The mosquitoes that fight dengue

Every rainy season, hospitals across Vietnam brace for dengue fever. The disease is spread by the Aedes mosquito, which breeds in any small pool of standing water, from flower pots to old tyres. There's no specific treatment, and a bad outbreak can fill hospital wards within weeks. For decades, the main defence has been chemical spraying, along with campaigns asking families to empty containers of water.

That approach has clear limits. Insecticides can harm other insects and animals, and mosquitoes slowly become resistant to them, so the same chemical works less well each year. Health workers go door to door, but they can't check every home. It's almost impossible to find every bottle cap full of rainwater in a crowded city. Scientists have spent years looking for a smarter, biological solution.

One of the most promising involves a tiny bacterium called Wolbachia. It lives naturally in many insects, but not usually in the Aedes mosquito. When researchers put Wolbachia into these mosquitoes, the insects became much worse at passing on viruses such as dengue and Zika. The bacterium is passed from mothers to their young. Once enough treated mosquitoes are released, it can spread through the local population by itself.

The method has now been tested in more than a dozen countries. Vietnam took part in some of the first field trials, on Tri Nguyen island off Nha Trang. The strongest evidence so far comes from Yogyakarta in Indonesia, where the city was split into areas that did or didn't receive the mosquitoes. The trial ran for more than two years. In the treated areas, dengue cases that needed hospital care fell by 86 per cent.

Not everyone liked the idea at first. Releasing millions of mosquitoes to fight a disease that mosquitoes carry sounds strange, and some residents worried that the insects would bite more. They don't, since they're ordinary mosquitoes with one important difference. The research teams spent months meeting local communities before any release. Wolbachia won't replace vaccines, clean water or good sanitation, and researchers are careful to say so. For cities that face dengue every year, though, it's a promising extra tool.