Map Shows Growing Insecticide Resistance in Africa's Malaria-Transmitting Mosquitoes

By Linda Carroll

June 29, 2020

(Reuters Health) - Newly developed maps of insecticide resistance among mosquitoes that carry malaria show that increasing numbers of the blood-sucking insects are becoming immune to the five most commonly used insecticides.

An international team of researchers analyzed data collected throughout mainland Sub-Saharan Africa and used it to construct a model that allowed them to map the spread of insecticide resistance among Anopheles gambiae complex mosquitoes, according to the report in PLoS Biology.

"We have seen rapid increases in insecticide resistance and there are important differences between locations," said study coauthor Catherine Moyes, an associate professor at the University of Oxford in the UK.

The researchers haven't yet correlated the dramatic rise in the number of insecticide resistant mosquitos with an increase in malaria cases, Moyes said in an email.

"There is anecdotal evidence for control failure linked to insecticide resistance and now that we have quantified trends in resistance the next step is to quantify the impact on infections," Moyes said.

"We are using this approach to develop tools to aid control programs in targeting the deployment of new, and more expensive, malaria control products," she said.

In 2018 there were 405,000 deaths due to malaria, with 67% of them in children under 5 years old, according to the World Health Organization. The vast majority of cases, 93%, and deaths, 94%, occurred in the WHO African Region.

To get a better sense of how serious resistance has become and where it is worst, the researchers analyzed information from a published database containing 6,423 observations of resistance phenotypes in A. gambiae populations at 1,466 different locations throughout mainland Sub-Saharan Africa between 2005 and 2017.

During the 12-year study period, resistance to pyrethroids - the only class of insecticides used in treated bednets - increased. In West Africa, for example, the proportion of mosquitoes resistant to deltamethrin rose from 15% in 2005 to 98% in 2017. A smaller rise, from 9% to 45%, was seen in East Africa.

Similar increases in resistance were seen with the insecticide DDT, which is often used for indoor spraying to kill malaria-transmitting mosquitos.

In West Africa, 53% of mosquitoes were resistant to DDT in 2005, rising to 97% by 2017. In East Africa, resistance to DDT rose from 32% in 2005 to 45% in 2017.

The new study is "really interesting," said Dr. Peter Hotez, dean of the National School of Tropical Medicine and a professor in the departments of pediatrics, molecular virology and microbiology at the Baylor College of Medicine in Houston.

"It's predictive mapping that shows where the hotspot areas of resistance are popping up," Dr. Hotez said. "It's extremely important and timely. While all the discussion this year has been about COVID-19, malaria is one of the most important infections of humankind and insecticide resistance is a global problem."

These types of data are really welcome, Dr. Hotez said. "It's a very useful approach," he said, adding that he hopes to see the maps validated with on-the-ground information.

"It provides some guidance to the geography of insecticide resistance and tells us where we need to target new approaches, including those being developed by IVCC (Innovative Vector Control Consortium) in Liverpool and some of the transgenic mosquito approaches," Dr. Hotez said.

SOURCE: PLoS Biology, online June 25, 2020.