Mosquitoes that carry dengue fever and other viruses have developed increasing resistance to insecticides in parts of Asia, and new ways to control them are desperately needed, new research warns. Health authorities routinely mist mosquito-infested areas with clouds of insecticide, and resistance has long been a problem, but the extent of the problem has not been well understood.
Japanese scientist Shinji Kasai and his team studied mosquitoes from several countries in Asia, as well as Ghana, and found that a series of mutations made some of them virtually impervious to popular pyrethroid chemicals such as permethrin.
Director of the medical entomology division of Japan’s National Institute of Infectious diseases Kasai said “In Cambodia, more than 90 percent of Aedes aegypti mosquitoes have a combination of mutations that lead to an extremely high level of resistance & the level of resistance we found in mosquitoes in Cambodia and Vietnam is quite different”.
He found that some strains of mosquitoes have 1000 times the resistance compared to 100 times the resistance before. That meant an amount of insecticide that would normally kill nearly 100 percent of the mosquitoes in the sample only killed about seven percent of the insects. Even a dose 10 times stronger killed only 30 percent of the super-resistant mosquitoes.
According to the World Health Organization Dengue can cause hemorrhagic fever and infects an estimated 100 to 400 million people annually, although more than 80 percent of cases are mild or asymptomatic. Several dengue vaccines have been developed, and researchers have also used a bacterium that sterilizes mosquitoes to deal with the virus. However, neither option is close to eradicating dengue fever, and Aedes aegypti mosquitoes transmit other diseases, including Zika and yellow fever.
Another species of mosquito
Resistance has also been found in another species of mosquito, Aedes albopictus, although at lower levels probably because it tends to feed outdoors, often on animals, and may be less exposed to insecticides than its human-loving Aedes aegypti counterparts. Research has found that several genetic changes have been linked to resistance, including two that occur near the part of mosquitoes targeted by pyrethroids and several other insecticides.
Levels of resistance varied, with mosquitoes from Ghana as well as parts of Indonesia and Taiwan still relatively sensitive to existing chemicals, especially at higher doses.
said Cameron Webb, associate professor and mosquito researcher at NSW Health Pathology and the University of Sydney said “But the research shows that “commonly used strategies may no longer be effective, there is increasing evidence that there may not be a place for current insecticide formulations to control key mosquito populations”.
He said new chemicals are needed, but authorities and researchers also need to think of other ways to protect communities, including vaccines.
When and where the resistance mutations emerged is still a mystery, but Kasai is now expanding the research elsewhere in Asia, examining more recent samples from Cambodia and Vietnam to see if anything has changed since the 2016-2019 study period.
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