Evidence suggests that germ-infected mosquitoes can greatly reduce the spread of dengue, but are difficult to "make" on a large scale.
At Singapore's National Environment Agency, more than 1 million mosquitoes buzz in plastic boxes in a breeding room that smells of fermented sugar.
Male mosquitoes do not bite people and feed on plant sap in the wild, but here they feed on sugar water. Meanwhile, their female mates lay eggs on strips of paper, which are half-immersed in trays that produce 24 million tiny black eggs each week.
All the mosquitoes here are Aedes aegypti, a species that can transmit a variety of viruses and diseases to humans, including dengue, yellow fever, Zika virus disease, and Japanese encephalitis. It is estimated that about 100 million~400 million people worldwide are infected with dengue fever every year, resulting in more than 21,000 deaths. However, these captive mosquitoes are disease-free, and they breed precisely to stop the spread of disease.
Specifically, Aedes mosquitoes in Singapore's National Environment Agency laboratory are infected with a bacterium called Wolbachia, which they will pass on to the next generation.
Wolbachia bacteria are everywhere in nature: including 60% of insect species, from butterflies, bees, dragonflies, to some mosquitoes. However, these bacteria are not naturally present in Aedes aegypti. Scientists have found that Aedes aegypti mosquitoes, infected with Wolbachia, no longer transmit dengue easily to humans. In addition, in some cases, the bacteria can interfere with the reproductive ability of mosquitoes. (But the exact mechanism behind it is not fully understood)
Since 2016, scientists at Singapore's National Environment Agency have been releasing male Aedes aegypti mosquitoes carrying Wolbachia bacteria. Although the project started on a small scale, by 2019 Singapore was releasing 2 million mosquitoes per week. Thanks to automation, this number increased to 5 million per week in 2022.
Now, after more than 6 years of release, wild Aedes aegypti mosquito populations have dropped by 98% and dengue cases by 88% in some parts of Singapore.
The World Health Organization has set a target to reduce the incidence of dengue by 60% by 2030 compared to 2016.
↑ Yishun, Singapore, researchers at Singapore's National Environment Agency are releasing male mosquitoes infected with Walbachia. To stop the spread of dengue, Singapore's National Environment Agency began a release program in 2016. They now release up to 5 million mosquitoes a week.
↑ Each week, captive-bred mosquitoes lay 24 million tiny black eggs on strips of paper, which are placed in small trays filled with water until they hatch into larvae. In the wild, when male mosquitoes carrying Wolbachia bacteria mate with wild females, their eggs rarely hatch.
↑ After hatching, count and transfer to larger trays. Previously, it was necessary to count the larvae manually, but now the counting is automated: millions of larvae are poured into the machine, and within minutes it calculates the 26,000 larvae needed to fill a tray.
↑ Larval racks in the NSA laboratory were properly preserved, and scientists even perfected the incubation process to keep diet, temperature and humidity perfectly constant.
↑ A plate of waiting pupae sorted by sex. If the difference in size between males and females is large enough, they can be separated by sieve.
↑ A staff member at Singapore's National Environment Agency scans a batch of pupae using an AI-based computer system, which then outputs a distribution curve of pupae size.
↑ At Singapore's National Environment Agency laboratory, visitors can put their hands in a breeding box full of male mosquitoes.
↑ Matthew Verkaik, Senior Research Officer, National Environment Agency, Singapore, releases a laboratory-recognized male Aedes aegypti mosquito. Since the implementation of the release plan, the number of wild Aedes aegypti mosquitoes in the intervention areas of Singapore has decreased by 98% and dengue cases by 88%.
↑ Each release tank will be filled with about 200 male mosquitoes carrying Walbachia.
↑ During the release of mosquitoes into a 12-story apartment building, the average household "gained" 6 mosquitoes. In Singapore, the community strongly supported the release plan of the National Environment Agency of Singapore. In a 2021 survey, 92% of households reported that they had no concerns about the release from their community.