Early warning of alpha-cypermethrin resistance in mosquitoes

Public-health programmes throughout the tropics use alpha-cypermethrin sprays to kill mosquitoes that transmit dengue, Zika and chikungunya. The insecticide acts quickly and is effective at low doses.

However, a laboratory strain of one of these mosquito species has started to withstand it. Researchers in New Delhi carried out a routine test, saw several insects survive a dose expected to kill almost all of them, and investigated why.

Mosquitoes survive diagnostic dose

The assessment used was the WHO bottle bioassay. Female mosquitoes were placed in glass bottles coated with the insecticide for one hour, then counted 24 hours later. Every bottle contained 20 mosquitoes.

At the diagnostic dose - the concentration used to screen for resistance - 97.91% of the mosquitoes died. The significance lies in the range into which that result falls.

The World Health Organization (WHO) classifies mortality of 90 to 98% as possible resistance requiring further investigation. A population is considered fully susceptible only from 98% mortality. This mosquito population was therefore not fully susceptible.

“We show that an Indian Ae. aegypti mosquito population demonstrated a 97.91% mortality rate when exposed to the recommended diagnostic dose of α-cypermethrin. This is an early sign these mosquitoes could be developing resistance to this insecticide,” says Dr. Rohit Lakhwani, the study’s first author at the University of Delhi.

One enzyme dominates

Insects produce defensive proteins called enzymes, which break toxins into harmless components that can then be removed from the body. The researchers examined five such enzymes.

One had a much stronger effect than the others. β-esterase targets the chemical bond that holds alpha-cypermethrin together. Following exposure, its activity increased by around 21-fold, rising from approximately 11 units per milligram of protein to 239.

Computer modelling supported this finding. The analysis assessed how strongly the insecticide attached to each enzyme, with β-esterase showing the tightest binding at −9.4 kcal/mol, ahead of the other four enzymes.

This was the first time alpha-cypermethrin had been assessed with this type of model against the enzymes of this mosquito.

Professor Sarita Kumar, the study’s senior author, outlines a process inside the insect in which the insecticide activates its natural defences, prompting cells to produce more of the enzymes that degrade it.

Other enzymes respond differently

The remaining four enzymes shifted in different ways. CYP450, a general detoxification enzyme, approximately doubled after exposure, and this increase was greater than would be expected by chance.

The activity of α-esterase rose by roughly seven-fold. AChE activity also increased slightly, although the change was small enough to have occurred by chance.

GST responded in the reverse direction. At the higher dose, its activity fell by about five-fold. The researchers propose that the dose may overwhelm the mosquito’s defences more rapidly than its cells can replenish them.

Laboratory mosquitoes limit conclusions

The mosquitoes were not collected in the wild. They originated from a colony maintained in a New Delhi laboratory since 2009, where they had been raised for years without insecticide exposure.

This is an important limitation. A colony that has not encountered insecticides would be expected to be fully susceptible. Those that survived may reflect normal variation between individuals or gradual genetic changes across many generations, rather than genuine resistance.

The results also concern a single population at one moment in time. Resistance differs according to region, climate and the intensity of spraying in an area, meaning mosquitoes elsewhere could already have higher levels of resistance.

India’s Department of Science and Technology (DST) funded the research. The authors declared no competing interests.

Resistance may still be reversible

“Because resistance develops at different speeds, it is not possible to predict when it will become so widespread that α-cypermethrin might not work anymore,” Kumar says.

“Resistance at the biochemical level can also reverse if that particular insecticide is no longer used.”

Several measures can reduce its spread. These include rotating insecticides, using chemicals that inhibit the mosquito’s defences, and removing breeding sites.

“Our study is valuable because it goes beyond showing that resistance exists; it helps explain how it develops at the molecular level,” Kumar says.

“It is critical that this resistance is not yet widespread. It provides an opportunity for public health authorities to implement resistance management strategies before the insecticide becomes completely ineffective.”

Early warning for mosquito control

The results provide an early indication that mosquito resistance to an important insecticide may already be emerging.

Monitoring these changes now could allow health authorities to adapt control strategies and preserve the effectiveness of existing mosquito sprays for longer.

As the study used laboratory mosquitoes, further research is required to establish whether identical changes are occurring in wild populations.

Nevertheless, identifying these warning signs early may help safeguard mosquito-control programmes against dengue, Zika and chikungunya.