Spread of insecticide resistance in Africa threatens to derail malaria prevention, study warns

Mosquitos are increasingly carrying genes that make them resistant to insecticides used across Africa to combat the spread of malaria, treat mosquito nets and clear breeding grounds.

A major study of the insect's populations, published as a letter in the journal Nature, found clear signs of the recent, widespread transmission of key resistance genes across Africa.

Two closely-related species of anopheles mosquito, which combined account for the majority of African malaria cases, are among the most genetically diverse species in the world, the research found.

This is likely to be the secret of their success at rapidly adapting to withstand insecticide treatments and other malaria controls.

The report comes as the World Health Organisation (WHO) released its annual report on malaria prevention warning that a lack of funding had seen decreasing death rates stall.

It identified insecticide resistance as a growing problem with 61 out of the 76 countries with an endemic malaria problem showing resistance to at least one of the four key insecticide groups.

This is because insecticide treatments for mosquito nets and the internal walls of homes in at risk regions are “the most commonly used method” of preventing bites and lower risk of contracting malaria by half.

In 2016 91 countries reported 216 million malaria cases, an increase of five million on 2015.

There were 445,000 malaria deaths, and these to have “stalled, and even reversed in some regions” after years of decline, the WHO says.

The report looked at mosquito populations across 15 sites across Africa and revealed key insights into how mosquitos migrate and develop resistance.

“The sustainability of malaria control in Africa is threatened by the rise of insecticide resistance in Anopheles mosquitoes, which transmit the disease” it said.

“Strong signals of recent selection were observed in insecticide-resistance genes, with several sweeps spreading over large geographical distances and between species.”

It notes that as early as 1899 global health experts were confidently suggesting that destroying anopheles mosquito breeding sites, wetlands and still bodies of water, could bring malaria under control.

But it “has proved resilient to a century of attempts to repress it” and the unexpected diversity revealed in this study could mean more modern tools are rendered ineffective.

Gene drives, a genetic engineering technology to rapidly spread desirable traits in a wild population, have been proposed as a future control for malaria.

This could be made possible by introducing genes for infertility, or which prevent mosquitos harbouring the malaria parasite, into the wild.

But the paper says: “High levels of natural diversity have practical implications for the development of gene-drive technologies for mosquito control.”

But natural variation in the mosquito population would make it harder to spread these traits, and would “undermine gene-drive efficacy in the field.”

One of the authors, Professor Martin Donnelly, from the Liverpool School of Tropical Medicine and honorary faculty at the Wellcome Trust Sanger Institute, which funded the research, said the research highlights “severe challenges” in controlling malaria.

“We have been able to see that a diverse array of genes linked to insecticide resistance are under very strong selection, confirming that they are playing an important role in the evolution of insecticide resistance in natural mosquito populations.”

While the study’s lead author Alistair Miles, from the University of Oxford and the Wellcome Trust Sanger Institute, said the data was a “unique resource” for designing better ways to control malaria in future.

Jo Lines, a professor of malaria control and vector biology at the London School of Hygiene & Tropical Medicine who was not linked to the study, said this "landmark" study was an "essential" resource.

“Over the last 15 years, the increased use of tools such as insecticide-treated nets has prevented more than 400 million cases of malaria," he said.

“Although work is taking place to combat insecticide resistance, it remains a neglected area. This research will provide us with a much better understanding of recent evolution in the world’s most important malaria vectors, and it could help us target novel control measures more effectively, enabling further progress in the fight against malaria.”