Mosquitoes now resistant to insecticides in countries neighbouring Thailand

In Thailand's neighbouring country of Cambodia and Vietnam, insecticide-resistant mosquitoes have been found, raising concerns.

There has been no research on mosquitoes in Thailand yet.

The Aedes aegypti mosquito is one of the main vectors of yellow fever, dengue and Zika virus.

See : Alert against intensifying dengue epidemics in Thailand

Insecticides are used to control populations of these insects in tropical and subtropical regions of the world.

Many of these insecticides belong to a class of chemicals called pyrethroids, which target the nervous system of insects and cause paralysis and death.

But a new study has found that some mosquitoes have developed resistance to pyrethroids.

In a study conducted by Shinji Kasai and colleagues at the National Institute of Infectious Diseases in Tokyo, 23 mosquito populations from Ghana, Taiwan, Vietnam and Indonesia were sampled.

A high dose of permethrin, a commonly used pyrethroid insecticide, was sprayed into each mosquito population.

Nevertheless, some populations have been more damaged than others.

In the most resistant population, which came from Vietnam, less than 20 % of the mosquitoes were killed.

Subsequently, Kasai and colleagues analysed the genomes of two mosquito populations that had been found to be particularly resistant to insecticides in Vietnam.

According to their findings, pyrethroid resistance was linked to a particular mutation, called L982W.

This mutation, along with three others previously linked to pyrethroid resistance, was found to be present in mosquito populations in Singapore and Cambodia that had demonstrated high levels of resistance to the insecticide.

Ten different strains of mosquitoes were found to be resistant to pyrethroids, some of which contained the L982W mutation combined with other mutations.

Approximately 78 % of the mosquitoes collected in Vietnam and Cambodia were of one of these strains.

These mosquitoes carrying the L982W mutation were able to tolerate 50 to 100 times more pyrethroids.

In addition, the team identified mosquitoes with a combination of mutations, including the L982W mutation, that were able to withstand 500 to 1000 times higher doses of pyrethroids.

The majority of mosquitoes collected in Phnom Penh, Cambodia, were of this strain.

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Kasai said neighbouring countries, such as China and Thailand, should determine whether these mosquitoes are also prevalent there.

"We need to determine whether these mutations are spreading.

According to David Weet man, from the Liverpool School of Tropical Medicine in the UK, the study highlights the danger of pyrethroid resistance.

"It remains to be seen whether the combination of mutants newly identified in this study poses a significant threat or has a high potential to spread," he says.

This indicates that control programmes that rely on pyrethroid spraying may need to consider other options.