NOTE: This is particularly interesting in relation to the work on creating transgenic mosquitoes for fighting malaria and dengue which are starting to be released into the environment.

EXTRACT: "It's a natural agent that exists within insects to block viruses," says Hoffmann. "We're not genetically modifying the mosquito or anything like that."
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Anti-viral bug could stop dengue spread
Anna Salleh
ABC, 25 August 2011 
http://www.abc.net.au/science/articles/2011/08/25/3301273.htm

Dengue transmission could be suppressed by infecting mosquitoes with natural virus-blocking bacteria, say researchers.

Professor Ary Hoffman of the Bio21 Institute at the University of Melbourne and colleagues, report their research on the Wolbachia bacteria in two papers in today's issue of the journal Nature.

"It's a natural agent that exists within insects to block viruses," says Hoffmann. "We're not genetically modifying the mosquito or anything like that."

Dengue is viral disease that affects about 50 million people a year globally, mainly in tropical cities.

The severity of outbreaks of the disease and its geographic range are on the increase and the mosquitoes that carry the disease are becoming resistant to pesticides used to kill them.

While dengue does not usually kill people, it does make them very sick and for people in developing countries this can have major economic consequences.

Hoffmann and colleagues have been exploring the potential to control dengue using a bacteria called Wolbachia, which occurs naturally in about 30 to 70 per cent of insects.

"It's a natural agent for suppressing viruses that insects have," says Hoffmann.
Infection advantage

Wolbachia lives in the cytoplasm of cells and is passed on to future generations by the mother through her egg.

It causes cytoplasmic incompatibility which gives the bacteria a powerful mechanism to invade a population of insects.

An infected mother can mate with either an infected or uninfected male, but an uninfected mother can only mate successfully with an uninfected male. If she mates with an infected male her offspring die.

This means infected females have a selective advantage because they have a greater choice of mates.

"It actually allows the bacterium to spread very quickly in a population," says Hoffmann.

From flies to mosquitoes

Hoffman first discovered cytoplasmic incompatibility in Drosophila while working in California in the 1980s.

Now, he and colleagues are trying to use this knowledge to try and suppress dengue transmission by mosquitoes.

"We've taken Wolbachia from flies and put it in mosquitoes," says Hoffmann.

In the first of the two new papers Hoffman and colleagues have shown that Wolbachia reduces the presence of dengue virus in Aedes aegypti mosquitoes.

They have also shown the bacteria can spread in a caged population of mosquitoes.
Field trial

In the second paper Hoffmann and colleagues tested whether they could spread Wolbachia in Aedes aegypti mosquitoes, which transmit dengue in the field.

The researchers had to account for the slightly negative effect the bacteria has on the mosquitoes' lifespan and egg numbers.

They calculated that for the Wolbachia bacteria to spread, at least 20 per cent of mosquitoes would have to be infected to start with.

Hoffmann and colleagues raised more than 100,000 infected mosquitoes and, after extensive public consultation and a CSIRO risk assessment, released them in two suburban areas not far from the northern Australian city of Cairns.

They then monitored the spread of the bacteria throughout the Aedes aegypti population by regular genetic tests of the mosquitoes.

The researchers managed to infect up to 100 per cent of the population of Aedes aegypti with the bacteria, says Hoffmann.

He says the next step is to test the effectiveness of Wolbachia on the actual spread of dengue.

The researchers plan to do this in countries such as Vietnam and Brazil, which have a major problem with dengue.

If it works, Hoffmann says infected mosquitoes could be collected in one area and used to seed infection in another area.

"The roll-out could be incredibly cheap because it could then become a community project," he says.

"That's why this is a particularly good technology to use in the developing world."

Hoffmann says while dengue could develop resistance to Wolbachia, the bacteria too would "co-evolve" to match this.

"It's in its interest to fight back against any changes in dengue," says Hoffmann.