WASHINGTON — An experiment to infect mosquitoes with bacteria that stop them from transmitting viruses appears to have helped reduce the spread of the deadly dengue virus in Indonesia, researchers reported on Wednesday. The modified mosquitoes thrived for three years, and cases of dengue were reduced by 77 percent in areas where they were introduced, the researchers reported in the New England Journal of Medicine. The mosquitoes are infected with bacteria called Wolbachia, which not only interfere with the ability of viruses to live in the bodies of the insects but which also control reproduction so that the mosquitoes only have Wolbachia-infected offspring. The result is a growing population of insects that don"t pass on viruses such as dengue, yellow fever and Zika. The study involved more than 8,000 people, about half of whom lived in areas where the modified Aedes aegypti mosquitoes had been living and breeding. Dengue fever was diagnosed in 9.4 percent of those living in areas with non-modified mosquitoes, and 2.3% of people living in areas where the modified mosquitoes had been released. "The protective efficacy of the intervention was 77.1 percent," the researchers wrote"There have been very few randomized trials of interventions against the dengue mosquito," Dr. Katie Anders of the World Mosquito Program, which helped sponsor the trial, said in a statement. "These trial results from Yogyakarta show conclusively that Wolbachia works to reduce dengue incidence and dengue hospitalizations," she added. The mosquitoes have also been tested in the Florida Keys and Australia. As many as 400 million people get infected with dengue every year, according to the US Centers for Disease Control and Prevention. The virus, which has four strains, sickens 100 million people a year and it kills 22,000 a year. "Indonesia has more than 7 million dengue cases every year," said Adi Utarini of the University of Gadjah Mada, who worked on the study. "We think there is a possible future where residents of Indonesian cities can live free of dengue." — CNN