ABOVE: A researcher hold up a plate of mosquitoes.

Mosquito-borne diseases such as malaria, dengue, and Zika sicken nearly 700 million people each year, and the preliminary results of a new study hint at a possible way of drastically minimizing the spread of such illnesses.

Researchers have infected Aedes aegypti mosquitoes—the species responsible for passing on many diseases—with bacteria called Wolbachia with the intent of reducing the insects’ ability to pass on dengue to people. When these modified mosquitoes were released in the Indonesian city of Yogyakarta, the rates of dengue dropped by 77 percent over two years, making it four times less likely that a person would contract the disease by the end of the study than in years past.

“It is a huge breakthrough,” Nicholas Jewell, a biostatistician at the University of California, Berkeley, who designed the study, said in an August 26 press release...

The Yogyakarta trial is the culmination of a decade of laboratory and field work based on  methods pioneered by Scott O’Neill, a microbiologist at the University of Monash and director of the World Mosquito Project (WMP), who first began manipulating mosquitoes in the 1990s. He had previously shown that Wolbachia reduced transmission of dengue by mosquitoes, and he later demonstrated that cases of dengue were lower in the Australian city of Townsville after Wolbachia-infected mosquitoes were released, although that study failed to include controls and tracked much smaller outbreaks than those in countries where dengue is endemic.

In the newest study, the first controlled trial to use the Wolbachia method, the team divided the city of Yogyakarta into 24 regions in a checkerboard pattern and released modified mosquitoes into 12 of them, retaining the remaining dozen as controls. In total, about 6 million mosquitoes were released over six months across an area that is home to around 312,000 people, The Guardian reports. These mosquitoes go on to mate (but not reproduce) with local wild populations, transferring the bacteria to naive mosquitoes. 

The scientists partnered with local clinics to track the number of people coming in with fevers that could be attributed to dengue, back-tracking their movements to see which of the areas of the city they lived in or had visited within the previous 10 days. After more than two years, 8,144 people between the ages of three and 45 were included to arrive at what the authors call the “golden number” of a 77 percent reduction in the rate of dengue diagnoses in clusters where the mosquitoes had been released. 

“This exciting result of the trial is a great success for the people of Yogyakarta,” Adi Utarini, a researcher of public health quality at the University of Gadjah Mad and co-principal investigator of the trial, says in the press release. “Indonesia has 7 million dengue cases every year. This trial result shows the significant impact the Wolbachia method can have in reducing dengue in urban populations.”

See “How Bacteria Interfere with Insect Reproduction

Scientists still do not fully understand how Wolbachia inhibits transmission of the virus that causes dengue when the two pathogens inhabit the same mosquito. Katie Anders, the director of impact assessment at the WMP, tells The Guardian it may be that the two compete for the same resources. “By having bacteria there it might prevent the virus from growing, and or [the bacteria] might elicit an immune response in the mosquito that’s protective against the virus,” Anders says.

Despite these unknowns, the team says the next step is to treat the rest of the city, releasing mosquitoes in areas that had previously served as controls. In addition, similar initiatives have been carried out in at least 11 other countries where dengue is endemic, according to the authors.

Singapore, an island city-state off the coast of Malaysia, is currently experiencing a record-breaking year for dengue, Reuters reports, with more than 26,000 reported cases. Local scientists are releasing up to six Wolbachia mosquitoes each week for every person living in the most high-risk areas. 

Moving forward, the WMP aims to release Wolbachia mosquitoes in areas covering 75 million people at risk of dengue in the next five years, Nature reports, increasing their reach to half a billion people in the next decade. 

Expansion of the program will depend upon an influx of funding spurred by this successful trial. Independent economists have suggested that the mosquito releases, estimated to cost up to $21 per person, will pay for themselves within a few years by reducing healthcare costs, lost income, and other losses attributed to dengue, Nature reports. And while the initiative has so far looked at dengue, the same technique has the potential to limit transmission for a host of other mosquito-borne diseases, including Zika, chikungunya, and yellow fever.

Immo Kleinshmidt, an epidemiologist at the London School of Hygiene and Tropical Medicine who was part of an independent board monitoring the trial, says the positive results will likely result in broad support. “I suspect that the demand for this intervention from dengue-endemic countries will result in widespread introduction of this method, with a good prospect of eventually eliminating the disease,” Kleinschmidt tells Nature. “The significance of this result is epochal.”

See “Dengue Cases Drop After Bacteria-Infected Mosquitoes Released

Interested in reading more?

mosquito, dengue, vector, transmission, Aedes aegypti, Wolbachia, bacteria, virus, Zika, yellow fever,  chikungunya

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