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Dengue’s link to El Niño

A mother watches over her son, who was diagnosed with dengue fever, at a hospital in Phnom Penh. A new study has found a link between rising temperatures and dengue outbreaks in the region.
A mother watches over her son, who was diagnosed with dengue fever, at a hospital in Phnom Penh. A new study has found a link between rising temperatures and dengue outbreaks in the region. Hong Menea

Dengue’s link to El Niño

As Cambodia contends with one of the strongest El Niño cycles in recent memory, a new study of millions of dengue cases over the past two-decades-plus has shown that spikes in the deadly disease across Southeast Asia may be linked to the weather phenomenon.

Upswings in dengue cases follow a cyclical pattern, and it had already been predicted that 2015 would see such an upswing, but one health official yesterday said the heightened temperatures brought on by El Niño – along with Cambodia’s unique developmental circumstances – will only serve to intensify the severity of the problem.

Though the exact effects of temperature changes on dengue fever are still under investigation, an international study published in the US-based Proceedings of the National Academy of Sciences this week found that surges in dengue cases occurred “synchronously” across Southeast Asia during El Niño years.

“Our analysis shows that periods of elevated temperatures can drive the occurrence of synchronous dengue epidemics across the region,” wrote researchers, which included American universities and health ministries across ASEAN.

“Multiannual dengue cycles (2-5 years) were highly coherent with the Oceanic Nino index and synchrony of these cycles increased with the temperature.”

El Niño, a weather phenomenon associated with high sea surface temperatures, will likely last through the end of this year, according to the UN Food and Agriculture Organization. However, the higher threat of dengue fever may last through the next two years, said Dr Huy Rekol, director of the National Center for Parasitology, Entomology and Malaria Control.

“I sent information to all provinces to be careful, because the situation this year and the next two years has a higher risk for dengue,” said Rekol, who assisted in compiling the PNAS study.

Steve Iddings, an environmental engineer with the World Health Organization, cautioned against drawing causal links, because the relationship between dengue and climate is “complex”. Nonetheless, he said, dengue, like other water-related diseases, is sensitive to changes in temperature and water cycles.

Dengue – which causes fever, aches, rashes and sometimes deadly hemorrhaging – is spread by mosquitoes. In warmer weather, the dengue virus’s life cycle inside mosquitoes speeds up, increasing the chance of infection, according to Iddings.

“Things happen quicker at warmer temperatures, and there are trends showing increasing temperatures,” he said.

According to the World Health Organization, there were 5,274 cases of dengue – including 16 deaths – reported in Cambodia this year as of August 3. The figure represents a nearly 50 per cent increase over the roughly 3,600 cases – with 21 fatalities – in all of last year, though 2014 had unusually low incidence of dengue.

Rekol cited even higher numbers, saying that Cambodia has seen 9,400 cases and 79 fatalities from dengue in roughly the same time period.

Over the course of a normal year, he said, the Kingdom sees about 10,000 dengue cases.

Iddings said that conditions in Cambodia in general heighten the risk of dengue infections, “because of the way the population is distributed, the risk of floods, low ability to escape heat and dependence on agriculture”.

According to Rekol, another dengue risk factor for Cambodia is increasing migration across Southeast Asia.

Since the majority of dengue cases don’t show any symptoms, people have no way of knowing if they are carrying or spreading the virus to others.

With ASEAN integration to come into force at the end of the year, Rekol said he worried that massive population movements might cause outbreaks.

“We are very concerned about how to manage this with all the people’s movement,” he said. “Maybe we need to have a strong surveillance system. But you need to spend huge money for that.”

Cambodians’ lack of access to effective, centralised water-treatment systems puts the country at an even greater risk, he said.

To reduce Cambodians’ chances of contracting the disease, Rekol said that people should reduce the number of mosquito larvae around their residence.

This means washing out water containers at least once or twice a week. He also suggested sleeping under mosquito nets and wearing clothes that minimise skin exposure.

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