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Bacteria could help control dengue fever

Controlling the spread of mosquitoes using bacteria that halve the insects' lifespan could virtually eliminate the transmission of dengue fever, which kills around 12,500 people a year. Traditional methods for controlling the spread of mosqui-to-borne disease, such as using bed nets and draining wetlands, are ineffective for the Aedes aegytpi mosquitoes that spread dengue fever virus because they bite during the day and thrive in urban areas.

Scott O'Neill, a geneticist at the University of Queensland in Brisbane, Australia, and his team has now developed a way to kill the mosquitoes before the dengue virus is mature enough to infect people if they are bitten. Dengue fever takes approximately eight to 10 days to incubate in mosquitoes, and therefore tends to be spread by older insects.


The team used a strain of the insect-infecting Wolbachia pipientis bacterium, which usually infects fruit flies and causes them to die early. By adapting the bacterium to infect A.aegtypi, the team hoped to cut the mosquito's lifespan.

After unsuccessful attempts to infect the dengue fever mosquitoes with the naturally occurring form of W. pipientis, the team grew the bacteria in a culture with the mosquito cells.

Over a period of three years, some of the bacteria adapted so that they could successfully infect female mosquitoes. The scientists found that the lifespan of the infected mosquitoes was around 30 days, roughly half the expected 60-day survival rate of laboratory-reared mosquitoes.

The team bred the infected females to produce whole populations of infected mosquitoes, which also lived for only 30 days. "We were able to show that when mosquitoes carry these bacteria, their adult lifespan is roughly halved," says O'Neill. The team's findings are published in Science.

Field studies

The researchers have now begun field studies with caged mosquitoes to see if they get similar results outside the lab. Dengue viruses can mature at different rates, making it difficult to predict the absolute age at which the mosquitoes need to be killed.

But O'Neill says that if the adapted Wolbachia also halve the average age of mosquitoes in the wild, transmission of the virus could be reduced to nearly zero. If the results of the cage studies are positive, O'Neill says the next step would be to try the same technique using wild, free-roaming mosquitoes. O'Neill says an advantage of his 'biopesticide' approach is that it is more likely to get regulatory approval than other ongoing attempts to control the spread of dengue fever by genetically modifying the disease-carrying mosquitoes.

That transgenic method is being researched by geneticist Luke Alphey at the University of Oxford, U.K., who founded a company called Oxitec to genetically modify the dengue fever mosquito. They modified mosquito larvae so that they need to be fed the drug tetracycline to survive. When theses lab-reared mosquitoes are released into the wild, they breed as normal but their offspring die because they don't have access to the drug, thereby reducing the total wild population size.

SOURCE: The Hindu Newspaper Dated: Thursday, January 29, 2009.
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