A bacterial method for the biocontrol of dengue fever.
Diseases transmitted by mosquitoes are responsible for the deaths of untold numbers of humans. Years of research, and millions of pounds, have been spent trying to control both the pathogens and the mosquitoes – yet many diseases are increasing in both prevalence and distribution.
A recent review from scientists at the University of Queensland and Monash University, Australia, describes a new method to potentially control the female Aedes aegypti mosquito, the vector of dengue fever – a severe and potentially life-threatening flu-like virus. No effective treatmens for dengue fever exists and an estimated 40% of the world’s population are at risk from the disease.
The paper describes a biological method for mosquito control, using the endosymbiotic bacteria Wolbachia. These microbes live within the cells of insects and are thought to be the world’s most common parasitic bacteria. Wolbachia do some very odd things, turning male insects into females, or enabling females to reproduce without males via parthenogenesis. They can also provide some advantages, aiding in insect nutrition or pathogen resistance.
Of note is the ability of some strains to drastically reduce the lifespan of the insect they infect. The Wolbachia strain wMelPop can shorten the lifespan of Drosophila by up to 50%. Charmingly known as ‘popcorn’, this strain replicates faster than the insect cells it lives in. This leads to a disproportionately high level of bacteria within the host cells, ultimately killing the insect.
It is this life-shortening ability of wMelPop that makes it an attractive tool for mosquito control. It takes about two weeks for the dengue virus to reach high enough levels within a female mosquito to be successfully transmitted to a human. If the mosquito can be killed before this then it may be possible to reduce disease transmission.
One challenge of this work has been to establish and maintain a stable wMelPop infection within an A. aegypti population, as this species of mosquito are not naturally infected by Wolbachia. To achieve this, researchers continually passaged the bacteria through an in vitro cell culture of a related mosquito species for almost four years. During this time the bacteria adapted to the intracellular environment of a mosquito cell and were able to survive within A. aegypti embryos.
Mosquitoes infected with this strain show around a 50% decrease in lifespan. Maternal transmission rates of almost 100% were achieved. Interestingly, the infection also induced the insect’s immune system, potentially increasing resistance to the dengue virus.
Currently the use of these infected mosquitoes to combat dengue fever is being tested in Australia. Semi-natural field studies, in a controlled environment, showed that the Wolbachia parasite can be transferred to wild mosquitoes, with the life-shortening and disease interference characteristics maintained.
A trial release of Wolbachia-infected mosquitoes will be undertaken in Queensland, Australia, during the 2011 wet season, with regulatory approval from the government and support from local communities. There is no evidence that Wolbachia can be transferred to humans via insect bites.
There is real hope that this new approach to mosquito control – which requires no pesticides – will provide the basis for a new bacterial approach to reduce the burden of dengue fever. A long-term goal of the work is to attempt to use the technique to control the Anopheles mosquito – the main vector of the malaria parasite.
Posted by Benjamin Thompson
Image Credit: Sanofi Pasteur on Flickr