S’pore headed for worst dengue year, with 30,000 cases projected

SINGAPORE — The ongoing El Nino phenomenon, along with a change in the type of dengue virus circulating among Singapore’s population, have led the authorities to forecast a historic high of 30,000 dengue cases this year, nearly a third higher than the previous record of 22,170 cases in 2013. 

And with the looming threat of the Zika virus — also spread by the Aedes aegypti mosquito — adding greater urgency to the need to control the mosquito population, the authorities will be taking early action. 

In a media briefing by the National Environment Agency (NEA), the Ministry of Health (MOH) and People’s Association (PA) on Thursday (Feb 18), the authorities announced that the annual Mozzie Wipeout Campaign, held usually around April, will be launched on Feb 28.

The PA has roped in more than 5,000 grassroots leaders and volunteers for house visits targeting areas with a high number of dengue cases, adding to the more than 5,800 NEA-trained volunteers who will advise residents on anti-mosquito breeding and dengue prevention tips.

An NEA spokesperson also said the agency would be implementing more stringent enforcement measures, such as increasing the frequency of inspections at major construction sites from quarterly to monthly. In preparation for the surge in cases, the NEA, which provides dengue diagnostic services to primary healthcare clinics and hospitals, has increased the number of laboratory staff and extended operating hours, by mobilising staff from other research groups.

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In an article on the dengue outlook for the year in MOH’s epidemiological news bulletin published last month, researchers noted that the effects of El Nino will contribute to a rise in cases in the coming months.

After El Nino peaked in December 1997, there was a spike in dengue cases in January 1998, followed by another in August.

The latest episode of El Nino, which quickens the breeding and maturation cycles of the Aedes mosquito, began last year, which saw over 11,200 reported dengue cases. 

There has been a 50 per cent increase in Aedes mosquitoes caught in the NEA’s Gravitraps in January compared to the same period last year. Household inspections also turned up 50 per cent more breeding sites as compared to the same period last January, with top breeding spots being domestic containers and flower pot plates or trays.

In addition, the DENV-2 virus serotype now accounts for two-thirds of all dengue cases, replacing DENV-1 as the predominant strain. Historically, a change in the predominant virus serotype is usually followed by a spike in dengue cases. The last serotype switch — from DENV-2 to DENV-1 — in 2013 was followed by a sharp dengue outbreak that year. 

Associate Professor Ooi Eng Eong, deputy director of the Duke-NUS Medical School’s Emerging Infectious Disease Programme, said that while dengue forecasting “is an imperfect science as we don’t fully understand all the determinants”, a high number of cases is to be expected this year based on dengue trends so far.

Infectious diseases professor Annelies Wilder-Smith from Nanyang Technological University’s Lee Kong Chian School of Medicine, concurred that the unusual climate conditions, combined with the virus serotype switch, have set the stage for a high epidemic year.

“Furthermore, Singapore has had further population growth. Population density enhances the biting rates and facilitates transmission of dengue virus. With an increasing population size, the absolute numbers of dengue also increase, even if the incidence remains the same,” she added.

More than 3,600 dengue cases have been reported in Singapore since January, with two deaths so far. 

In previous years, the NEA has stepped up inspections when the number of dengue cases spiked. In 2014 — when there were 18,335 reported cases — the NEA deployed 850 dengue inspectors who conducted more than 3.5 million inspections and destroyed over 18,400 mosquito breeding habitats. Last year, In 2015, the NEA conducted more than 1.4 million inspections and uncovered more than 19,000 instances of mosquito breeding.