London School of Hygiene & Tropical Medicine

African malaria mosquitoes considered to be 'highly resistant' to insecticides may not be as invincible as previously thought, according to a new study published in the Proceedings of the National Academy of Sciences.

Although unaffected in the short term, the mosquitoes' long-term survival is reduced by almost 50% following exposure to insecticides on bednets, which reduces their disease transmission potential by two thirds.

The research, conducted by the University of Glasgow and Liverpool School of Tropical Medicine, may help explain why Long Lasting Insecticidal Nets still remain an effective tool in the battle against malaria, even in areas where most of the mosquitoes that spread the disease are not killed on immediate contact with them.

So to what degree does this resistance reduce the effectiveness of vector control efforts?   Does it render them completely useless from the start, or does it just make them somewhat less effective, or effective for a shorter period?  Dr Jo Lines, Reader of Malaria Control and Vector Biology at the London School of Hygiene & Tropical Medicine, provides expert comment on the findings.

"Over the last 15 years in Africa, the burden of disease and death due to malaria has been substantially reduced, and most of this reduction is attributable to increased coverage of anti-mosquito interventions, especially insecticide-treated nets (ITNs) and insecticide-spraying of houses.   However, these gains could easily be lost: they depend on our continued efforts and ability to suppress the malaria-transmitting vector mosquitoes.   Unfortunately, these mosquitoes are evolving rapidly: insecticide resistance is now present throughout Africa and has reached very high levels in some places

"There are various mechanisms by which ITNs might continue to suppress malaria transmission even if the mosquitoes are not immediately killed by the insecticide.   This study looked at one such mechanism: the possibility that the insecticide might produce delayed and possibly cumulative effects on survival.   The authors demonstrated in the laboratory that such delayed-mortality effects do occur when resistant mosquitoes are repeatedly exposed to conventional test doses of insecticides.   They then used transmission models to confirm that if such delayed effects occur in nature, they would indeed be expected have a major effect on transmission, maintaining at least some of the effectiveness of insecticidal interventions despite resistance in the vectors.

"This study shows that the effects of resistance are not all-or-nothing, and is a reminder that we are now deeply committed to an arms race against the mosquitoes.   In much of Africa, malaria transmission is normally and naturally very intense, but for the last few years it has been suppressed to low levels, leaving the human population with little or no immunity against malaria.   We know that if such areas were suddenly left without effective vector control, there would be catastrophic epidemics of resurgent malaria, with massive loss of life.   This study helps us to understand how we can avoid losing the arms race in the short and medium term."