Infecting Mosquitoes With Bacteria To Keep Them From Infecting Us With Dengue Fever

BY Nidhi SubbaramanWed Aug 24, 2011

Dengue fever affects 50 million people, with no cure in sight. But maybe prevention could work instead: Scientists have found a way to get mosquitoes sick with a bacteria that prevents them from carrying the disease.

To you, dengue fever might sound like something out of a 19th century novel. But to large parts of the world, it's a continual issue. WHO estimates that dengue infects 50 million people each year, with 2.5 billion people at risk of the infection. Dengue is not fatal in most cases, but it is painful and results in lost productivity. It also spreads quickly, carried by the Aedes aegypti mosquito. There is no cure, so people in dengue-prone areas manage the disease by managing mosquitoes and hoping for the best.

But scientists in Australia have successfully tested a new weapon in the battle against dengue. In two studies published today in the journal Nature, they show that a certain strain of a bacteria called Wolbachia has shown new promise as a cost-effective way to control the mosquito-transmitted disease.

For some years now, researchers like Scott O’Neill, the Dean Faculty of Science at Monash University, have been working on one such control method--a naturally occurring bacterium which destroys the ability of infected mosquitoes to function as carriers. Wolbachia infects up to 70% of insect species, but for reasons as yet unknown, it doesn’t naturally infect disease-carrying mosquitoes like A. aegypti.

But O’Neill and his colleagues were able to get the dengue-quashing bacterium into mosquitoes during confined cage tests, as well as in a first-of-a-kind open release in isolated Australian towns. In the field trials, the researchers released lab-bred, Wolbachia-infected mosquitoes near people’s homes once a week for 10 weeks, and tracked the spread of the bacterium through the population. With data from their confined release, they were optimistic about its success. “It’s like having an ace in your sleeve, its like having a great hand,” O’Neill said at a press briefing. “We were pleased as punch when it came out as good as it did.”

Mosquitoes infected with Wolbachia infiltrated the wild population and passed on the anti-dengue bug to growing numbers of the wild population, almost completely penetrating the wild population when the experiment came to an end. “What we’re doing is creating a vaccine for the mosquito rather than for the people” and a vaccine that can be deployed over very large areas, very inexpensively, without treating humans at all,” O’Neill said.

For a biological control method like this one to be effective, it’s important that the trait--Wolbachia infection, in this case--spreads and stays in the mosquito wild population. And that’s exactly what happened in the tests.

A wily bug, Wolbachia sticks around by affecting the mating abilities of the mosquito. Uninfected male and female mosquitoes breed normally, but when an uninfected male mates with an infected female, their spawn is not viable. On the other hand, female mosquitoes pass on the bacterium to their kids no matter what kind of male they mate with, resulting in a next generation that is significantly Wolbachia-filled (and thus, dengue-free).

Other studies have shown the dengue-stomping effect that Wolbachia has on would-be mosquito carriers in the lab. On the heels of this set of studies, this team of researchers are working on field tests in cities where dengue exists as a recurring threat. Field trials in locations in Thailand, Vietnam, Indonesia, and Brazil are being planned, where the effect of the Wolbachia-carrying mosquitoes will be measured alongside incidences of dengue infections.

“Until we go through that exercise, we’ve got to be cautious,” says Ary Hoffman, a professor at the University of Melbourne, and another researcher on the study. “It’s early days, we’re certainly hopeful.”

[Image: Flickr user wildxplorer]