unprecedented step of offering screening for all residents at that particular HDB block (Block 203, Ang Mo Kio Avenue 3) – it is clear from the news report that at least a chest X-ray (the SATA Commhealth bus is a mobile X-ray unit) will be offered, although it is less clear if interferon-gamma release assay (IGRA) testing for latent tuberculosis will be performed as well.
Many people will naturally think that the transmission of the bacterium occurred in the lift – many HDB lifts are old, slow and have poor ventilation despite upgrading. But if that is the case, we would have become aware of more such outbreaks in the past.
It bears thinking what the MOH screening operation is aiming to achieve, besides being more transparent about reporting outbreaks and having “appeared to have done something active”. The chances of mobile chest X-rays picking up lung lesions that are not TB are fairly high, and these persons (and their families) would be subject to considerable anxiety and concern while waiting for further TB investigation results. Of course, picking up cases of MDR-TB or even “ordinary TB” will be a good outcome, preventing further transmission, but it is not completely clear that the clinical benefits outweigh the costs in this situation. The use of IGRAs (or Mantoux tests) is equally controversial – it would not be possible to determine whether someone with a positive IGRA has MDR-TB or ordinary drug-sensitive TB in this case, since the epidemiological link to MDR-TB is very weak, and drug-sensitive TB remains fairly common in Singapore. The cost-benefit ratio is also unclear – only 10% of latent TB cases go on to develop active disease, and there is little concrete evidence with regards to the efficacy of drug treatment for latent MDR-TB. Someone with a positive IGRA test in this situation will have to live under the potential shadow of an MDR-TB infection for many years, if not a lifetime.