Monash UniversityFighting dengue

Fighting dengue

Bacterium putting the bite on mozzies

Mosquitoes that transmit the dengue virus could be rendered harmless under a biological control program that infects them with an insect-specific bacterium.

In the tropics and subtropics, health authorities brace for the yearly impact of dengue fever, one of the world’s most debilitating, often fatal, diseases – and one of the most difficult to control.

The approach of the monsoon season finds authorities in many countries implementing their annual community-awareness programs, the measures of which include, in countries such as Pakistan, the enforcing of strict laws requiring property owners to eliminate dengue larvae breeding grounds.

Controlling dengue fever primarily involves killing the Aedes aegypti mosquito that spreads the virus among people, as well as draining open water in which the mosquito breeds, distributing repellent, and taking action to drive the mosquito and its deadly cargo away from populated areas. This, however, is a near-impossible task, evidenced by approximately 400 million people affected by the disease each year.

Now, an international research team is engaged in a radical new approach:  deliberately seeding dengue-plagued areas with mosquitoes.

These are no ordinary mosquitoes, though. They carry a bacterium called Wolbachia – one of the most prolific bacterial parasites in nature, hosted by about two-thirds of all insect species. When Wolbachia infects a mosquito, it prevents the dengue virus from taking hold and subsequently being transmitted to humans.

Professor Scott O’Neill, the Eliminate Dengue program leader and Dean of Science at Monash University, had the idea of using the bacterium as a control agent while working on Wolbachia in mosquitoes as a PhD student. He has since enjoyed what he terms “the privilege” of seeing something grow from a concept to an international collaborative research program that could lead to a significant reduction in the spread of this virus.

Having proven in small field trials that the release into the wild of Wolbachia-infected mosquitoes leads to the spread of the bacterium through mosquito populations, the challenge for the research team is to expand this method across large urban areas.

“At the moment we’re focusing on developing methods to scale up so we can deploy over whole cities of 500,000 people,” Professor O’Neill says.

Following ongoing small-scale trials in Indonesia, Vietnam, Brazil, Colombia and Cairns in tropical north Queensland, a city-wide trial is now underway in Townsville, also in north Queensland, where the local community has been enlisted to assist by breeding and releasing infected mosquitoes.

Professor Cameron Simmons, an infectious diseases specialist at the University of Oxford and the University of Melbourne, and a collaborator on the program, says community involvement and acceptance is one of the research program’s greatest strengths. “The research program has been exemplary in the way it’s gone around engaging communities and treating the communities as being very important stakeholders in delivering the Wolbachia dengue control method,” he says.

This has required communicating a complex concept: that the release of infected mosquitoes will serve as a biological control that does not itself eradicate the Aedes aegypti mosquito, but does prevent dengue transmission.

“This is quite a challenging message to get across to communities who for decades have been told that the way to control dengue is just to kill all mosquitoes,” Professor Simmons says. These approaches have by and large been unsustainable, with little evidence that they have reduced dengue transmission.

The Wolbachia dengue control method has the advantage of being a short-run intervention. Unlike previous mosquito-control efforts, which relied on constant vigilance and ongoing eradication, in theory no further action is needed once Wolbachia has established itself in a mosquito population.

It’s yet to be proven whether the intervention will lead to a drop in dengue infections, but there is every indication it will. “Models and the predictions that come from [the trials] say we should be able to completely stop dengue transmission where we can deploy the Wolbachia-carrying mosquitoes,” Professor O’Neill says.

Professor Simmons says the speed at which the research has progressed is testament to its potential. “It’s gone from the lab to field trials in northern Australia to field trials [now underway] in Indonesia and Vietnam … a whole suite of countries where dengue is endemic will be putting their hands up to find out more about the Wolbachia control method and how they can get involved.”

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