1/2 It takes a mosquito to fight a mosquito － by Tina Rosenberg NYT Jan. 10, 2019 蚊退治
After humans, mosquitoes are the most dangerous animals on the planet. For much of the world, a mosquito bite can mean serious illness or death from malaria, yellow fever, chikungunya, Zika and dengue.
The solution has been － well, we haven't had a good solution. Insecticide-treated bed nets and pesticides applied inside homes protect against the malaria-carrying anopheles mosquito, which bites at night. But Aedes aegypti bites during the day. That’s the mosquito that carries Zika and dengue, which infects 400 million people per year. No usable vaccine exists for either disease.
So we spray large areas with pesticides, which affects the whole ecosystem; it's an ax, when we need a scalpel. And any pesticide eventually becomes useless when mosquitoes evolve to resist it. But now there's a new response: We can create good mosquitoes － those infected with bacteria that block or greatly reduce transmission of disease － and use them to vanquish the bad ones.
Townsville, Australia, is a beautiful city of 193,000 people, on the Great Barrier Reef in Australia’s tropical northeast. While dengue is not endemic, it had been possible to catch it in Townsville when travelers from other countries imported the virus to Townsville, where local Aedes aegypti mosquitoes then spread it to others.
But in 2011, the World Mosquito Program at Monash University in Melbourne began releasing good mosquitoes in Townsville. Between 2011 and 2014, releases grew to cover practically the whole city. To reassure a public hesitant about signing up to live with even more mosquitoes, the program held meetings and events to explain the project and ask households to participate. Thousands of households got their own Mozzie Box, a special bucket with eggs and mosquito food, to raise and release the good mosquitoes. And elementary schoolchildren got Mozzie Boxes － with a magnifying glass and a booklet － as part of citywide science project.
Imported cases of dengue kept rising in Townsville during the mosquito releases. But local transmission of dengue virtually stopped. Four people caught dengue without having traveled abroad, compared with 54 in the equivalent period before the program. And only one of the four lived in an area where good mosquitoes had taken over; that person had been highly mobile, and so could well have become infected in a different neighborhood.
What’s a good mosquito? It's one infected with bacteria called Wolbachia. It's harmless to humans and vertebrates, and occurs naturally in most insect species.
Wolbachia does not occur naturally in Aedes aegypti. Scientists had tried to inject into the mosquito’s eggs, but had no luck until 2005, when Zhiyong Xi, then a doctoral student at the University of Kentucky, found the way. It wasn't genetic modification. The key was that these mosquitoes, when born from infected eggs, are infected themselves － including their own eggs. In that way, the infection continues.
A Wolbachia-infected female will pass the infection to her offspring whether or not her mate is infected. An uninitiated female, however, can't have offspring with an infected male; those eggs will not hatch. And females only mate once. This gives Wolbachia-infected mosquitoes a transmission advantage.
Researchers in a variety of locales enduring tropical diseases are testing several ways to employ Wolbachia. The World Mosquito Program aims to replace the local mosquito population with a Wolbachia-infected population, the infected mosquitoes forming a larger and larger proportion of the total with each generation.
The program releases both male and female Wolbachia-infected mosquitoes. It places traps to catch mosquitoes, and stops releases when the percentage of mosquitoes found to be infected reaches a certain point. In Townsville, that was 50 percent for two consecutive weeks.
The infected mosquitoes’ transmission advantage was enough to tip the population. In most mosquito-release areas in Townsville, all or nearly all mosquitoes are now Wolbachia-infected, and that has remain stable. (In a few neighborhoods, however, Wolbachia’s control has fluctuated and the program has had to release additional mosquitoes.)
It's great for public health that a single treatment campaign provides lasting effects. That will help governments do it cheaply. And the job will have to be done by governments, because there's little profit in it, said professor Scott O'Neill, who was dean of science at Monash University in Melbourne and now directs the mosquito program. “We have no business model.”
The program is working with governments to run field tests in 12 countries, including Australia, Vietnam, Indonesia, Brazil. Columbia and Sri Lanka. In Rio de Janeiro, the project covers 1.2 million people, and in Medellin, it covers 2 million: in both, scaling up was quick in response to Zika. In several places, the program is working on national coverage. So far, results have been reported only from Australia and Yogyakarta, Indonesia. But those have been excellent.
Dr. O'Neill said that he and his colleagues had worried about communities’ reaction to the program. “But people who understand dengue and live in transmission areas are horrified and scared,” he said. “I live in Saigon, where transmission is through the roof. We're not seeing any community pushback.”
Wolbachia does not occur naturally in Aedes aegypti. Scientists had tried to inject into the mosquito’s eggs, but had no luck until 2005, when Zhiyong Xi, then a doctoral student at the University of Kentucky, found the way. It wasn't genetic modification. The key was that these mosquitoes, when born from infected eggs, are infected themselves － including their own eggs. In that way, the infection continues.
A Wolbachia-infected female will pass the infection to her offspring whether or not her mate is infected. An uninitiated female, however, can't have offspring with an infected male; those eggs will not hatch. And females only mate once. This gives Wolbachia-infected mosquitoes a transmission advantage.
Researchers in a variety of locales enduring tropical diseases are testing several ways to employ Wolbachia. The World Mosquito Program aims to replace the local mosquito population with a Wolbachia-infected population, the infected mosquitoes forming a larger and larger proportion of the total with each generation.
The program releases both male and female Wolbachia-infected mosquitoes. It places traps to catch mosquitoes, and stops releases when the percentage of mosquitoes found to be infected reaches a certain point. In Townsville, that was 50 percent for two consecutive weeks.
The infected mosquitoes’ transmission advantage was enough to tip the population. In most mosquito-release areas in Townsville, all or nearly all mosquitoes are now Wolbachia-infected, and that has remain stable. (In a few neighborhoods, however, Wolbachia’s control has fluctuated and the program has had to release additional mosquitoes.)
It's great for public health that a single treatment campaign provides lasting effects. That will help governments do it cheaply. And the job will have to be done by governments, because there's little profit in it, said professor Scott O'Neill, who was dean of science at Monash University in Melbourne and now directs the mosquito program. “We have no business model.”
The program is working with governments to run field tests in 12 countries, including Australia, Vietnam, Indonesia, Brazil. Columbia and Sri Lanka. In Rio de Janeiro, the project covers 1.2 million people, and in Medellin, it covers 2 million: in both, scaling up was quick in response to Zika. In several places, the program is working on national coverage. So far, results have been reported only from Australia and Yogyakarta, Indonesia. But those have been excellent.
Dr. O'Neill said that he and his colleagues had worried about communities’ reaction to the program. “But people who understand dengue and live in transmission areas are horrified and scared,” he said. “I live in Saigon, where transmission is through the roof. We're not seeing any community pushback.”
