Insecticide exposure did not just make mosquitoes sick; it supercharged their immune systems against malaria parasites.Scientists have discovered that pyrethroid insecticides, which are commonly used in bed nets and indoor sprays, do more than just kill mosquitoes.
They make mosquitoes (those that are exposed to insecticide but survive) less likely to carry and transmit malaria by activating immune responses that kill the disease parasite during development inside the insect.
The study, which has not been peer reviewed, is available on bioRxiv.
It is titled, "Exposure to pyrethroid insecticides modulates immunity of Anopheles against Plasmodium falciparum."
“These findings highlight an overlooked secondary mode of action for pyrethroids and reinforce the importance of sustained pyrethroid usage in Insecticide Treated Nets, even in the face of high levels of pyrethroid resistance in mosquitoes,” the authors wrote.
This discovery offers a sigh of relief in Kenya and other African countries, where insecticide resistance in mosquitoes has raised fears about the long-term effectiveness of bed nets. Malaria kills about 10,000 Kenyans every year, according to the Kenya Malaria Indicator Survey 2020.
The study zeroed in on pyrethroids, the only insecticides approved for use in insecticide-treated nets (ITNs).
They are meant to kill mosquitoes, but over time, many Anopheles mosquitoes (the main malaria vectors) have developed resistance to these chemicals, which led scientists to investigate whether exposure still had any beneficial effects for malaria control.
The team used resistant Anopheles mosquitoes from Burkina Faso and found that even a single, non-lethal exposure to permethrin—a common pyrethroid—increased levels of reactive nitrogen species (RNS) in mosquito tissues.
These highly reactive molecules are part of the insect’s immune arsenal and are known to attack invading parasites.
“This RNS increase activates a part of the mosquito’s immune system known as the IMD pathway,” the authors explained. “It leads to nitration around the midgut epithelium—the tissue the malaria parasite must cross—and destroys the invading ookinetes,” which are early forms of the parasite.
The researchers likened this to fortifying the mosquito’s gut wall, preventing the malaria parasite from establishing infection in the insect’s body.
The scientists said they further gave mosquitoes a dietary supplement called L-arginine, a compound that boosts nitric oxide, a key RNS molecule.
Remarkably, this feeding method mimicked the immune boost seen with insecticide exposure, confirming that RNS plays a critical role in fighting the malaria parasite.
They found that both L-arginine-fed and insecticide-exposed mosquitoes had fewer parasites in their guts, lower infection prevalence, and reduced ability to transmit malaria.
“This shows that the reduction in parasite development was not just a fluke of insecticide exposure,” the authors noted. “It’s a consistent immune response tied to RNS generation.”
The most dramatic effect came early in the infection cycle. When mosquitoes had elevated RNS levels, the malaria ookinetes—the parasite form that crosses the mosquito gut—were fragmented and destroyed before they could mature into infectious forms.
One of the study’s key discoveries was that pyrethroid exposure led to a proliferation of immune cells called granulocytes, which are responsible for key defensive functions.
“We observed a rise in granulocytes from 10.8 per cent in control mosquitoes to nearly 32 per cent after insecticide exposure,” the researchers wrote.
In other words, insecticide exposure didn’t just make mosquitoes sick; it supercharged their immune systems against malaria parasites.
To understand the real impact of this immune activation, the scientists created a mathematical model simulating human and mosquito populations over one year. They found that treating just 50 per cent of mosquitoes with a sub-lethal dose of permethrin could reduce malaria infections in children under five by 12.8 per cent. At 100 per cent coverage, infections dropped by more than 30 per cent.
“These results suggest that pyrethroids may still benefit malaria control even when they don’t kill mosquitoes, by reducing the parasite load in the surviving insects,” the authors said.
The researchers are from Heidelberg University and the Bernhard Nocht Institute for Tropical Medicine, Germany.
