MANILA — Long-lasting insecticide-treated bed nets have been a lifesaving tool against malaria, but increased resistance against pyrethroids — the key chemical ingredient used — had researchers looking for alternatives or adjustments. Now, they may have found one.
In a major trial in Uganda, researchers found that although traditional pyrethroid-treated bed nets continue to be effective, households given bed nets with a combination of pyrethroids and the chemical piperonyl butoxide experienced 27% fewer cases of malaria among children ages 2 to 10. These households also had 80% fewer malaria-carrying mosquitoes.
“We need to guard against complacency, mosquito genomes are highly diverse, and it is almost inevitable that resistance will emerge.”— Martin Donnelly, head of vector biology department, Liverpool School of Tropical Medicine
“The results from Uganda show that by using an insecticide formulation that interferes with the mechanisms mosquitoes use to defeat pyrethroids, we are seeing a significant restoration of pyrethroid efficacy — not back to pre-resistance levels, but enough to buy us critically needed time while we develop new insecticides and interventions,” said Martin Donnelly, co-leader of the evaluation project. The results of the trial were presented Thursday at the annual meeting of the American Society of Tropical Medicine and Hygiene.
Nets with piperonyl butoxide have been used on a small scale in countries such as Tanzania, but Donnelly told Devex that many health ministers in Africa are awaiting clear guidelines from the World Health Organization.
The combination formula received a conditional recommendation from WHO in 2017, and its Vector Control Advisory Group is expected to make another assessment in 2020 to determine whether to provide a full recommendation for the intervention.
The success of a recent experiment offers a potential pathway.
Other insecticide-treated nets under review by the WHO advisory group include one containing a combination of pyrethroid and chlorfenapyr, an insecticide used in agriculture; and another with pyrethroid and pyriproxyfen, an insect growth regulator that sterilizes female mosquitoes.
“Staff at the Liverpool School of Tropical Medicine are also looking at redesigning the shape of the nets to exploit mosquito behavior — mosquitoes commonly land first on the top surface of nets,” Donnelly added.
But he stressed that the aid and research community must continue searching for long-term solutions.
“There are already pyrethroid resistance mechanisms in the vector populations which are not affected by [piperonyl butoxide],” said Donnelly, who is an expert in the genetics of insecticide resistance at the Liverpool School of Tropical Medicine.
“Our message is that we need to guard against complacency, mosquito genomes are highly diverse, and it is almost inevitable that resistance will emerge,” he said.
There have been increasing concerns about the impact of pyrethroid resistance on anti-malaria efforts globally. Donnelly said it’s hard to quantify, but WHO reports in recent years have shown stagnation in global progress against malaria, which “is at least partially coincident with observed increases in pyrethroid resistance,” he said.
Emphasizing the importance of insecticide-treated bed nets in the fight against malaria, Donnelly said that “between 2000 and 2015, an estimated 215 [million] cases of malaria were averted by three interventions. It is estimated that [insecticide-treated bed nets] were responsible for [approximately] 68% of this total.”