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How Zika got here could remain a mystery, say experts

SINGAPORE — Exactly how the Zika virus found its way to Singapore and triggered a local outbreak could remain a mystery, experts say, given that existing tests are unreliable for infections that are more than two weeks old.

Thermal fogging being carried out in the vicinity of Block 102, Aljunied Crescent on Aug 28. Photo: Ooi Boon Keong

Thermal fogging being carried out in the vicinity of Block 102, Aljunied Crescent on Aug 28. Photo: Ooi Boon Keong

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SINGAPORE — Exactly how the Zika virus found its way to Singapore and triggered a local outbreak could remain a mystery, experts say, given that existing tests are unreliable for infections that are more than two weeks old.  

Nonetheless, the infectious diseases experts whom TODAY spoke to believe that Patient Zero — or the source of the outbreak — was likely to have been infected with the virus overseas and brought it back to the country — specifically to the Aljunied Crescent and Sims Drive area where a cluster has emerged.

A mosquito could have bitten the person, got infected, and then transmitted the virus to other people. While transmission is possible through sexual intercourse, it is a less common means of infection.

Associate Professor Hsu Li Yang, programme leader of the antimicrobial resistance programme at the Saw Swee Hock School of Public Health, said: “If a compatible mosquito bites someone who has the virus in his or her blood, the mosquito becomes infected. After about a week, the virus can be detected in the mosquito’s saliva, and the mosquito can then infect all the other people that it bites.”

“If these other infected people are bitten by other mosquitoes during the days when the virus is circulating in their blood (which is usually three to 12 days after being bitten by an infected mosquito), the virus will be passed to these mosquitoes, and the cycle of infection will continue,” he said.

On the sexual transmission of Zika, Prof Hsu noted that based on reported cases, the virus can remain in semen for more than six weeks. “Almost all cases of Zika sexual transmission — with one exception — have been from men to women,” he said.

Prior to the Ministry of Health’s (MOH) announcement over the weekend that Singapore has discovered dozens of cases of locally transmitted Zika infection, a 48-year-old man who travelled to Brazil was diagnosed in May as the Republic’s first Zika patient. The Singapore permanent resident made a full recovery and tested negative for Zika before he was discharged. His family members also had not reported any symptoms of the mosquito-borne disease.

Infectious diseases expert Leong Hoe Nam, who diagnosed the first Zika case at Mount Elizabeth Novena Hospital, said: “This is a very mild disease. The infected person might not even see a doctor, and if he didn’t raise the alarm, he could become a source of infection. He might have thought that the travel made him unwell, had some rest and probably recovered after a couple of days. Meanwhile, he got bitten by a mosquito while he was ill, and the whole cycle started.”

By now, Patient Zero would be very hard to track down, said Dr Wong Sin Yew, an infectious diseases physician at Gleneagles Medical Centre. Tracing of cases would need to be done and a transmission link has to be established. “At present, we don’t have a good, reliable test for infections that are beyond two weeks,” he said.

Dr Wong said that based on MOH’s recommendation, blood and urine tests are carried out within the first week of infection. If it is beyond the first week, a urine test is used, because the virus tends to stay in the urine for a longer period of time.

Results of the urine and blood tests can be obtained at a laboratory within three or four hours, said the experts. But Dr Wong noted that test results could take days, if there is a huge number of cases.

While some have questioned the delay in the discovery and announcement of the Zika cases, Dr Leong stressed that the focus is on protecting Singaporeans and “not (playing) the blame game”. “The gatekeepers, who are now the family physicians, must be aware that Zika is mild and we must investigate (the suspected cases), especially (those) in a cluster,” he said.

The experts urged people to protect themselves from mosquito bites by using repellent or wearing more clothing, and not travel to Zika-infected countries. Pregnant women in their first trimester should take extra precautions as the risk to the foetus is highest, Prof Hsu said.

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