Disease-carrying mosquitoes are on the defensive. Cases of dengue fever, which is transmitted by the insects, plummeted in areas of Indonesia, Vietnam and Brazil in the months after researchers released Aedes aegypti mosquitoes that were modified to be resistant to dengue virus.

The findings, presented on 21 November at the annual meeting of the American Society of Tropical Medicine and Hygiene in National Harbor, Maryland, come from experimental releases of mosquitoes that carry Wolbachia bacteria, which block the replication of mosquito-borne pathogens such as the dengue, chikungunya and Zika viruses. The Wolbachia infection then spreads through local mosquito populations. The efforts were led by the World Mosquito Program (WMP), a global consortium that is testing whether these releases can prevent mosquito-borne diseases.

Releases of Wolbachia -infected mosquitoes in 2016 near Yogyakarta City, Indonesia, led to a 76% reduction in cases of dengue fever over 2.5 years, compared with rates in areas where mosquitoes were not released. Two parts of Niterói, Brazil — a city home to around 500,000 people — experienced similar drops in dengue cases after releases in 2018. And declines also occurred after a smaller-scale 2018 release near Nha Trang, Vietnam, WMP researchers reported.

The WMP is working on a larger, better-controlled study across Yogyakarta City, which should determine more conclusively whether Wolbachia mosquitoes can reduce dengue cases. Results are expected next year.