Harmless bite (Image: Patrick Lorne/Sunset/Rex Features)

Some 300,000 mosquitoes with the potential to block the spread of dengue fever have been released in Australia, in a large-scale trial of one of the most promising techniques to rid the world of the disease.

Dengue fever infects around 100 million people in the tropics each year, killing 40,000 people annually. Insecticides and nets provide the most effective means to control the disease at present, says Scott O’Neill at Monash University in Melbourne, Australia, but the dengue virus’s range continues to grow. In 2009, for instance, it reached Buenos Aires in Argentina for the first time, while France reported its first locally acquired case of dengue fever in 2010.

Last year, O’Neill and colleagues announced plans to release Aedes aegypti mosquitoes infected with a fruit-fly bacterium called Wolbachia. The bacterium makes the mosquitoes less able to carry the dengue virus, and it could therefore limit dengue transmission if it were to become widespread in the mosquito population.


Fast spreader

In principle Wolbachia can spread quickly as infected male mosquitoes produce viable offspring only if they breed with Wolbachia-infected females. What’s more, all the offspring of infected females will carry Wolbachia, whether the male parent is a carrier or not.

To test this, O’Neill’s team began releasing the mosquitoes in Queensland in northeastern Australia, which has experienced relatively severe dengue fever outbreaks in recent years. First they released around 2500 A. aegypti mosquitoes carrying Wolbachia into two outdoor enclosures which mimicked a typical north Queensland backyard – complete with potted tropical plants and trees. For every infected mosquito, one uninfected mosquito was also released.

After 30 days, the entire population in one cage had become infected with Wolbachia, and after 80 days all the mosquitoes in the second cage were also infected.

On the back of that success, the team released almost 300,000 Wolbachia-infected mosquitoes in two towns in northern Queensland: Yorkeys Knob and Gordonvale.

‘Pleased as punch’

After four months, the team found that all of the mosquitoes they trapped in Yorkeys Knob and 90 per cent of those trapped in Gordonvale were carrying Wolbachia. “We were pleased as punch,” says team member Scott Ritchie at James Cook University in Cairns, Queensland.

However, two weeks later infection rates had fallen to 95 per cent in Yorkeys Knob and 81 per cent in Gordonvale. The researchers speculate that this might reflect the onset of the dry season, which brought uninfected mosquitoes from surrounding areas into the towns.

O’Neill’s team also monitored neighbouring towns to see whether Wolbachia spread beyond the release area. Evidence of spread could prove legally problematic if one country wants to use Wolbachia in an anti-dengue strategy but a neighbouring country does not. However, infected larvae were detected beyond the town limits on just three occasions. “We don’t expect Wolbachia to establish in those mosquito communities,” O’Neill says.

He says the team is now ready for the final test: releasing Wolbachia-infected mosquitoes in areas with a high incidence of dengue. Pending government approval, the team will begin trials in Thailand, Vietnam, Indonesia or Brazil within twelve months.

Journal reference: Nature, DOI: 10.1038/nature10355