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Published On February 5, 2021

DISCOVERIES OF 2020

Progress on a Different Plague

A novel use of bacteria could blunt the spread of dengue and other mosquito-borne diseases.

The COVID-19 pandemic brought a vast mobilization of scientific ingenuity. At the same time, work on other medical threats continued to push forward. This week, Proto looks at the most transformational non-COVID-19 research of 2020.

Dengue is a brutal foe, threatening 40% of the global population and a leading cause of serious illness and death in several Asian and Latin American countries. The disease is caused by a virus carried by female Aedes aegypti mosquitoes, a hardy and wide-ranging insect that can also carry chikungunya, yellow fever, Zika and other debilitating diseases. So one of the most promising advances to come out of the past year aims for the vector herself—an introduction of a bacterium, Wolbachia, into her system. In an Indonesian trial, areas of the city Yogyakarta where Wolbachia was introduced saw dengue rates fall by 77%.

“Traditional methods have relied on suppressing the mosquito population through insecticide spraying, reduction of breeding sites around houses and other approaches,” says public health researcher Katie Anders, director of impact assessment for the World Mosquito Program, based at Monash University in Melbourne, Australia, which sponsored the “Applying Wolbachia to Eliminate Dengue” (AWED) trial. “But that has really been a losing battle in dense urban environments.” Even if you push down mosquito numbers to very low levels, she says, the insects that remain continue to spread disease.

Wolbachia occurs naturally in many insects, and the idea of introducing the bacterium into Aedes aegypti came after years of research by Scott O’Neill, founder and director of the World Mosquito Program. Bacteria taken from fruit flies were injected into mosquito eggs, in the hopes that the Wolbachia-carrying mosquitoes would have shorter lifespans. Although it took thousands of attempts, researchers in 2008 succeeded in getting the mosquitoes to survive and pass along Wolbachia to their offspring. Aedes aegypti with Wolbachia turned out to have another trait that captured the researchers’ attention: The bacteria fortuitously made the mosquitoes more resistant to infection with dengue and thus less likely to transmit it.

Smaller experiments with the release of Wolbachia mosquitoes had been underway since 2011. For the AWED trial, launched in 2017, researchers randomized 24 areas of Yogyakarta and introduced Wolbachia-bearing Aedes aegypti into half of them. Within six months, virtually all of the mosquitoes in those areas carried Wolbachia. The researchers hoped to show that people in the Wolbachia neighborhoods were 50% less likely to have an acute dengue infection than those in the untreated areas. To reach 77% efficacy was “stunning,” Anders says.

Dengue is endemic in tropical climates, and an estimated 40% of the global population is at risk of contracting it. But other diseases caused by mosquito-borne viruses are also on the rise, and the Wolbachia approach appears to be effective in stopping their spread as well, Anders says. Now the World Mosquito Program is introducing Wolbachia to the rest of Yogyakarta and hopes to establish the special mosquitoes in dengue-threatened areas with 50 million people within the next five years. “What’s really exciting is how sustainable this approach is,” says Anders. “Once Wolbachia is in a mosquito population, it seems to stay there, sustaining public health benefits for years if not decades, without a further input of resources.”