The potential impact is huge. If resistance renders pyrethroids in mosquito nets and sprays useless, the toll could be 120,000 malaria deaths and as many as 26 million new cases a year, according to the WHO.
“It’s a serious problem,” said Bruce Hammock, professor of entomology at the University of California, Davis, and member of the UC Malaria Research and Control Group.
Malaria kills an estimated 660,000 people – mainly children – every year.
Image: US Embassy
“Sadly it started building up in the 1950s, before there were even pyrethroids,” he said.
The culprit was DDT, the big gun in the first global war on malaria. Begun in 1955, widespread and massive spraying had remarkable initial success, with nearly 40 nations, primarily in Europe and the Americas, eradicating the disease.
But the pesticide’s ubiquity – it was also widely applied to crops – unleashed resistant mosquitoes, and by the 1970s, deaths in the remaining malarial countries rebounded to the same or even higher levels than before. Three decades later, as campaigns to eradicate malaria with pyrethroids ramped up, the trigger for resistance had already been pulled.
“Many of the resistance mechanisms to DDT cross over to pyrethroids,” Hammock explained. With cross-resistance, immunity acquired to one pesticide can transfer to a completely different chemical if the two have the same mode of action. DDT and pyrethroids both target the insect nervous system.
“So when we started using pyrethroids widely, we began to see resistance immediately because we’d already selected for it with DDT,” he said.
DDT use may rise
Ironically, with pyrethroids fading, DDT now stands as the primary chemical alternative. Earlier this month, the African Union's Heads of State and Government passed a resolution endorsing DDT for malaria control throughout its 54 member nations.
One of the original “dirty dozen” persistent organic pollutants, the compound was banned under a United Nations treaty except for limited use to control insect-borne disease. That exception was intended to be temporary: four years ago, the United Nation’s Environment Programme (UNEP) announced its goal of a 30 percent drop in DDT use by 2014, with total phase-out by the early 2020s. Now, rising pyrethroid resistance will likely derail that plan.