Wolbachia was first discovered in 1924, in a different species of mosquito. At first, it seemed so unremarkable that scientists ignored it for decades. But starting in the 1980s, they realized that it has an extraordinary knack for spreading. It passes down mainly from insect mothers to their children, and it uses many tricks to ensure that infected individuals are better at reproducing than uninfected ones. To date, it exists in at least 40 percent of all insect species, making it one of the most successful microbes on the planet.
But Aedes aegypti isn’t one of Wolbachia’s natural hosts. Scott O’Neill, the founder of the WMP, spent decades trying to get the latter to stably infect the former, and his team finally succeeded in 2006. Five years later, it released 300,000 of its special mosquitoes into two suburbs of Cairns, Australia. Within four months, 80 to 90 percent of the local mosquitoes were full of Wolbachia. Dengue cases fell, but it was hard to say if that was due to the mosquitoes or some other factor. To prove that Wolbachia could really make a dent in dengue, the team needed to set up a randomized controlled trial. It did so in Yogyakarta.
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First, WMP’s Indonesian chapter spent years earning public trust. The Wolbachia approach is unorthodox, and involves releasing live mosquitoes into people’s backyards. Beyond setting up community meetings and WhatsApp hotlines, “we opened up our entomology lab for people to see the technology for themselves,” Utarini told me. By the time the researchers started their trial in 2017, their surveys showed that the endeavor had support from 88 percent of the public. The team divided a large portion of the city into 24 zones and released Wolbachia-infected mosquitoes in half of them. Almost 10,000 volunteers helped distribute egg-filled containers to local backyards. Within a year, about 95 percent of the Aedes mosquitoes in the 12 release zones harbored Wolbachia.
From January 2018 onward, the team checked on people in the city who showed up to primary-care clinics with a fever, and tested them for dengue. This continued until March 2020, when the pandemic brought the trial to a premature end—but fortunately not before it had enough participants. The team found that just 2.3 percent of feverish people who lived in the Wolbachia release zones had dengue, compared with 9.4 percent in the control areas. Wolbachia also seemed to work against all four dengue serotypes, and reduced the number of dengue hospitalizations by 86 percent.
Even then, these already remarkable numbers are likely to be underestimates. The mosquitoes moved around, carrying Wolbachia into the 12 control zones where no mosquitoes were released. And people also move: They might live in a Wolbachia release zone but be bitten and infected with dengue elsewhere. Both of these factors would have worked against the trial, weakening its results. That those results exist, let alone that they are so impressive, is surprising. “We don’t have strong evidence of how effective most [dengue] interventions are,” says Gonzalo Vazquez-Prokopec of Emory University, who studies vector-borne diseases. “When you see trucks spraying insecticides, we don’t know how many dengue cases that prevents.”