A new study warns that mosquitoes that spread dengue and other viruses are becoming more resistant to pesticides in some regions of Asia, and that new approaches are urgently required to combat this problem. Insecticide fogging is a routine practice by health officials in mosquito-infested regions, and although resistance has been a worry for some time, the full scope of the issue has only just been grasped.
Mosquitoes in many Asian nations and Ghana were studied by Japanese scientist Shinji Kasai and his colleagues, who discovered that a sequence of changes had rendered some of them nearly immune to common pyrethroid-based pesticides like permethrin.
Some mosquito strains demonstrated resistance that was 1,000 times as high as the previous 100-fold threshold. This meant that just around 7 percent of mosquitoes were killed at doses of pesticide that would ordinarily kill about 100 percent. Just 30% of the ultra-resistant insects were killed with a treatment 10 times greater. Mosquitoes in Cambodia and Vietnam have quite diverse resistance levels.
The World Health Organization (WHO) estimates that between 100 and 400 million individuals are infected with dengue each year, with over 80% of those infections being either mild or asymptomatic.
A number of dengue vaccines have been created, and a microbe that sterilises mosquitoes has been employed to combat the virus. Neither strategy has been effective enough to completely eliminate dengue, and Aedes aegypti mosquitoes also spread Zika and yellow fever.
Another mosquito species, Aedes albopictus, was shown to be resistant as well, but at lower levels. This may be due to the fact that Aedes albopictus feeds mostly on animals rather than humans, and hence may be less often exposed to pesticides.
Several genetic alterations were associated with resistance, notably two that are located near the mosquito organ that is the target of pyrethroid and other pesticides.
Mosquitoes showed varying degrees of resistance to current pesticides; some, like those found in Ghana, Indonesia, and Taiwan, are still vulnerable, especially to larger dosages.