"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
WHO.
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.
