A new tool to fight the world’s deadliest animal—the malaria-carrying mosquito—may soon become more widely available. Yesterday, the World Health Organization (WHO) endorsed using a new kind of bed net treated with insecticides. It combines two chemicals to more effectively kill the mosquitoes that transmit the parasite that causes malaria, a disease that killed an estimated 619,000 people in 2022, most of them young children in sub-Saharan Africa.
Insecticide-treated bed nets protect people from malaria in two ways. They prevent mosquitoes from reaching the person sleeping under the net, and they kill the mosquitoes that land on them, reducing the overall population of the deadly insects. Widespread use of the nets has been one of the main reasons malaria rates fell by an estimated 40% between 2000 and 2015. But in recent years, resistance to the main insecticide used to treat nets, pyrethroid, has been spreading. That has contributed to the rebound of malaria in many places.
“It’s high time” for the WHO recommendation, says Patrick Kija Tungu, an entomologist at the National Institute for Medical Research in Tanzania. “We need new nets” that don’t rely only on pyrethroid, he says.
The new nets endorsed yesterday are treated with pyrethroid and a second chemical called chlorfenapyr. It is a relatively new insecticide that targets mosquitoes’ mitochondria, inducing muscle cramps and preventing them from moving or flying.
This is the first time WHO has given its full recommendation to a new insecticide formulation. Last year, it gave a limited recommendation to another new type of net, which combines pyrethroid with a second substance, piperonyl butoxide (PBO), that blocks mosquitoes’ ability to break down pyrethroid. The combination was designed to help overcome some of the insects’ growing resistance. But those pyrethroid-PBO nets cost significantly more than pyrethroid-only nets, and WHO said the data didn’t make clear that the nets reduced malaria enough to justify the extra expense.
The extra killing power of the chlorfenapyr, however, seems to pay off. Two large recent studies in Tanzania and Benin showed the new-generation nets, known as IG2, reduced the incidence of malaria in children by nearly half compared with pyrethroid-only nets. That meant the savings in treatment costs significantly outweighed the higher price of the nets. That was enough for WHO to endorse use of the nets in areas with pyrethroid resistance. “The fact that the trials showed very similar, really strong evidence, gives the WHO more freedom to strongly recommend these nets,” says Corine Ngufor, an entomologist at the London School of Hygiene & Tropical Medicine who helped lead the study in Benin.
Chlorfenapyr was first introduced in the United States in 2001 for use on nonfood crops in greenhouses, but it is not allowed to be sprayed on fields because it can be toxic to birds and aquatic animals. Its use in bed nets, however, is considered environmentally safe because the treated nets hang indoors and have limited contact with the wider environment.
As the nets are rolled out, it will be important to use techniques that can slow the development of resistance to chlorfenapyr, Ngufor says. That might mean combining the use of the new nets with indoor insecticide spraying or alternating the use of IG2 nets with pyrethroid-PBO nets for a few years at a time. Resistance to chlorfenapyr is certain to emerge at some point, she says. “We need to find ways to manage that.”
Source: Science Mag