The residents of a block of flats in Ang Mo Kio Avenue 3 are being urged to undergo screening for tuberculosis (TB), after six of their neighbours were diagnosed with the same drug-resistant strain of the disease over a four-year period.

Last night, grassroots volunteers, officers from the Ministry of Health (MOH) and Ang Mo Kio GRC MP Koh Poh Koon knocked on the doors of the 160 units of Block 203 to inform residents of the situation, which MOH described as "highly unusual".

Three of the six TB patients lived in the same unit but the other three were from different households. All four households said they did not know and had not interacted with one another.

This stumped investigating doctors as TB is typically spread through "close and prolonged contact", said Associate Professor Benjamin Ong, director of medical services at MOH.

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Even among members of the same household, transmission could take days or weeks to occur.

"It is unusual for TB to be spread by casual, brief exposure," Prof Ong told reporters yesterday. The ministry has been unable to find out how the same strain of TB could have spread among the six.

The cluster came to light when a sharp-eyed doctor at Tan Tock Seng Hospital's TB Control Unit noticed that a patient diagnosed with multi-drug resistant TB lived in the same block as earlier cases and alerted the ministry.

The first patient was diagnosed with the disease in February 2012. The last was diagnosed last month.

The group comprises five men and one woman aged between their early 20s and 70. They are no longer infectious and cannot spread the disease, said Dr Jeffery Cutter, director of MOH's communicable diseases division.

Three have completed treatment, and the other three are undergoing treatment in hospital.

Screening of their close contacts, such as family members and colleagues, has also been carried out. MOH said, without specifying the number, that some were found to have the latent form of the disease, which cannot be spread.

TB is caused by bacteria that usually attack the lungs. It is spread via droplets in the air when someone with an active infection coughs.

Of those who are infected, only one in 10 will actually develop an active infection. The vast majority have latent infections without symptoms, and will not spread the disease.

Normal TB infections can be completely cured within six to nine months, but the use of antibiotics has led to the rise of drug-resistant strains that are harder to treat.

MOH is providing voluntary free screening for residents at the void deck of the 11-storey block from today until Sunday as a precautionary measure, to pick up any undiagnosed or latent TB infections.

Told of the situation last night, some residents expressed concern while others said they were not worried as they did not interact much with their neighbours.

When approached by reporters, Dr Koh, who is also Minister of State for Trade and Industry and National Development, said he would comment on the issue later.

MP Koh Poh Koon visits resident at Block 203 Ang Mo Kio Ave 3