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SCIENCE & ANIMALS: Bacteria-infected mosquitoes may fight dengue



                                  PART 1


------------------------------- GENET-news -------------------------------

TITLE:   SPECIALIZED MOSQUITOES MAY FIGHT TROPICAL DISEASE

SOURCE:  The Associated Press, USA

AUTHOR:  Malcolm Ritter

URL:     http://hosted2.ap.org/APDEFAULT/bbd825583c8542898e6fa7d440b9febc/Article_2011-08-24-Dengue%20Fever/id-d5e891f204da448dbe4948c96c6602e2

DATE:    24.08.2011

SUMMARY: "[Researchers] showed that Aedes aegypti mosquitoes, the chief carriers of the dengue virus, resist spreading that virus if they are infected with a particular kind of bacteria. Then they tested whether these resistant mosquitoes could displace their ordinary cousins in the wild, thus reducing the number of dengue-spreading mosquitoes. [...] For the experiment, scientists released more than 140,000 resistant mosquitoes over 10 weeks in each of two isolated communities near Cairns in northeastern Australia, starting last January. By mid-April, monitoring found that resistant mosquitoes made up 90 percent to 100 percent of the wild population."

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SPECIALIZED MOSQUITOES MAY FIGHT TROPICAL DISEASE

Scientists have made a promising advance for controlling dengue fever, a tropical disease spread by mosquito bites. They?ve rapidly replaced mosquitoes in the wild with skeeters that don?t spread the dengue virus.

More than 50 million people a year get the dengue virus from being bitten by infected mosquitoes in tropical and subtropical areas, including Southeast Asia. It can cause debilitating high fever, severe headaches, and pain in the muscles and joints, and lead to a potentially fatal complication. There?s no vaccine or specific treatment.

Some scientists have been trying to fight dengue by limiting mosquito populations. That was the goal in releasing genetically modified mosquitoes last year at sites in Malaysia and the Cayman Islands.

Australian scientists took a different tack, they report in Thursday?s issue of the journal Nature.

First, they showed that Aedes aegypti mosquitoes, the chief carriers of the dengue virus, resist spreading that virus if they are infected with a particular kind of bacteria. Then they tested whether these resistant mosquitoes could displace their ordinary cousins in the wild, thus reducing the number of dengue-spreading mosquitoes.

The resistant mosquitoes have an advantage in reproduction. Resistant females can mate with either resistant or ordinary mosquitoes, and all their offspring will be resistant. But when ordinary females mate with a resistant male, none of the offspring survive.

For the experiment, scientists released more than 140,000 resistant mosquitoes over 10 weeks in each of two isolated communities near Cairns in northeastern Australia, starting last January. By mid-April, monitoring found that resistant mosquitoes made up 90 percent to 100 percent of the wild population.

The result is a ?groundbreaking first step,? Jason Rasgon of the Johns Hopkins Bloomberg School of Public Health in Baltimore wrote in a commentary accompanying the paper. Rasgon, who did not participate in the study, said the next hurdle is to test the idea in areas where dengue is spread constantly, rather than sporadically as in Australia. Researchers will also have to show it works against varied strains of the dengue virus, he said.



                                  PART 2

------------------------------- GENET-news -------------------------------

TITLE:   BACTERIUM OFFERS WAY TO CONTROL DENGUE FEVER

SOURCE:  Nature News, UK

AUTHOR:  Natasha Gilbert

URL:     http://www.nature.com/news/2011/110824/full/news.2011.503.html

DATE:    24.08.2011

SUMMARY: "A team led by Scott O?Neill, Dean of Science at Monash University in Melbourne, Australia, believe they may have the answer. [...] they reveal a strain of the bacterium Wolbachia pipientis that can stop the dengue virus from replicating in its mosquito host. They go on to show that this bacterium can rapidly spread through wild A. aegypti populations, suggesting it could be a viable control mechanism for dengue fever. ?The presence of Wolbachia in mosquitoes completely blocks the ability of the dengue virus to grow in mosquitoes,? O?Neill says."

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BACTERIUM OFFERS WAY TO CONTROL DENGUE FEVER

Transmitted by the mosquito Aedes aegypti, dengue fever kills around 12,500 people a year. Unlike for malaria, bed nets are not effective in combating dengue as A. aegypti is active during the day. And concerns about rising resistance to insecticides has spurred the search for alternatives.

A team led by Scott O?Neill, Dean of Science at Monash University in Melbourne, Australia, believe they may have the answer. In work published in Nature1,2 today, they reveal a strain of the bacterium Wolbachia pipientis that can stop the dengue virus from replicating in its mosquito host. They go on to show that this bacterium can rapidly spread through wild A. aegypti populations, suggesting it could be a viable control mechanism for dengue fever.

?The presence of Wolbachia in mosquitoes completely blocks the ability of the dengue virus to grow in mosquitoes,? O?Neill says.

First principles

In previous work, O?Neill and his colleagues showed that infecting female mosquitoes with the Wolbachia strain wMelPop-CLA could cut their lifespan by half. The aim was to fight dengue fever by killing infected mosquitoes early, before the virus could mature enough to be passed on to people. But this approach, which also reduces infected mosquitoes? rate of reproduction by 56%, is problematic, as it also limits infected mosquitoes? ability to pass the bacterium on.

The solution, the group reports today, could lie in wMel. In caged field experiments, mosquitoes infected with this strain had lifespans, reproductive rates and offspring viability similar to that of uninfected controls.

Further, the team did not find virus in the saliva of wMel-infected females, suggesting that the bacterium could block transmission of the disease. And, because it does not harm mosquitoes, the bacterium is more likely than its predecessor to spread successfully through wild populations.

?It?s an environmentally friendly approach that does not affect the mosquitoes, just the virus,? says Flaminia Catteruccia, a molecular entomologist at Imperial College London, whose work has focused on controlling the malaria-carrying mosquitoes Anopheles gambiae.

How wMel stops the virus from replicating is ?not fully understood?, says O?Neill. But he says mounting evidence suggests that the bacteria ?compete for limited sub-cellular resources required by the virus for replication?.

Into the wild

In an open field trial in two relatively remote areas in Australia, the team released more than 300,000 adult mosquitoes infected with the wMel Wolbachia strain into wild A. aegypti populations over a period of 9?10 weeks. Five weeks later, nearly all the wild mosquitoes tested were infected.

?This is the first case where wild insect populations have been transformed to reduce their ability to act as vectors of human disease agents,? the authors write. They now plan to run large trials over the next 2?3 years to test the approach in countries where dengue fever is endemic. If the trials go well, their method could be implemented as a control mechanism ?immediately afterwards?, says O?Neill.

References

Walker, T. et al. Nature http://dx.doi.org/10.1038/nature10355 (2011).

Hoffmann, A. A. et al. Nature. http://dx.doi.org/10.1038/nature10356 (2011).