People once thought mosquitoes, and the diseases they carry, could be controlled using insecticides like DDT. Unfortunately, besides the environmental consequences, resistance spread rapidly and the mosquitoes were back as numerous as ever. In recent years, we've been investigating a variety of smarter approaches, ones that might actually provide long-term protection against disease. For the first time, a large-scale trial of a next-generation technique has worked, offering the potential to free the world from dengue, yellow fever, and zika.

The Wolbachia bacterium infects a wide variety of insects, mosquitoes included. It comes in many different strains, a handful of which have properties that prevent the development, and therefore transmission, of viruses. Twenty-five years ago, while still a student, Monash University’s Professor Scott O’Neill proposed that if we could get the right strain of Wolbachia into disease-carrying mosquito populations, we might have a lasting solution.

After extensive lab testing for suitable strains, numerous trials have been conducted, many still in progress. Some show promise but have been quite small, with serious question marks over whether they could protect entire cities.

Now, however, the World Mosquito Program, led by O’Neill, have announced success in eliminating dengue fever from Townsville, northern Australia’s largest city. Aedes aegypti mosquitoes, the primary dengue carriers, were infected with the wMel Wolbachia strain in labs in Melbourne and Townsville. They were bred for three generations, before being released at dozens of known mosquito breeding sites, where they bred with natives spreading the bacterium.

In the four years since the trial began, only four people have contracted dengue in Townsville, despite 51 people acquiring the disease elsewhere before traveling there, potentially spreading the disease if bitten by local mosquitoes. Previously, every similar length period this century had at least 54 locally acquired cases and an average of 98, notes a report on Gates Open Research. At least three of the four cases were acquired before the release program was complete.

O’Neill told IFLScience that trials are underway in much larger and more dengue-prone cities, including Rio de Janeiro and Yogyakarta, but it will be about 18 months before results are in. Meanwhile, the Townsville findings are creating optimism. So far, he said, Wolbachia has been maintained even in areas where releases were done seven years ago.

The technique stands in contrast with another recent experiment in the same area, where male mosquitoes were sterilized using a different Wolbachia strain and released into the environment. This resulted in an 80 percent decrease in numbers of A. aegypti mosquitoes, but the program needs frequent repetition or the population will bounce back.

The two approaches are compatible O’Neill told IFLScience: sterile releases don’t affect wMel concentrations where it is already present.

O'Neill's work doesn’t free people from the flying blood-suckers but leaves a dengue-blocking Wolbachia strain that should last years or decades, so the $12 per head cost for Townsville residents will not need to be repeated.

“It lays the foundation for future deployments in larger cities, globally, where we’re targeting a reduced cost of US$1 per person," O’Neill said in a statement.

Dengue is responsible for less than 2 percent of the mosquito-induced deaths in the world, most of which are from malaria. However, it causes horrific pain, hospitalizing half a million people a year, and can leave people unable to work for weeks, with devastating effects on struggling families. Although its mechanism is not well understood, wMel has also been found to block the viruses for yellow fewer, zika, and chikungunya.

O’Neill told IFLScience other teams are investigating using Wolbachia to block the malaria parasite. This research shows progress, he said, but is far more complex and further from widespread application.