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Dengue cases, already at record, likely to rise

SINGAPORE — It is barely halfway through January and Singapore is already seeing a record number of dengue cases, an unusual trend for this time of the year. Today (Jan 12), the National Environment Agency (NEA) warned that figures are likely to rise as the weather heats up.

Dengue cases, already at record, likely to rise

A banner alerting residents of the number of dengue cases in the neighbourhood in front of Shunfu Ville, along Shunfu Rd. Photo: Koh Mui Fong

SINGAPORE — It is barely halfway through January and Singapore is already seeing a record number of dengue cases, an unusual trend for this time of the year. Today (Jan 12), the National Environment Agency (NEA) warned that figures are likely to rise as the weather heats up.

The proportion of dengue cases due to the DENV-2 serotype, a common type of dengue virus here, has also risen sharply and now accounts for two-thirds of all dengue cases here, up from about half of all cases just a month ago, the agency said in an advisory. The DENV-1 serotype has accounted for most cases here since March 2013.

The NEA attributed the spike in cases to an increase in the Aedes mosquito population and a “slightly warmer-than-usual year-end weather due to the El Nino phenomenon”, which shortens the dengue virus’ incubation periods as well as the mosquitoes’ breeding and maturation cycles.

“This change in the main circulating dengue virus may be an early indicator of a future dengue outbreak, unless measures are taken to suppress the Aedes mosquito population,” the NEA said.

Between Jan 3 and 9, there were already 554 dengue cases — 96 more than the 458 cases seen the week before that, which was the highest recorded in 2015. Another 121 cases surfaced from Jan 10 to 3.30pm on Jan 11. The figures are higher than in past periods: In the first week of January 2013, there were about 125 cases, and in the same period in 2014, there were about 425.

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Associate Professor Ooi Eng Eong, deputy director of the Duke-NUS Medical School’s Emerging Infectious Disease Programme, explained that to have DENV-2 resurfacing now — after it was the predominant dengue virus from 2007 to 2013 — may not be due to a lack of herd immunity, which is the threshold proportion of a population that has been infected before an epidemic dies out.

Herd immunity may drop with new birth cohorts and influx of people, but Assoc Prof Ooi believes there are other factors at play. “The replication of the dengue virus genome is error-prone, and occasionally gives rise to strains that spread more effectively in populations,” he said.

The good thing is that those who had been infected with the DENV-2 virus would not get it again, he added.

Infectious diseases professor Annelies Wilder-Smith from Nanyang Technological University’s Lee Kong Chian School of Medicine said this switch in serotype from DENV-1 to DENV-2 “is not unusual”, and could be the reason behind the continued dengue outbreak. “For now, we see an outbreak that is unusual for this time of the year, and it could indeed herald a larger outbreak year for 2016,” she added.

Two dengue types most common here:

There are four different dengue serotypes, with DENV-1 and DENV-2 being the two most common ones here.

The DENV-2 virus is more complex than other serotypes, because it changes structure in the human body, preventing previous antibodies from binding to new structures. Various strains can emerge as the virus mutates, with some spreading more easily than others.

In 2013, Singapore faced its worst dengue epidemic: Over 22,000 people were infected and seven died when the dengue serotype being transmitted switched from DENV-2 to DENV-1. Clinical trials are expected to begin at the end of this year after a team of scientists and engineers in Singapore said they have engineered an antibody that can neutralise all four virus serotypes. 

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