Malaria is a leading cause of disease and death in tropical and subtropical regions. Use of insecticidal nets, which are draped over beds while people sleep, has significantly reduced the incidences of malaria, along with the population of mosquitoes that transmit this disease. Unfortunately, long-term use of insecticides has led to the evolution of a mosquito population that is immune to them.

A recent paper published in PNAS indicates that this might not be as grim as it seems. Mosquitoes that are resistant to the insecticides still suffer ill effects, including shortened lifespans and reduced ability to transmit malaria. So insecticide-laden sleeping nets should have a strong effect on transmission of malaria, even among insecticide-resistant mosquito populations.

While most mosquitoes die within hours of exposure to the insecticides on bed nets, these experiments were designed to examine the populations of mosquitoes that survive for 24 hours or longer after exposure. As the mosquito populations are thinned by use of insecticides, more and more of the surviving generations of mosquitoes will fall within this group with prolonged post-exposure survival, so understanding these insects is important to future malaria prevention efforts.

The researchers ran a series of experiments with multiple mosquito strains known to have insecticide resistance. These strains were exposed to insecticides either daily or every four days, or they were given a combination of exposure to insecticides plus a blood meal. The researchers used a Bayesian survival model to explore expected post-exposure survival, to predict the effects of delayed mortality, and to estimate the transmission potential of the insects.

The authors found that the mean daily survival was significantly lower among mosquitoes exposed to the insecticide when compared to mosquitoes that weren’t exposed, even if the exposed group exhibited insecticide resistance. Though the effects of delayed mortality varied by mosquito strain, all mosquito strains experienced delayed mortality (ranging from 72 hours to ten days) from repeated exposures to the insecticide nets. This finding suggests that the mosquitoes that continue to survive repeated exposures to the insecticide nets still have shortened life spans, which should hamper their ability to spread malaria.

The authors think that their findings explain a critical malaria paradox: though insecticide resistance is becoming more and more common among mosquitoes in subtropical and tropical regions, the number of malaria cases continues to fall. The delayed mortality seen here could account for this discrepancy—though the mosquitoes may not be dying immediately upon contact, repeated exposure to the insecticide reduces their lifespan and their ability to find humans to feed on.

Unfortunately, as the mosquito population becomes increasingly immune to the insecticides used in these nets, the delayed mortality effect of repeated exposures is likely to decline considerably; inevitably, some other method of reducing malaria transmission will be needed. But for now, these insecticide-laden nets remain effective at reducing transmission both by normal mosquitoes and those that are insecticide-resistant.

PNAS, 2016. DOI: 10.1073/pnas.1603431113 (About DOIs).