How bacteria-infected mosquitoes reduced dengue cases

Dengue infections are on the rise in many parts of the world, thanks to climate change and the expansion of the range of Aedes aegypti mosquitoes. While some public health agencies are slowly rolling outtwo licensed vaccines, a few countries are also testing a novel intervention to battle these mosquitoes and the diseases they spread.

Researchers at the nonprofit World Mosquito Program (WMP) have been releasing mosquitoes infected with a bacterium called Wolbachia as a way to control dengue. On Nov. 16, at the American Society of Tropical Medicine and Hygiene annual meeting in New Orleans, Katie Anders, the WMP’s director of impact assessment, presented results from Niterói, a city in Brazil where Wolbachia-infected mosquitoes were first deployed in 2015.

Released in phases, with citywide coverage completed in 2023, the infected Aedes aegypti mosquitoes have helped protect Niterói’s residents against dengue year after year, Anders said in her talk. Since mid-2019, her team noted a 90% reduction in dengue cases. In 2024, when the rest of Brazil saw record dengue cases, Niterói likely averted thousands of them. Chenoa DeFreece, a biologist with the Fresno Mosquito and Vector Control District in California, who attended the talk and wasn’t involved in the WMP’s work, found the results “very encouraging.”

Unlike many other insects, Aedes aegypti mosquitoes typically don’t harbor naturally occurring Wolbachia in their cells. These bacteria compete with viruses for resources, thus limiting a pathogen’s ability to replicate and transmit diseases such as dengue, Zika fever, and chikungunya.

Because Wolbachia is maternally transmitted from one generation to the next, “it drives itself into the local Aedes aegypti population,” Anders told C&EN. Once most mosquitoes are infected with Wolbachia, the bacterium population is self-sustaining, she added.

In northern Australia, where the WMP first began its work, the team found that 95% of the population of Aedes aegypti mosquitoes remained infected with Wolbachia 10 years after the earliest insects were released, in 2011. One concern, though, is that extreme heat can reduce Wolbachia levels in mosquitoes. But scientists have seen recovery within months after heat waves. They’re also testing other Wolbachia strains that have better thermal tolerance, Anders said.