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Last Updated: Tuesday, 25 July 2006, 14:11 GMT 15:11 UK
Study raises malaria block hopes
The malaria parasite is spread by mosquitoes
Scientists have made a key breakthrough in understanding the genetics of a parasite they hope could be used to block the spread of malaria.

The Wolbachia bacterium can manipulate the way insects reproduce so that it is passed down the generations.

Researchers believe a genetically modified version of the parasite could stop insects transmitting the parasite which causes malaria.

The University of Bath study features in the journal Genetics.

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Malaria kills over a million people a year and is second only to tuberculosis in its impact on world health.

Wolbachia bacteria infect as many as 80% of the world's insects, including mosquitoes.

They are able to alter the sperm of infected males to prevent them successfully reproducing with uninfected females.

As a result, infected females tend to produce more offspring, which are also infected with the parasite, ensuring it thrives in future generations.

GM insects

Researchers believe it should be possible to use genetically-modified Wolbachia to spread genes that render mosquitoes unable to transmit the plasmodium malaria parasite throughout the insect population.

The Bath team, working in collaboration with the University of Chicago, have identified two of the genes that Wolbachia manipulates when it infects the fruit fly Drosophila simulans.

Researcher Dr Ben Heath said: "This is a major breakthrough in our understanding of the genetic basis of Wolbachia infection.

"In recent years there has been great interest in using transgenic Wolbachia as a way of modifying natural populations of insects such as mosquitoes which transmit malaria.

"However this would always be difficult to achieve without a full understanding of the genetics of how Wolbachia interacts with its host insect."

Part of the problem in studying Wolbachia is that it lives inside the cells of its host insect, and cannot survive in isolation.

It has also proved difficult to pin down the changes it makes to the development of sperm because they are so subtle they can be difficult to trace.

Sperm changes

The researchers compared the active genes in infected and uninfected male fruit flies to determine which were being switched on as a result of Wolbachia infection.

They identified two genes - zipper and lgl - which they found worked together to change the make up of sperm cells, making it impossible for them to fertilise uninfected eggs.

However, the Wolbachia in infected eggs seems to be able to reverse these changes, rendering the sperm cells fertile again.

The researchers are now looking at the mechanisms present in other insect species.

Dr Pierre Guillet, of the World Health Organization Global Malaria Programme, said the mosquito population had proved itself very adaptive, and responsive to environmental change.

He said there was no guarantee that introducing a specific genetic change would have much impact in a population which was subject to the effects of many other natural selection processes.

"This is very good science, but I must warn against jumping too quickly to optimistic conclusions," he said.

"What works through in a tube or a mosquito cage is much more complicated in the field."

Dr Guillet said much more work was needed into the environmental factors controlling malaria spread.

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