Mosquitoes, which transmit dengue fever and other sometimes serious viral diseases, have developed high resistance to insecticides in parts of Asia, and new methods to stem their spread are urgently needed, according to a recently published Japanese study.
Spraying insecticides in mosquito-infested areas is a common practice in tropical and subtropical regions. Resistance was already a problem, but the extent of the problem was not precisely known until now.
Japanese scientist Shinji Kasai and his team have studied mosquitoes from several Asian countries and Ghana and found genetic mutations that make some of them immune to common insecticides like permethrin.
“In Cambodia, more than 90% of Aedes aegypti mosquitoes, the main vectors of dengue, zika, chikungunya and yellow fever viruses, have a combination of mutations that result in extremely high levels of resistance,” said Mr. Kasai, who was interviewed by AFP.
This director of the Department of Medical Entomology at the Japan National Institute of Infectious Diseases discovered that certain species of mosquitoes that were supposed to be 100% killed by insecticides were only 7% killed. And even a tenfold higher toxic dose only killed 30%.
Resistance levels vary by region. According to Mr Kasai, for example, they are “completely different” between Cambodia and Vietnam.
His work also found that existing insecticides were still working in Ghana, parts of Indonesia and Taiwan.
Resistance to insecticides has also been observed in the “tiger mosquito” Aedes albopictus, but to a lesser extent.
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This study, published in late December by the journal Science Advances, shows that “commonly used strategies may no longer be effective in controlling populations of harmful mosquitoes,” said Professor Cameron Webb, an expert from the University of Sydney who was interviewed by AFP.
New chemicals are needed, but authorities and scientists also need to consider new methods of protection, such as vaccines, according to Webb.
Few dengue vaccines are currently available, that of the Japanese pharmaceutical company Takeda was approved by Indonesia last year and then by the European Union, while the use of the French Sanofi is very limited because it can aggravate the disease in people who have never had it have had this virus before.
According to the World Health Organization (WHO), dengue fever cases have increased dramatically worldwide over the past 20 years, with between 100 and 400 million infections per year. More than 80% of cases are mild or asymptomatic, but life-threatening complications exist.
Mr Kasai, who fears that the super-resistant mosquitoes he has identified will spread to other parts of the world “in the near future”, also recommends more variations in insecticides, but the problem is that their mode of action is often similar.
Alternatives include stepping up efforts to eliminate mosquito breeding grounds or sterilizing male mosquitoes with Wolbachia bacteria, an innovative method that has already yielded encouraging results on the ground.
Exactly where and when insecticide resistance mutations occurred in mosquitoes remains a mystery.
Mr. Kasai is now expanding his research to other parts of Asia and is also examining more recent samples from Cambodia and Vietnam to see if anything has changed from his previous study, which looked at the period 2016-2019.