Releasing mosquitoes into the corridors of apartment complexes might seem like an unusual strategy for a city fighting its worst recorded outbreak of dengue, a painful disease spread between humans by mosquitoes. But the thousands of little insects discharged last week werent your average mosquitoes.

They were bred in a laboratory to carry a substance not commonly found in this type of mosquito: bacteria called Wolbachia. When the bacteria-laden male mosquitoes are released into the open and mate with naturally-born females, the resultant eggs wont hatch.

The outcome is reduced number of dengue cases in the areas where the lab-bred insects were released, according to Singapores government.

Scientists and governments are expanding high-tech solutions like these as the threat from the dengue virus grows. Some are using genetically engineered mosquitoes; others are zapping them with X-ray beams to sterilize them.

The World Health Organization says roughly half the worlds population is at risk of catching dengue, a viral infection that causes an intense flulike illness that is sometimes lethal. Growing urbanization and bulging cities have given mosquitoes vast human populations to feast on. Reported cases of the disease increased from about 500,000 in 2000 to 4.2 million in 2019, with tropical countries such as Brazil, Indonesia and the Philippines especially hard-hit.

Global warming could spread the disease further as both dengue-carrying mosquitoes and the virus itself thrive in warmer climates.

Dengue is transmitted by the female Aedes aegypti mosquito, which also spreads other diseases like Zika, which can cause severe birth defects when pregnant women are infected, and chikungunya, which causes fever and joint pain. Public-health campaigns have traditionally focused on simple solutions, such as encouraging people to empty stagnant water from household objects such as vases, pails and watering cans, where mosquitoes lay eggs. Insecticides are also used in dengue-prone areas.

But mosquitoes have developed immunity against common insecticides and dengue cases are rising globally. That is why scientists turned to altering or modifying the mosquitoes themselves.

In Singaporewhich has long suffered from dengue outbreaksspecialized mosquito-breeding began with mosquito eggs shipped from Michigan. A team led by Zhiyong Xi, a professor at Michigan State Universitys Department of Microbiology and Molecular Genetics used long, thin glass needles to inject Wolbachia into mosquito eggs, resembling tiny grains of dirt, that had been laid 90 minutes before. Upon hatching, the larvae also contained the bacteria.

That first generation passed the Wolbachia bacteria on to its descendants, birthing a new line of bacteria-infused mosquitoes whose eggs were shipped to Singapore to found the city-states colony.

Before the offspring could be released, the females needed to be separated from the males, which dont bite or transmit the dengue virus. Sex-sorting is critical because Singapores program hinges on mating males that contain the bacteria with females that dont. If both sexes carried the bacteria, the mosquitoes would successfully procreate, thwarting the programs goal of reducing the local mosquito population.

A machine developed by Verily, an Alphabet Inc. company focused on life sciences, uses automated mechanical sieves to separate female mosquito pupaewhich are generally largerfrom male ones. This step removes about 95% of females, the company says.

A computer vision system is used to identify any females the sieve may have missed. The system looks for the females distinct proboscis or mouth, antenna and other anatomical clues, flagging it for removal. Verily says substantially fewer than one in a million mosquitoes it releases is female, keeping Wolbachia from being inherited in the wild mosquito population.

Not all Wolbachia mosquitoes released in Singapore are sieved through Alphabets machine. Others are subjected to low-dose X-ray irradiation using a specific methodology Singapore developed in collaboration with the International Atomic Energy Agency. The irradiation sterilizes female mosquitoes, so that any that are inadvertently released will be unable to reproduce and spread Wolbachia to future generations.

Singapores government says that in parts of the city where its males have been released there were 65% to 80% fewer dengue cases compared with areas where the mosquitoes werent released. Mosquitoes are now being discharged in 5% of the citys public housing blocks. The releases are slated to expand to 15% of them by 2022.

Other programs want the Wolbachia to be inherited widely in wild populations. That is because those programs have found that the bacteria has another feature: It strongly reduces the Aedes aegypti mosquitoes ability to transmit dengue to humans.

The World Mosquito Program, a nonprofit active in a dozen countries in Asia, the Pacific and Latin America, released lab-bred bacteria-containing mosquitoesboth male and femalein the city of Yogyakarta in Indonesia. It counted on the fact that female mosquitoes will produce offspring that also have the bacteria, meaning the dengue-blocking feature is passed down.

Its trial showed a 77% reduction in dengue cases in areas where the mosquitoes were released compared with areas where they werent, the nonprofit said in August.

This method is much simpler than Singapores technique, which involves complex sex-sorting. But some scientists say releasing females with Wolbachia is potentially irreversible. If the Wolbachia turns out to have unintended consequences, it would be very difficult to extract the bacteria from the mosquito population, they say.

One laboratory study found that carrying Wolbachia enhanced the infection rate of West Nile virus in the Culex tarsalis species of mosquito, which is endemic to North America. Its a big black box," said Jason Rasgon, professor of disease epidemiology at Pennsylvania State University, arguing more research should be done on Wolbachias effects on the transmission of other diseases before further large-scale releases.

Cameron Simmons, a director at the World Mosquito Program, said many governments have conducted risk-assessments of its approach. On balance Wolbachia represented a negligible risk compared to doing nothing," he said.

One company is going in a different direction altogether: genetic engineering. Oxitec, a U.S.-owned biotechnology company with research bases in the U.K. and Brazil inserts a new gene in eggs that makes female mosquitoes die shortly after hatching while they are still in the larval stage of development.

Last year, Oxitec conducted a trial of its latest gene-modified version, which it calls OX5034, in Indaiatuba, Brazil, near So Paulo. For the trial, the company produced OX5034 eggs at a factory in Brazil and distributed them at release points around the municipality. When the eggs hatched, the females died before they could become adults capable of flying and biting.

The males, which reached adulthood, mated with local wild females, passing along the female-killing genes, reducing Aedes aegypti mosquito numbers by about 95%, Oxitec said.

The company received U.S. federal approvals in May for pilot releases in Florida, which the company expects to begin next year.

Oxitec says the genes they have added are self-limiting, which means that after a few generationsabout three to four monthsthe female-targeting gene is bred out of the species. Municipalities that wish to continue with the approach would carry on releasing OX5034 eggs to keep the mosquito population in check, it said, and those that dont would still have an off-ramp.

Jeffrey Powell, a biology professor at Yale University, sees drawbacks to the gene-modification approach. He said the need for periodic rereleases would get expensive, and over time wild mosquitoes may adapt to avoid mating with Oxitecs genetically doomed males. There is no evidence it is doing anything bad," he said of the genes Oxitec has introduced into mosquitoes. Its a complete unknown." He said he felt more comfortable with the use of Wolbachia, which is found naturally in many mosquito species.

Oxitec says it has released about one billion mosquitoes in the past decade and has no evidence female mosquitoes selectively mate with non-Oxitec males.

Theres no ecological footprint; theres no persistence," said Kevin Gorman, who heads field operations for Oxitec. Its not going to permanently change the environment at all."

Write to Jon Emont at jonathan.emont@wsj.com

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How to fight the deadly dengue virus? Make your own mosquitoes - Livemint

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