Aug. 24, 2011 -- By releasing 300,000 live mosquitoes, scientists say they've freed a remote region of Australia from the threat of dengue fever.
Before releasing them, the researchers deliberately infected the mosquitoes with a strain of a common insect bacterium, Wolbachia pipientis, that makes them incapable of spreading the dengue virus.
The four types of dengue virus sicken 50 million people each year. Those who get a second infection with a different strain are at risk of life-threatening dengue hemorrhagic fever.
The only way a person can get dengue is via the bite of a dengue-infected mosquito. A. aegypti is the mosquito that spreads the dengue virus. The hope is that this relatively inexpensive strategy can end the pandemic of dengue fever sweeping tropical regions of the world, says study leader Scott O'Neill, PhD, of Monash University in Victoria, Australia.
"It is a very exciting result that this strain of wolbachia really reduces the ability of dengue virus to grow in mosquitoes," O'Neill said at a news conference held to announce the findings. "And if dengue cannot grow in mosquitoes, it cannot infect people."
Wolbachia bacteria are very common. Some 70% of all insect species in the world are naturally infected, says study researcher Scott Ritchie, PhD, of James Cook University in Cairnes, Australia.
"Wolbachia already infects many mosquito species that bite people but don't spread dengue," Ritchie said at the news conference.
There are two reasons why wolbachia infects so many insects: how it spreads and how it affects the insect. The bacterium is spread by an infected female to all of her offspring. And the bacterium kills all the offspring of an uninfected female that mates with an infected male. So as the bacterium spreads in an insect population, infected females have a tremendous evolutionary advantage.
Once a mosquito population has a certain level of wolbachia infection, nearly 100% of that population soon carries the bacteria. In an indoor lab, O'Neill's team showed that this is exactly what happens when mosquitoes carrying the wMel strain of wolbachia are released in a population of uninfected mosquitoes.
Residents' Decision: Release Mosquitoes or Risk Dengue
It seems odd that anyone would welcome the release of hundreds of thousands of live, blood-sucking mosquitoes in their neighborhoods. But that's exactly what happened in the Queensland towns of Yorkey's Knob and Gordonvale.
O'Neill's team held numerous town meetings and distributed literature describing the experiment. Residents were enthusiastic, according to study researcher Ary Hoffman, PhD, of the University of Melbourne.
"People are tired of dengue and fearful of it. They are quite eager to see new control measures," Hoffman said at the news conference. "We were overwhelmed by the support of the community."
Beginning in January, at the start of Australia's wet season, the researchers began weekly releases of 10,000 to 22,000 bacteria-infected mosquitoes at nearly 200 sites in each town. They eventually released 303,900 mosquitoes.
The plan worked. Nearly 100% of mosquitoes in Yorkey's Knob and over 80% of mosquitoes in Gordonvale became infected with the dengue-inhibiting bacteria.
"Once releases ended, wolbachia continued to increase, until by the end of the wet season we had almost complete penetration," O'Neill said. "We expect these regions should have a much reduced risk of dengue transmission within them."
That remains to be seen. But if it holds true, the researchers already are making plans for regions with year-round dengue transmission.
"We want to conduct large field trials and are looking for approval in Thailand, Vietnam, Indonesia, and Brazil," O'Neill said.
If it works, mosquito releases won't have to continue indefinitely.
"The really interesting thing about wolbachia is it is potentially a one-off intervention and should be able to maintain itself in the mosquito population," O'Neill said. "We expect it to persist and offer control of dengue after a single intervention."
O'Neill and colleagues report their findings in two papers published in the Aug. 25 issue of the journal Nature.