More than three quarters of Aedes aegypti mosquitoes sampled in Vietnam and Cambodia came from strains extremely resistant to pyrethroid insecticides
Mosquitoes extremely resistant to insecticides have been discovered in Cambodia and Vietnam, raising concerns about the implications for controlling infectious diseases.
Aedes aegypti mosquitoes are major carriers of yellow fever, dengue fever and the Zika virus. The insects are common in tropical and subtropical regions of the world and their population numbers are largely controlled using insecticides.
Many of these belong to a class of chemicals called pyrethroids that target the nervous system of insects and cause paralysis and death.
Pyrethroid resistance is a widely recognized problem in controlling mosquito populations, but it’s unclear how big the problem really is.
Shinji Kasai of the National Institute of Infectious Diseases in Tokyo and his colleagues sampled 23 mosquito populations from Ghana, Taiwan, Vietnam and Indonesia.
The team sprayed mosquitoes in each population with a large dose of permethrin, a commonly used pyrethroid insecticide, which should theoretically kill 99 percent of the insects.
Chemicals sprayed to kill mosquitoes for Zika virus prevention in the Vietnamese city of Danang. Archive photo
However, some populations died from the insecticide in greater numbers than others. Less than 20 percent of mosquitoes in the most resistant population, from Vietnam, were killed.
Kasai and his colleagues then analyzed the genomes of two populations of mosquitoes from Vietnam with particularly high resistance to the insecticide. They found that a specific mutation called L982W was linked to pyrethroid resistance.
The researchers looked for this mutation, as well as three others previously associated with pyrethroid resistance, in mosquito populations from Singapore and Cambodia that showed high levels of resistance to the insecticide. They found 10 different mosquito species — some of which contained L982W in combination with other mutations — that were resistant to pyrethroid.
They estimate that more than 78 percent of the mosquitoes they collected in Vietnam and Cambodia belonged to one of these tribes. Mosquitoes with the L982W mutation showed a 50- to 100-fold increase in the amount of pyrethroid they could resist.
The team also identified mosquitoes with a combination of mutations, including L982W, that could survive 500 to 1,000 times higher doses of pyrethroid. More than 90 percent of the mosquitoes collected in Cambodia’s Phnom Penh belonged to this species.
Kasai says neighboring countries, such as China and Thailand, should determine whether these insecticide-resistant mosquitoes are also present there. “We need to see if these mutations spread.”
David Weetman of the Liverpool School of Tropical Medicine in the UK says the study highlights the danger of pyrethroid resistance.
“Whether the newly identified mutant combination in this study poses a greater threat or has a greater spreading potential is unclear,” he says. “This will depend on the balance between fitness benefits and costs in the wild, for which evidence – apart from fairly high frequencies in Vietnam and Cambodia – is lacking.”
“It does suggest that control programs that rely on pyrethroid spray should consider alternatives, although this should probably already have been the case given the widespread nature of pyrethroid resistance, albeit perhaps not at the level identified in the study,” he says.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abq7345
By By Jason Arunn Murugesu @. newsscientist.com. Read original story here