The mosquito Aedes aegypti sucks the blood of people from all over the tropics, and exchanges it for the virus that causes dengue fever – a disease that afflicts 40 million people every year. The mosquito has proven to be a tough adversary and efforts to drive it from urban settings have generally failed in the long-term. So how do you fight such an accomplished parasite? Simple – use a better parasite. In fact, try the most successful one in the world, a bacterium called Wolbachia.
Wolbachia‘s success rests on two traits. First, it targets the most diverse group of animals on the planet, the insects, infecting the majority of species and about one in eight individuals. Second, it spreads like wildfire by using several extremely self-serving strategies, all of which screw over male insects in some way or other. Wolbachia passes from one generation to the next in the eggs of infected females. But without similar access to sperm, males are useless to it and has evolved a number of ways of dealing with that. Sometimes it kills males outright before they’re even born; sometimes it turns them into females.
In other subtler cases, it ensures that infected males can only mate successfully with infected females. If they try to breed with uninfected ones, the embryos die at an early stage of development. This strategy is known as “cytoplasmic incompatibility” and while it’s still unclear how it works, there’s no doubt that it does. It gives infected females (who can mates with any male they like) a competitive advantage over uninfected females, who are restricted to uninfected males. With this upper hand, massive swathes of a given population eventually become Wolbachia-carriers.
Conor McMeniman and colleagues from the University of Queensland have found a way to use that to their advantage. They have found a strain of Wolbachia that can halve the lifespan of the Aedes mosquito and that induces complete cytoplasmic incompatibility. If introduced into a natural population, it should invade with tremendous zest.
Shortening a mosquito’s lifespan may seem like a flimsy victory, but McMeniman recognises it as an important one. Only old mosquitoes really pose a threat to human health because it takes about two weeks for an individual to become infectious after it first sucks up a mouthful of infected blood. The virus first need to reproduce in its gut before travelling back to its salivary glands, where it can spread further. Because mozzies are short-lived anyway, most die before they reach that point, which means that any technique that slashes their already limited lifespan will have a huge impact on controlling the diseases they carry.