The cost of the program in Townsville was $13 per person covered, but that was because it's sprawling city. In a place like Rio, where people live closer together, the cost is $4 per person. That might save Rio money over all. It’s a onetime cost. And treating dengue and Zika are expensive; babies with Zika-related microcephaly may need lifelong care. Dr. O'Neill said he hoped to get the cost down to $1 per person.
The program’s strategy of replacing bad mosquitoes with good ones has some disadvantages as well. Unfortunately, it doesn't reduce the number of mosquitoes or their bites.
Also, there's a trade-off. The better Wolbachia does at blocking disease, the more it weakens the mosquito. Weaker mosquitoes are less able to spread into the population and maintain their hold. The world Mosquito Program is using the best Wolbachia strain possible, said Dr. Xi, who is not involved in the program. “But the technology needs to be improved so we are able to develop a mosquito that can completely block the virus and still be able to spread into population.”
Dr. Xi, who is now a professor at Michigan State University, uses Wolbachia in a different way: to eliminate the mosquito population. He heads a research collaboration between M.S.U. and Sun Yat-sen University, in which the universities built a mosquito factory in Guangzhou, China. Dr. Xi also is leading a project with Autonomous University of Yucatan to start another factory, in Merida, Mexico.
The China factory can produce 60 million Wolbachia-infected male mosquitoes per week, he said. Unlike the replacement strategy, an elimination strategy releases only males. Sex-sorting is expensive and difficult. Essentially, mosquitoes are put through a giant sieve, which separates males from the larger females. But releasing only males make it possible to clear an area of all mosquitoes: When the existing females, which are all uninfected, mate with an infected male, their eggs won't hatch. So the population is gradually reduced.
This strategy creates a double attack on dengue and Zika: The infected mosquitoes don't transmit disease, and their population dwindles to the point where they can't keep an epidemic going. It could also get a warmer welcome from local residents, as the only mosquitoes released are males, which don't bite.
It has some disadvantages. Sex-sorting is not only costly, it's imperfect － and letting even a few females through can be fatal to the project. More important, the gains don't last. Mosquitoes will fly in from untreated areas and re-establish a colony, so the project must start anew every year.
“If you have to release mosquitoes every year to keep the numbers down, it becomes a business,” said Dr. Andrea Gloria-Soria, an evolutionary biologist who is an agricultural research scientist at the Connecticut Agricultural Experiment Station. “Which is what I don't like. These problems are mostly third-world country problems, and there's no money there.”
In Guangzhou, Dr. Xi is working with local officials to test a combination strategy. For the last three years, mosquitoes have been infected with Wolbachia and then zapped with low-dose radiation, which sterilizes the females. He said that the mosquito population has dropped by 90 percent on average, and some areas have completely eliminated mosquitoes.
Most Americans don't worry about dengue and Zika － yet － but would love to see fewer mosquitoes. Dr. Stephen Dobson, a professor of entomology at the University of Kentucky, founded MosquitoMate, a for-profit company that works with mosquito-abatement program in the United States. MosquitoMate’s largest projects are in Fresno, Calif., and Miami, with more than 150 areas treated in each city. One unexpected partner in Fresno is Verily, the life-sciences division of Google’s parent company, Alphabet. Verily is trying to use machines and algorithms to raise, sort, release and algorithms to raise, sort, release and monitor Wolbachia mosquitoes.
Dr. Dobson said that mosquito populations drop by 70 to 95 percent － the larger the area, the stronger the effect. But it's expensive. He sells the service to individual homeowners in Lexington, Ky., who will pay $1,500 for weekly mosquito releases this summer.
Any time we play God with nature, we need to worry about the consequences. But Wolbachia is natural － most insect species already carry it. And any possible environmental impact is far less than what we create by spraying. No scientist I talked to worried that Wolbachia could help mosquitoes transmit new diseases such as H.I.V. － which, let us give thanks, is not mosquito-borne. Mosquitoes are very particular about which viruses they can transport. “Mosquitoes aren't flying syringes,” Dr. Dobson said.
What’s much more likely is that the benefits of Wolbachia won't last. At some point, pathogens are all but certain to evolve to resist Wolbachia, just as they evolve to resist insecticides. Dr. Gloria-Soria said that the elimination strategy might be better able to stave off resistance, because it must start over each year. “You could put in slightly different strains of Wolbachia,” she said. “It's like getting a new insecticide.”
But she said that pathogens could increase their use of other, less efficient, kinds of mosquitoes. “If you eliminate Aedes aegypti from Australia, you still have Aedes albopictus,” she said. “Other mosquitoes who play minor roles could become a better vector once the niche is open. That’s how nature works.”
