Trojan horse mosquitoes released in Qld

By Katherine Nightingale 18 December 2010
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Scientists aim to combat dengue fever with the spread of ‘vaccine’ bacteria.

SCIENTISTS WILL RELEASE mosquitoes that have been ‘vaccinated’ against the dengue virus into the wild for the first time in northern Queensland today.

The experiment is part of a $16 million program aiming to slash cases of dengue fever by targeting the main carrier of the virus, the Aedes aeygpti mosquito. There are 50-100 million cases of dengue fever worldwide each year and there were around 1000 cases in Far North Queensland last summer.

The vaccinated mosquitoes have been infected by a common bacteria called Wolbachia. No one is exactly sure how it blocks dengue infection in mosquitoes – it could act like a vaccine by priming the mosquito immune system or it might compete for essential nutrients within the mosquito, says Professor Scott Ritchie of James Cook University in Cairns.

Infecting wild mosquitoes

About 6000-7000 mosquitoes – or roughly 10 mosquitoes per house – will be released each week for 12 weeks in the Cairns suburbs of Gordonvale and Yorkeys Knob, says Scott, a collaborating scientist on the project. Researchers will trap mosquitoes periodically to see how well the bacterium has established itself in the existing mosquito population.

Scott says the hope is that by the end of the trial, Wolbachia will have matched its performance in the laboratory and infected 100 per cent of the mosquitoes.

The strategy exploits Wolbachia‘s unusual way of ensuring its survival. The bacteria can only be passed on by the female mosquito through her eggs to her offspring. But if an uninfected female mates with an infected male, her eggs will fail to hatch – giving an extreme competitive advantage to infected eggs.

The trials were cleared in late September after extensive risk analysis by the CSIRO and the Australian Pesticides and Veterinary Medicines Authority.

The researchers, led by Scott O’Neill of the University of Queensland, have spent months talking to the communities and assuring them that the risks to humans and the environment are negligible.

Little cause for concern

There is little cause for concern, says Scott Ritchie, as Wolbachia is already found in 70 per cent of insect species.

The technology would be useful to Australia but the big wins could be made in countries where dengue is present year-round. The team plans to carry out a similar test in Vietnam next year, where it is more likely to show whether the strategy actually reduces dengue cases.

Professor John McBride, a clinical microbiologist also at James Cook University though not involved in the research, says dengue control strategies are desperately needed. “Right now, what people are doing to control dengue hasn’t worked. Dengue is getting worse, not better. It’s a formidable foe.”

He describes the Wolbachia strategy as “elegant”, adding: “If this realises its full potential it’ll be a dramatic intervention, a giant leap forward in the battle against dengue fever.”

John says that, if successful, the strategy would most likely be used alongside others, such as potential vaccines. “Impact on the number of dengue cases will be the proof of the pudding,” he concludes.