Asia shows way to fight dengue as global spread looms

Mar 12, 2008 11:48 AM EDT

Clarissa Poon was one of an estimated 50 million people who contracted mosquito-borne dengue fever last year. She spent an agonizing week on a drip in a Bangkok hospital as she battled the potentially deadly disease.

"There was not a single moment when I wasn't aching everywhere, dizzy and nauseous. I was so weak I couldn't even stand," said Poon, who caught the illness during a family holiday at a beach resort in Thailand.

"My kids were very worried because the mother of one of their friends died," she added.

From Africa to Asia to Latin America, around 2.5 billion people live in areas that are at risk of dengue fever, a viral disease spread by the Aedes aegypti mosquito. There is no vaccine or drugs to treat the illness which killed an estimated 22,000 people last year, most of them children.

Due to international travel and climate change, the Aedes aegypti mosquito's habitat is spreading.

In January, health officials warned that the disease was poised to move across the United States. It has been spreading aggressively in Latin America and the Caribbean, reaching epidemic levels last year.

Dengue is endemic in Southeast Asia where a tropical climate and monsoon rains provide ideal conditions.

Strategies developed in places such as Singapore might provide vital information for other countries seeking to combat the virus and the mosquitoes that spread it. Family doctors in Singapore look out for patients with suspicious symptoms. When cases are confirmed, researchers try to nail down the specific dengue virus subtype, of which there are four, and the location of the outbreak.

"You need to monitor what (subtype) is going around ... You want to limit the damage, the fatalities," the World Health Organisation's advisor in Asia, John Ehrenberg, told Reuters.

While dengue and malaria share geographical patterns, dengue is more dangerous because its mosquito carriers thrive indoors. Mosquitoes that carry malaria are rarely found in urban areas.

Dengue fever is endemic in more than 100 countries in Africa, the Americas, eastern Mediterranean, Southeast Asia and western Pacific. Of the 50 million people who contract the disease every year, about one percent get potentially deadly severe dengue hemorrhagic fever (DHF), which requires hospitalization.

There is no cure or vaccination for dengue fever. Sufferers such as Poon, face an increased likelihood of developing DHF if they contract the disease again, which is not uncommon for those living in the tropics where the mosquito carriers flourish.


International travel has made the spread of dengue inevitable, experts say.

"There is always a risk for the borders ... In central America, you have a lot of people moving up north," Ehrenberg said. "There is a risk of people moving in with dengue."

Ehrenberg says there is little to stop dengue from spreading. He compares it to West Nile virus which appeared in New York in 1999 and then spread across the United States, Canada and Mexico. West Nile killed 98 people in the United States last year.

"As you can see with West Nile virus, there is hardly anything you can do to control its spread in the U.S. It's all over the place now. There's always the risk of introducing, when the climatic conditions are right," Ehrenberg said.

Both dengue and West Nile are spread by mosquitoes.

"It's a neglected disease because no one pays attention in between outbreaks, except in places like Singapore, where there is very good surveillance," Ehrenberg said.

In Singapore, health workers aggressively control breeding sites by regularly spraying pesticides in parks and gardens. Government inspectors fine people for allowing water to build up in flower pots which is a favourite breeding site.

Singapore reported 14,000 dengue cases in 2005, but that fell to 3,597 cases in the first half of 2007, according to the


With 42,456 cases in 2006 and 45,893 in 2005, Thailand figures near the top of the dengue list. Fanned out across the country are 500,000 volunteers who educate villagers on mosquito control, chiefly by removing stagnant pools of water.

Kitti Pramathphol, head of Thailand's dengue control, said more inspections would be made to remove potential breeding sites before the rainy season in June and July, when the disease peaks.

"Its eggs can hide in crevices and survive for a year without water in tropical climates and in normal temperatures. Once there is rain or water, they will hatch into larvae," he said.

Compared to its cousin, the Culex mosquito, the Aedes aegypti is considered a weaker species.

"It is slender and has thin wings. Culex likes to breed in drain water, but Aedes will die in such dirty water. It likes rain water, relatively clean water," Pramathpol said.

"It is usually indoors and has problems surviving outdoors," Pramathphol said, adding that another strategy was to trap it indoors with insecticide-laced curtains.

Drugmaker Novartis AG has designed a drug which it hopes can combat all four dengue viruses.

"If the safety is acceptable, we hope to go into human testing, hopefully next year," Paul Herrling, head of corporate research, said in a telephone interview.