Fighting TB by 'fingerprinting' its DNA

SINGAPORE - The Ministry of Health (MOH) is looking at using a DNA fingerprinting process called genotyping to study all tuberculosis cases here.

Genotyping would allow the authorities to understand better how TB is being transmitted in Singapore and intervene early to prevent the infectious disease from spreading.

The process can help identify those in the same chain of recent transmissions, and is an indicator of TB control.

TB, a contagious bacterial infection that involves the lungs, has been affecting more people in recent years.

Last year, there were about 1,580 TB cases reported, as indicated by preliminary figures from MOH. This was slightly higher than the 1,533 cases in 2011.

An MOH spokesman said that the ministry is "assessing the feasibility of introducing TB genotyping routinely to all diagnosed TB cases in Singapore".

TB genotyping is already being used in a small way here for selected cases, such as those patients with drug-resistant TB, said Associate Professor Sonny Wang, director of Tan Tock Seng Hospital's Tuberculosis Control Unit.

MOH bears the cost of about $300 per patient.

Drug-resistant TB is harder to cure as it does not respond to the usual treatment for the disease.

TB genotyping involves fingerprinting the DNA of the TB bacteria in infected persons. If two people have the same TB genotype, it could mean one of them spread the disease to the other.

Said Prof Wang: "The goal of TB control is to interrupt transmission. So if there are too many similar fingerprints, it would mean we have not been successful in that." He added that TB genotype surveillance could also help to determine where the transmission is occurring.

Singapore's TB control programme focuses on early detection and diagnosis, effective treatment, and prevention through the prompt identification and treatment of infected contacts.

In 2008, TB incidence rates spiked to 1,451 cases for the first time here in almost a decade, after an all-time low of 1,256 cases the year before.

A recent MOH paper on changing TB trends among Singapore residents between 2002 and 2011 pinpointed the ageing population as one of the drivers of this rise in TB incidence rates.

The elderly population represents a large pool of latent TB infection, the study noted.

A person with a latent TB infection has the bacteria but does not show the symptoms for years, sometimes decades.

Although non-contagious, about 10 per cent of those with latent TB infections will develop active TB during their lifetime. Age-related chronic diseases such as diabetes can lower an elderly person's immunity and increase the risk of progression from latent TB to the active form.

But the study noted that in the past five to six years, age-specific TB incidence rates have increased in residents aged 20 to 59, and appear to have plateaued in the above-60 group.

Delaying treatment, not completing treatment, missed diagnoses, and Singapore's high immigrant population are some other reasons for the growing TB numbers.

Details of the ministry's review of its TB prevention programme will be announced in due course, said its spokesman.

melpang@sph.com.sg

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