Researchers released around 300,000 mosquitoes in the Cairns suburbs of Yorkeys Knob and Gordonvale in far north Queensland, Australia. The mosquitoes have been specially treated with the potential to block the spread of dengue fever in a large-scale trial conducted by Scott O'Neill and group from the Monash University in Melbourne.

O'Neill and colleagues announced last year their plan to release Aedes aegypti mosquitoes infected with a fruit-fly intracellular bacterium called Wolbachia. This mild strain of Wolbachia lessens the capability of mosquitoes to carry the dengue virus, without shortening the life span of the mosquitoes. Theoretically, widespread infiltration of these treated mosquitoes into the general mosquito population would limit the transmission of dengue.

To test the principle, O'Neill chose to release mosquitoes in Queensland, in northeastern Australia, which had severe dengue fever outbreaks in recent years. They first released 2,500 Wolbachia-carrying A. aegypti into two outdoor enclosures mimicking Queensland backyards. They also released one uninfected mosquito for every Wolbachia infected mosquito. After 30 days, all mosquitoes in the first cage had Wolbachia, and after 80 days all the mosquitoes in the second cage were also infected.

The promising test led to the large-scale trial aimed at stopping dengue. Dengue is said to infect approximately 100 million people in the tropical regions, causing fever, rashes, muscle and joint pains, and headaches. Death rates are known to reach 40,000 people every year. Despite efforts to curb of dengue-carrying mosquitoes through insecticides and traps, the virus continues to spread around the world. And there are still no drugs to combat this dangerous disease.

Results of the Yorkeys Knob and GordonvaleTrial

Four months after the release of the mosquitoes, the researchers found that all of the mosquitoes in Yorkeys Knob and 90 per cent of those in Gordonvale were carrying Wolbachia.

But, two weeks later the percentage of infected mosquitoes had fallen to 95 per cent in Yorkeys Knob and 81 per cent in Gordonvale. According to the researchers, this turnout might be due to the onset of the dry season, which brought uninfected mosquitoes from surrounding areas into the towns.

Another point of concern was the infection of Wolbachia mosquitoes beyond the release area. Infected larvae were found outside the town limits on three occasions. Unintended spread of the Wolbachia mosquitoes is deemed legally problematic if one county uses Wolbachia as against dengue, while a nearby country does not."We don't expect Wolbachia to establish in those mosquito communities," O'Neill says.

"The next step for us in Australia will be continuing to monitor the field experiments that we have and also some expansion of those in the next wet season," said O'Neill. This final trial is likely to occur within 12 months, pending government approval. It would involve the release of Wolbachia-infected mosquitoes in other countries, including Thailand, Vietnam, Brazil, and Indonesia as target areas. These countries are known to have high levels of dengue transmission and outbreaks.