Our research program is developing an innovative biological approach to manipulate mosquito populations to make them incapable of transmitting dengue viruses between people. If successful it will greatly reduce our reliance on conventional insecticide-based mosquito control. The approach is centered on a naturally occurring bacterial agent known as Wolbachia pipientis. This bacterium is estimated to occur naturally in 20-70% of all insect species. Considering that scientists estimate that there might around 5 million different insect species on the planet, Wolbachia is extremely common in the environment. We know that in addition to well-known insects like Birdwing butterflies, Wolbachia also occurs naturally in many mosquito species that bite humans. Interestingly it does not occur naturally in the mosquito species that are known to be of major importance in transmitting pathogens like malaria and dengue between people.
Our research program is investigating whether we can use Wolbachia strains that occur naturally in fruit flies to influence the ability of the mosquito Aedes aegypti to transmit dengue viruses between people. We have shown that the fruitfly strains of Wolbachia we are working with have a number of properties in the mosquito that will interfere with its ability to transmit dengue. The first is that one of these Wolbachia strains causes a reduction in adult mosquito lifespan. Since only old mosquitoes are known to be able to transmit dengue viruses, reducing mosquito lifespan will greatly reduce dengue transmission. Another effect that has recently been discovered is that a number of different Wolbachia strains reduce the ability of insects to support virus infection. We have shown that fruitfly Wolbachia can prevent the mosquito from becoming infected with dengue viruses and as such cannot transmit the virus between people.
An interesting feature of Wolbachia strains is that they have an inbuilt mechanism to invade insect populations. The bacterium is not infectious and does not pass from insect to insect like a pathogen. Instead it lives inside insect cells and is passed from mother to offspring through the insect egg. It is able to spread into insect populations by influencing the reproduction of the insect. Male insects when infected with Wolbachia are unable to successfully fertilize the eggs of female insects that do not contain Wolbachia, reducing the reproductive output of these females and indirectly benefiting female insects that contain the Wolbachia infection. As a result Wolbachia can spread quite quickly into an insect population. Using this mechanism we should be able to introduce Wolbachia into Aedes aegypti populations by seeding small numbers of mosquitoes that contain the Wolbachia infection into the environment and then it should spread into the rest of the mosquito population and render the mosquitoes unable to transmit dengue to humans.
At the present time we have demonstrated that we can transfer Wolbachia from the fruitfly into the Aedes aegypti mosquito. In the mosquito different introduced Wolbachia strains confer different properties including influencing the lifespan of the mosquito and blocking the ability of dengue viruses to grow in the mosquito. We are now in the process of determining how well these Wolbachia strains can invade mosquito populations in contained semi-field trials. If these experiments show promising results we plan to move to open field-testing in Nth Queensland, Australia, where Aedes aegypti occurs naturally and spreads dengue between people. Before these open field-testing experiments proceed the project is undergoing a comprehensive risk assessment and seeking formal government regulatory approval to ensure it has been evaluated as being safe to people and the environment. We are also engaging with communities to explain the project and incorporate concerns into our research program. Community support will be a critical determinant to whether the approach moves to open field-testing. In addition to developing the Wolbachia method in Australia we are also working with local scientists and field sites in Vietnam and Thailand where we hope to also test the approach. If field-testing proves positive we hope to be able to roll out a new approach to dengue control that is effective and better for our environment by reducing our dependence on current insecticide-based approaches. The approach should be fully compatible with other methods of control such as future dengue vaccines that have yet to be developed and we hope will make a contribution to reducing the risk of acquiring dengue for the estimated 2.5 billion people that currently live in dengue transmission areas.