‘Super-Resistant’ Mosquitoes Can Survive Insecticides in Southeast Asia

Aedes aegypti, a mosquito that carries yellow fever, dengue and the Zika virus, is considered one of the world’s deadliest creatures. Now, it may be becoming a bit more dangerous. 

In a new study published Wednesday in Science Advances, researchers collected mosquitoes in several countries and found that most of those from Cambodia and Vietnam had a mutation linked to the resistance of a commonly used insecticide. The mutation, called L982W, has been previously observed in mosquitoes, but never at such a high rate, reports Joel Achenbach for the Washington Post.

“It is important to be aware that the insecticides we normally use may not be effective against mosquitoes,” Shinji Kasai, author of the study and senior research scientist at the National Institute of Infectious Diseases in Japan’s Department of Medical Entomology, tells ABC News’ Julia Jacobo. 

For one part of the study, Kasai and his colleagues collected samples of 23 populations of A. aegypti from Ghana, Taiwan, Vietnam and Indonesia. They compared those mosquitoes to samples of an insecticide-susceptible strain, Aedes albopictus, collected from the same areas. They sprayed both mosquitoes with two different doses of permethrin—a pyrethroid insecticide that targets the nervous system—and looked at their mortality rates. Except for one population in Taiwan, all A. aegypti populations showed a lower mortality rate than A. albopictus. 

The lower dosage—which is equivalent to a 99 percent lethal dose for the less-tolerant A. albopictus strain—resulted in lower than 20 percent mortality in nearly all A. aegypti populations, except three Taiwanese and one Ghanaian population. After raising the dose by ten times, only two populations from Ghana had a 100 percent mortality rate. In one population from Vietnam and one in Indonesia, the mosquitoes still had a mortality rate of less than 30 percent. 

The researchers then analyzed some of the mosquitoes’ DNA to find mutations related to insecticide resistance. After examining other samples from Singapore and Cambodia, they found that more than 78 percent of mosquitoes sampled in Cambodia and Vietnam had the L982W mutation.

They also discovered mosquitoes with a combination of mutations that included L982W, making them “super-resistant,” with “substantially higher levels of pyrethroid resistance than any other field population ever reported,” the authors write in the study. More than 90 percent of the mosquitoes in Phnom Penh, Cambodia, had this extreme resistance, making it “nearly impossible to control A. aegypti with pyrethroids in this region.” 

The L982W mutation has not yet been identified outside of Southeast Asia, but the Post reports the insects are likely to spread, perhaps through international trade. 

“Aedes mosquitoes can inhabit anywhere. They like artificial water containers including jars, used tires, plastic cups, basins and pods,” Kasai tells the Post. “I think it is impossible to eliminate such water containers.”

Insecticide-resistant mosquitoes are a growing public health concern. In the past 50 years, dengue cases have increased 30-fold, the authors write in the study. The CDC estimates around 400 million people are infected with dengue each year, and about 21,000 die. With no affordable vaccine or medication available, insecticide is the only way to manage dengue epidemics, per the authors. 

The new study demonstrates the danger of insecticide resistance in mosquitoes, says David Weetman, a vector biologist at the Liverpool School of Tropical Medicine in England, to New Scientist’s Jason Arunn Murugesu.

“Whether the newly identified mutant combination in this study represents a greater threat or has greater potential for spread is unclear,” Weetman, who was not involved in the study, tells New Scientist. “This will depend on the balance of fitness benefits and costs in the wild, for which evidence—beyond quite high frequencies in Vietnam and Cambodia—is lacking.”

“It does suggest that control programs dependent on pyrethroid spraying should consider alternatives,” he says to the publication.

Margaret Osborne | | READ MORE

Margaret Osborne is a freelance journalist based in the southwestern U.S. Her work has appeared in the Sag Harbor Express and has aired on WSHU Public Radio.