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An “Aedes aegypti” mosquito pictured at the Department of Medical Entomology at the National Institute of Infectious Diseases in Tokyo. Photo: Shinji Kasai via AFP

Asia’s super-resistant mutant mosquitoes swat away insecticides: ‘we have to think of a solution’

  • A team of Japanese scientists found mutations in mosquitoes from Cambodia and Vietnam that make some virtually impervious to popular insecticides
  • It’s sparked concern that new control methods need to be found before the mutations spread to mosquitoes elsewhere in the region – and the wider world
Mosquitoes that transmit dengue and other viruses have evolved growing resistance to insecticides in parts of Asia, and novel ways to control them are desperately needed, new research warns.

Health authorities commonly fog mosquito-infested areas with clouds of insecticide, and resistance has long been a concern, but the scale of the problem was not well understood.

Japanese scientist Shinji Kasai and his team examined mosquitoes from several countries in Asia as well as Ghana and found a series of mutations had made some virtually impervious to popular pyrethroid-based chemicals like permethrin.

“In Cambodia, more than 90 per cent of Aedes aegypti mosquitoes have the combination of mutations that results in an extremely high level of resistance,” Kasai said.

Cambodian children look at a larger-than-life mock up of a mosquito mounted on a health ministry truck in Phnom Penh. Insecticide resistance was found to be particularly high among Cambodian mosquitoes. Photo: Reuters

He found some mosquito strains had 1,000-fold resistance, compared to the 100-fold seen previously.

That meant insecticide levels that would normally kill almost 100 per cent of mosquitoes in a sample killed only around seven per cent of the insects.

Even a dose 10 times stronger killed just 30 per cent of the super-resistant mosquitoes.

“The resistance level that we found in mosquitoes in Cambodia and Vietnam is totally different,” said Kasai, director of the Department of Medical Entomology at Japan’s National Institute of Infectious Diseases.

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Dengue can cause haemorrhagic fever and infects an estimated 100 to 400 million people a year, although over 80 per cent of cases are mild or asymptomatic, according to the World Health Organization.

Several dengue vaccines have been developed, and researchers have also used a bacteria that sterilises mosquitoes to tackle the virus.

But neither option is yet close to eradicating dengue, and Aedes aegypti mosquitoes carry other diseases, including Zika and yellow fever.

Resistance was also detected in another type of mosquito, Aedes albopictus, though at lower levels – possibly because it tends to feed outdoors, often on animals, and may be exposed to insecticides less than its human-loving Aedes aegypti counterparts.

We’re worried that the mosquitoes with the mutations … will spread to the rest of the world in the near future
Shinji Kasai, Japanese infectious-disease researcher

The research found several genetic changes were linked with resistance, including two that occur close to the part of mosquitoes targeted by pyrethroid and several other insecticides.

Resistance levels differed, with mosquitoes from Ghana as well as parts of Indonesia and Taiwan still relatively susceptible to existing chemicals, particularly at higher doses.

But the research shows “commonly employed strategies may no longer be effective,” said Cameron Webb, an associate professor and mosquito researcher at NSW Health Pathology and the University of Sydney.

“There is growing evidence that there may not be a place for current insecticide formulations in controlling populations of key mosquito pests,” Webb said.

A health worker sprays chemicals in Colombo, Sri Lanka, to control dengue-carrying mosquitoes. Mutations have made some mosquitoes highly resistant to insecticides. Photo: Xinhua

He said new chemicals are needed, but authorities and researchers also need to think of other ways to protect communities, including vaccines.

“We have to think about rotating insecticides … that have different target sites,” added Kasai, whose research was published last month in the journal Science Advances.

“The problem is that we don’t have so many different kinds that we can use.”

Other options include more efforts to remove breeding sites.

Experts slap down proposal to rid China of mosquitoes

When and where the mutations for resistance emerged is still a mystery, but Kasai is now expanding the research elsewhere in Asia and examining more recent samples from Cambodia and Vietnam to see if anything has changed from the 2016-2019 study period.

“We are worried that the mosquitoes with the mutations that we found in this study will spread to the rest of the world in the near future,” he said.

“Before that, we have to think of a solution.”