Many mosquitoes are now insecticide-resistant in Vietnam and Cambodia

Mosquitoes that are 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 found widely 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, which target the insects’ nervous systems, causing paralysis and death.

Pyrethroid resistance is a widely acknowledged issue when trying to control mosquito populations, but it is unclear how big a problem it actually is.

Shinji Kasai at the National Institute of Infectious Diseases in Tokyo and his colleagues sampled 23 populations of mosquitoes from Ghana, Taiwan, Vietnam and Indonesia.

The team sprayed mosquitoes in each population with a large dose of permethrin, a commonly used pyrethroid insecticide, that should in theory kill 99 per cent of the insects.

However, some populations died in greater numbers from the insecticide than others. Less than 20 per cent of mosquitoes in the most resistant population, which came from Vietnam, were killed.

Kasai and his colleagues then analysed 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 that had previously been linked to pyrethroid resistance, in mosquito populations from Singapore and Cambodia that had exhibited high levels of resistance to the insecticide. They found 10 different strains of mosquitoes – some of which contained L982W combined with other mutations – that were resistant to pyrethroid.

They estimated that more than 78 per cent of the mosquitoes they collected from Vietnam and Cambodia belonged to one of these strains. Mosquitoes with the L982W mutation showed a 50 to 100-fold increase in the amount of pyrethroid they could withstand.

The team also identified mosquitoes with a combination of mutations, including L982W, that could survive 500 to 1000 times higher doses of pyrethroid. More than 90 per cent of the mosquitoes collected in Phnom Penh in Cambodia belonged to this strain.

Kasai says neighbouring countries, such as China and Thailand, should determine if these insecticide-resistant mosquitoes are prevalent there too. “We need to see if these mutations are spreading.”

David Weetman at 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 represents a greater threat or has greater potential for spread is unclear,” he says. “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 programmes dependent on pyrethroid spraying should consider alternatives, though this probably should have been the case already given the widespread nature of pyrethroid resistance, albeit perhaps not to the level identified in the study,” he says.