Mosquitoes carry dangerous diseases like dengue. Scientists had a new idea. Instead of killing mosquitoes, they gave them a bacterium called Wolbachia. Many insects already have it naturally. With Wolbachia, the virus cannot grow well inside the mosquito.

In a city in Indonesia, scientists released these mosquitoes in half of 24 areas. In those areas, dengue cases fell by about 77%. It was a smart way to fight a disease.

Dengue is a disease spread by mosquitoes in tropical and subtropical areas. Instead of trying to kill mosquitoes, researchers came up with the opposite idea: giving them a bacterium called Wolbachia, which lives naturally in many insects such as butterflies and flies. In 2009, a research team reported that when the dengue mosquito carries Wolbachia, viruses such as dengue do not grow well inside it.

What is more, Wolbachia spreads by itself, because all the eggs of a female with the bacterium inherit it. In a large trial in Yogyakarta, Indonesia, the city was divided into 24 areas, and mosquitoes with Wolbachia were released in half of them. Dengue cases there fell by about 77%.

Mosquitoes are often called the deadliest animals on Earth because of the pathogens they transmit, and dengue is a major threat across the tropics and subtropics. Rather than attempting to eliminate mosquitoes, scientists have pursued a counterintuitive strategy: introducing Wolbachia, a bacterium naturally found in many insects, into the main dengue-carrying mosquito, which does not normally have it.

A 2009 study showed that Wolbachia limits infection with dengue and other pathogens inside the mosquito. The bacterium also spreads on its own, because all offspring of infected females inherit it, while eggs from uninfected females that mate with infected males rarely hatch. In 2011, Wolbachia was shown to have become established in mosquito populations released in Australia.

The strongest evidence came from a 2021 trial in Yogyakarta, Indonesia, where 24 areas were randomly assigned either to receive Wolbachia mosquitoes or not. Confirmed dengue cases were about 77% lower, and hospitalizations about 86% lower, in the treated areas. Because assignment was random, differences such as local hygiene are unlikely to explain the result. Even so, whether the effect will last, given possible influences such as rising temperatures and viral adaptation, requires long-term monitoring.