Malaria is a major cause of illness and death in tropical and subtropical regions. The use of insecticidal nets, which are draped over beds while people sleep, has significantly reduced the incidence of malaria and 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 article published in PNAS indicates that this may not be as grim as it seems. Mosquitoes resistant to the insecticides still suffer ill effects, including reduced lifespan and reduced ability to transmit malaria. So insecticide-laden sleeping nets should have a strong effect on malaria transmission, even among insecticide-resistant mosquito populations.
While most mosquitoes die within hours of exposure to the insecticides on mosquito nets, these experiments were designed to examine the populations of mosquitoes that survive 24 hours or more after exposure. As mosquito populations are depleted through insecticide use, more and more of the surviving generations of mosquitoes will fall within this group with longer post-exposure survival, so understanding these insects is important for future efforts to prevent malaria.
The researchers conducted a series of experiments with multiple mosquito strains known to have resistance to insecticides. These strains were exposed to insecticides daily or every four days, or they received a combination of insecticide exposure plus a blood meal. The researchers used a Bayesian survival model to examine expected survival after exposure, to predict the effects of delayed mortality and to estimate the insects’ transmission potential.
The authors found that mean daily survival was significantly lower in mosquitoes exposed to the insecticide compared to mosquitoes not exposed, even if the exposed group showed insecticide resistance. Although the effects of delayed mortality varied by mosquito species, all mosquito strains experienced delayed mortality (ranging from 72 hours to 10 days) from repeated exposure to the insecticide nets. This finding suggests that the mosquitoes that continue to survive repeated exposure to the insecticide nets still have shorter lifespans, which should hinder their ability to spread malaria.
The authors think their findings explain a critical malaria paradox: Although insecticide resistance is becoming more common in mosquitoes in subtropical and tropical regions, malaria cases continue to decline. The delayed mortality observed here could explain this discrepancy – while the mosquitoes may not die 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 exposure is likely to diminish significantly; it is inevitable that another method is needed to reduce the transmission of malaria. But for now, these insecticide-laden nets remain effective at reducing transmission from both normal mosquitoes and insecticide-resistant mosquitoes.
PNAS2016. DOI: 10.1073/pnas.1603431113 (About DOIs).