How do I combat the lethal dengue virus? Make your individual mosquitos


The release of mosquitoes into the corridors of apartment complexes seems like an unusual strategy for a city battling the worst dengue outbreak, a painful disease that mosquitoes transmit between people. But the thousands of tiny insects released last week weren’t your average mosquito.

They were grown in a laboratory to carry a substance that is not common in this type of mosquito: bacteria called wolbachia. When the bacteria-laden male mosquitoes are released outdoors and mate with naturally born females, the resulting eggs do not hatch.

The result is fewer dengue cases in the areas where the laboratory-grown insects were released, according to the Singapore government.

Scientists and governments expand such high-tech solutions as the dengue virus threat increases. Some use genetically modified mosquitoes; others zap them with x-rays to sterilize them.

The World Health Organization says roughly half the world’s population is at risk of developing dengue fever, a viral infection that causes an intense flulike disease that is sometimes fatal. Increasing urbanization and crowded cities have given mosquitoes a huge human population. Reported cases of the disease rose from around 500,000 in 2000 to 4.2 million in 2019, with tropical countries like Brazil, Indonesia and the Philippines being particularly hard hit.

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

Dengue fever is transmitted by the female Aedes aegypti mosquito, which also spreads other diseases such as zika, which can cause serious birth defects in infected pregnant women, and chikungunya, which causes fever and joint pain. Public health campaigns have traditionally focused on simple solutions, such as: For example, encouraging people to drain stagnant water from household items such as vases, buckets and watering cans that mosquitoes lay eggs in. Insecticides are also used in dengue-prone areas.

But mosquitoes have developed immunity to common insecticides, and dengue cases are increasing around the world. Because of this, scientists have changed or modified the mosquitoes themselves.

In Singapore, which has long suffered from dengue outbreaks, specialized mosquito breeding began with mosquito eggs shipped from Michigan. A team led by Zhiyong Xi, a professor at Michigan State University’s Department of Microbiology and Molecular Genetics, injected Wolbachia into mosquito eggs that resembled tiny grains of dirt that had been laid 90 minutes earlier using long, thin glass needles. When the larvae hatched, they also contained the bacteria.

This first generation passed the Wolbachia bacteria on to their offspring and gave birth to a new line of bacteria-infused mosquitoes whose eggs were shipped to Singapore to form the city-state’s colony.

Before the offspring could be released, the females had to be separated from the males, who do not bite or transmit the dengue virus. Sorting by gender is crucial as the Singapore program relies on the mating of men who contain bacteria and women who don’t. If both sexes carried the bacteria, the mosquitoes would reproduce successfully and defeat the program’s goal of reducing the local mosquito population.

A machine developed by Verily, a life sciences company of Alphabet Inc., uses automated mechanical screens to separate female mosquito pupae – which are generally larger – from males. This step removes about 95% of women, the company says.

A computer vision system will be used to identify any women who may have missed the sieve. The system looks for the woman’s pronounced trunk or mouth, antenna, and other anatomical clues and marks them for removal. Truly, it is said that significantly less than one in a million mosquitoes it releases are female, which prevents Wolbachia from being inherited by the wild mosquito population.

Not all Wolbachia mosquitoes released in Singapore are screened through Alphabet’s machine. Others are subjected to low-dose X-ray irradiation using a specific method that Singapore developed in collaboration with the International Atomic Energy Agency. The irradiation sterilizes female mosquitoes so that accidentally released mosquitoes cannot reproduce Wolbachia and pass it on to future generations.

According to the Singapore government, parts of the city where the males were released had 65% to 80% fewer cases of dengue than areas where the mosquitoes were not released. Mosquitoes are now being released in 5% of the city’s public apartment blocks. Publications are to be expanded to 15% by 2022.

Other programs want the Wolbachia to be widespread in wild populations. This is because these programs found that the bacterium had another trait: it greatly reduced the Aedes aegypti mosquito’s ability to transmit dengue fever to humans.

The World Mosquito Program, a nonprofit that operates in a dozen countries in Asia, the Pacific, and Latin America, has released bacterial mosquitoes that contain both male and female bacteria in the Indonesian city of Yogyakarta. Female mosquitoes have been calculated to produce offspring that also contain bacteria, which means that dengue blocking function is passed on.

The study found a 77% reduction in dengue cases in areas where the mosquitoes were released compared to areas where they weren’t, the nonprofit said in August.

This method is much simpler than Singapore’s technique, which involves complex gender sorting. However, some scientists say the release of women with Wolbachia may be irreversible. If the Wolbachia is found to have unintended consequences, it would be very difficult to extract the bacteria from the mosquito population, it is said.

A laboratory study found that wearing Wolbachia increased the rate of West Nile virus infection in the Culex tarsalis mosquito species endemic to North America. “It’s a big black box,” said Jason Rasgon, professor of disease epidemiology at Pennsylvania State University, arguing that Wolbachia’s effects on the transmission of other diseases should be studied more closely before further large-scale releases.

Cameron Simmons, director of the World Mosquito Program, said many governments have carried out risk assessments of his approach. “All in all, Wolbachia presented negligible risk compared to doing nothing,” he said.

One company goes in a different direction overall: genetic engineering. Oxitec, a US biotechnology company with research bases in the UK and Brazil, inserts a new gene into eggs that causes female mosquitoes to die shortly after hatching while they are still in the larval stage of development.

Last year, in Indaiatuba, Brazil, near São Paulo, Oxitec conducted a test of its latest genetically modified version, which is called the OX5034. For the trial, the company produced OX5034 eggs in a factory in Brazil and distributed them to release points in the community. When the eggs hatched, the females died before they could grow up, which could fly and bite.

The males that reached adulthood mated with native wild females, passed on the female killing genes, and reduced Aedes aegypti mosquito counts by about 95%, Oxitec said.

The company received US federal approval in May for pilot versions in Florida, which are expected to begin next year.

According to Oxitec, the genes they added are self-limiting, which means that after a few generations – around three to four months – the female target gene will be bred from the species. Communities that wish to continue this approach would continue to release OX5034 eggs to keep the mosquito population in check, and those that would not have an exit yet.

Jeffrey Powell, a biology professor at Yale University, sees disadvantages in the gene modification approach. He said the need for regular re-releases would become expensive, and over time, wild mosquitos could adapt to avoid mating with Oxitec’s genetically condemned males. “There’s no evidence it’s doing anything bad,” he said of the genes Oxitec introduced into mosquitoes. “It’s completely unknown.” He said he was more comfortable using Wolbachia, which is naturally found in many species of mosquitoes.

Oxitec claims to have released around a billion mosquitoes in the past decade, and there is no evidence that female mosquitoes selectively mate with non-Oxitec males.

“There is no ecological footprint. There is no persistence, “said Kevin Gorman, who leads field operations for Oxitec.” It won’t permanently change the environment at all. “

Write to Jon Emont at

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