Biological adaptations, as Darwin taught us, are produced by natural selection and favor an improved ability to survive within a given environment. A new study provides evidence of a new and unfortunate adaptation in the mosquito population of West Africa. Interbreeding of two malaria mosquito species in the country of Mali has resulted in a new hybrid — a "super mosquito" that is resistant to insecticide. What’s most painful to consider here is the fact that this particular adaptation is a response to a significant man-made change to the West African environment: widespread use of life-saving bed nets.

Mosquito-Borne Illness

Malaria is a mosquito-borne illness, which, according to the most recent World Health Organization report, affects 97 countries and territories and places an estimated 3.2 billion people at risk. Sadly, an estimated 453,000 children died before their fifth birthday in 2013 due to malaria, which is not only treatable but also preventable. However, there is some good news: The expansion of malaria interventions between 2000 and 2013 have helped to reduce worldwide malaria incidence by 30 percent, while global mortality rates from the illness plummeted by an estimated 47 percent.

How has this substantial decline in deaths come to pass? In part, through the use of insecticide-treated nets as a preventive measure. In Africa alone, an estimated 214 million long-lasting insecticidal nets (LLINs) were delivered to malaria-endemic countries during 2014, boosting the overall total number of LLINs in the region (since 2012) to 427 million. While most everyone would consider this a beneficial achievement, medical entomologists and epidemiologist understood from the start that it would just be a matter of time before insecticide resistance emerged.

For the current study, then, a team of researchers led by Dr. Gregory Lanzaro, professor in the School of Veterinary Medicine at UC Davis and lead author, investigated the African situation. Soon enough, they discovered what they feared: Anopheles gambiae, a major malaria mosquito, is interbreeding with isolated pockets of another malaria “vector,” Anopheles coluzzii. While entomologists originally believed these two mosquitoes to be the "M and S forms" of the same species (A. gambiae), now they are recognized as separate species.

By mating, these species have created a new “super mosquito,” at least “super with respect to its ability to survive exposure to the insecticides on treated bed nets," said Lanzaro, who noted how the new research “provides convincing evidence indicating that a man-made change in the environment — the introduction of insecticides — has altered the evolutionary relationship between two species, in this case a breakdown in the reproductive isolation that separates them.”

Having observed growing resistance for some time, the new insecticide-resistant species comes as no surprise to the researchers. "Recently it has reached a level at some localities in Africa where it is resulting in the failure of the nets to provide meaningful control, and it is my opinion that this will increase," Lanzaro said. He explained this means there is now “an urgent need to develop new and effective malaria vector control strategies.”

In a press release, Lanzaro outlined the new strategies in development, including new insecticides and biological agents — imagine: mosquito-killing bacteria and fungi! Scientists are also working on genetic manipulation aimed at either killing the mosquitoes or at least altering their ability to transmit malaria.

Source: Norris LC, Main BJ, Lee Y, et al. Adaptive introgression in an African malaria mosquito coincident with the increased usage of insecticide-treated bed nets. PNAS. 2015.

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