An experimental method to control dengue fever has virtually eliminated the tropical infection from many test villages in Vietnam. The researchers hope other dengue-prone regions can adopt the successful procedure, which involves using a shrimp-like predator to control the mosquito that transmits the virus. But, the technique has limits.
Dengue fever, a potentially deadly illness, is the world's most common insect-delivered infection. A mosquito that breeds in water containers and other standing pools, spreads it.
The World Health Organization says dengue fever's prevalence has grown dramatically in recent decades because of increased air travel, high population growth, overcrowding in cities, and the deterioration of public health services, including mosquito control.
"We're talking about a situation that has escalated now to global pandemic terms," said Professor Brian Kay of the Queensland Institute of Medical Research in Brisbane, Australia. He says the dengue virus is now common in more than 100 countries of the tropics and sub-tropics, posing a risk to two-fifths of the world's population.
"50 million cases a year and probably 15,000 to 20,000 people dying of dengue hemorrhagic fever [annually] and the situation getting much worse," he said.
But experiments by Mr. Kay and Vu Sinh Nam of Vietnam's ministry of health show it is possible to block dengue transmission by a novel control method.
As outlined in the journal Lancet, they populated large water cisterns inhabited by mosquito larvae with one-millimeter-long shellfish that eat the larvae, before they hatch. Mr. Kay says the mosquito has been eradicated in most Vietnamese villages where the technique was tested, with no cases of dengue fever reported since 2002.
"We never expected we'd get such good results," Mr. Kay said. "Forty two out of 46 communes are now totally free of the dengue mosquito and, of course, totally free of dengue. This is just unheard of in a place like Vietnam."
In Thailand, researchers reported in December that the tiny shelled predator, a marine creature with the monstrous name Mesocyclops, killed between 98 and 100 percent of the juvenile dengue mosquitoes in laboratory water containers. When combined with a pesticide, Mesocyclops kept the numbers of larvae lower for more weeks than when used alone.
But there are limits to this predator technique. Simon Hales of the Wellington School of Medicine in New Zealand says, to be efficient, it relies on treating only big water containers inhabited by a large number of mosquito larvae. He notes that it also depends on a public health effort, backed by active participation of the people, to monitor the containers, which might be easier in the countryside than in cities.
"It's not clear that the strategy will actually be effective in some of the places where dengue is the most serious problem," he said. "We're talking about very large cities with very poor services, probably not a very coherent community structure, and where a lot of the mosquito breeding is occurring in very small, discarded containers. In the Vietnam study, they got around this problem by simply removing all of the small containers. Now that obviously requires a very coordinated and dedicated community response."
Brian Kay agrees that the dengue control method he studied in Vietnam works best in areas where piped water is unavailable and where the public is mobilized to fight the problem.
"But we definitely believe that other strategies are needed," he said. "I'm part of a large team at the moment that is investigating the possibility of using bacteria capable of shortening the lifespan of mosquitos and we hope to superimpose this second innovative method on top of the existing one."
Simon Hales in Wellington, New Zealand says that with a dengue fever vaccine still at least a decade away, such low technology approaches are necessary for developing countries. But in the long run, he says, the best solution to such infections is alleviating poverty.
"Dengue is very much linked to poverty, poorly planned urbanization in which communities don't have running water and reliable sanitation systems," he said. "Those issues need to be solved to manage dengue on a global scale."