An Australian city has seen local cases of dengue fever plunge after it was blanketed with mosquitoes modified to block transmission of the virus.

Over 28 months beginning in October 2014, researchers and community members intentionally released roughly 4 million Aedes aegypti mosquitoes over 66 square kilometres in the northeastern city of Townsville. The insects carried Wolbachia bacteria that block them from transmitting dengue, Zika and some other disease-causing viruses.

A team led by microbiologist Scott O’Neill at Monash University in Clayton, Australia, tracked the mosquito release — the first time the strategy has been trialled across an entire city. Wolbachia -infected mosquitoes quickly spread the bacteria to local populations. In an inner-city suburb called Belgian Gardens, for instance, nearly 100% of mosquitoes carried Wolbachia one year after the release period. The results were posted to Gates Open Research on 1 August ¹ .

Townsville, which has a population of around 187,000, has faced periodic dengue outbreaks since 2001. In the 44 months after the releases began, however, local authorities recorded just 4 locally acquired dengue cases, compared with 54 locally acquired cases over the 44 preceding months. (During the same period after the release, 51 imported cases were reported.)

Similar trials with Wolbachia -infected mosquitoes are being carried out in Yogyakarta, Indonesia, which has a much higher incidence of dengue than Townsville. These trials include control areas in which mosquitoes have not been released, so they should provide stronger evidence that the strategy can reduce dengue cases. Ongoing releases in greater Rio de Janeiro in Brazil and Bello and Medellín in Colombia will test whether the strategy can succeed in dense metropolises where dengue is rife.