The National University Hospital (NUH) is recalling 178 paediatric patients - including 131 under the age of two years - who had been cared for by a nurse now confirmed to have tuberculosis (TB).
They include 34 children who have received a transplanted organ, so are on immunosuppression drugs and therefore at higher risk than normal children.
Associate Professor Daniel Goh, Head of Paediatrics at NUH, said the risk of any of the children contracting TB from the nurse is "very low", especially as she had donned a mask while working. But the hospital is not taking any chances and is recalling patients who had been in ward 47 since July.
The nurse was treated at a GP clinic for her cough in July, but despite having a chest X-ray, was not diagnosed with TB. She was given antibiotics and seemed to get better.
But the cough persisted. Last month, she sought treatment again and was given a CT scan which showed a possible TB patch in her lung the size of a 50-cent coin.
She told the hospital on Wednesday last week and was tested for TB. On Friday, the results confirmed that she had the disease. Fortunately, it is the normal and not the multidrug-resistant variety.
Dr Goh said his team spent the weekend trawling through the patient database to identify patients who might have had long exposure to her while warded, as well as those who are deemed at higher risk because of their age or disease.
NUH chief executive Joe Sim said: "We fully understand the anxiety of the parents and are taking this matter seriously."
Paul Tambyah, a senior infectious diseases expert at NUH, said TB transmission depends on the amount of exposure, the bacterial load of the carrier and the individual's immune system.
He said there is a one in 10 chance of getting the bug if a person with TB coughs at them for two hours. Of those who get the bacteria, one in 10 would get the disease in their lifetime. This doubles for those with low immunity.
The first patients were at the hospital for screening on Tuesday and more will be screened over the coming weeks.
They will have a chest X-ray to check if they have TB, and blood tests if they are five years and older, and/or skin tests to see if they have the bug latent in them.
Any child diagnosed to have caught the bug will be given treatment, which has a greater than 90 per cent chance of suppressing TB.
Crystal Lim, 28, whose son was in ward 47 for three weeks in August, was shocked when the doctor called to ask her to take her son back for tests.
Her son had a liver transplant in October last year but was back in hospital for treatment to a 3cm abscess on his buttocks. The doctor had explained that because of his transplant, he was at a higher risk.
She is worried but not angry with the hospital or the nurse.
"I would be angry if the hospital did not do anything and the patients started getting TB. But they are doing something," she said.
As for the nurse, Madam Lim said: "She did not do it purposely. All the nurses look after the children very well. I hope the doctors can help her get well."
The nurse, who is on medical leave for two weeks, will be able to resume work as being on treatment means she is not contagious.
Colleagues in the ward have all been tested and found negative.
So far this year, 1,252 new TB cases have been diagnosed in Singapore.
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