In 2014 it seemed the Papua New Guinea (PNG) had malaria elimination in its sights. The average malaria prevalence in the country had dropped from 16% in 2008 down to just 1% in 2014.
But then, the numbers returned with a vengeance. For a while, it was unknown what had caused this rebound in cases, but a Nature Communications paper published in 2020 suggested that reduced efficacy of long-lasting insecticidal nets was to blame.
Now, a new paper published in the Malaria Journal by the same team has zeroed in on the cause, suggesting it was a change to the ‘binder’ of the nets that had caused them to become much less effective at killing the mosquitoes.
“One of the ingredients in the coating formulation (that’s not the insecticide) is called the binder. So that’s basically the glue that sticks the insecticide to the net fabric,” Associate Professor Stephan Karl, a James Cook University and the PNG Institute of Medical Research entomologist specialising in malaria tells Cosmos. He was the senior author on both papers.
“That was changed from one particular chemical to another chemical, and that coincided directly with the change in the mortality of the mosquitoes.”
Long-lasting insecticidal nets are one of the cheapest ways for countries with malaria to keep the mosquito-spread parasite under control. They have the double protection of not only creating a physical barrier between the person under the net and any hungry mosquitoes, but also killing any mosquitoes that come too close.
“That provides community protection to people that might not be able to use nets or are not under a net at a particular point in time,” says Karl.
In the 2020 study, the team found that almost all the nets manufactured after 2013 did not kill enough mosquitos to fulfil the required World Health Organisation bioefficacy standards, but every single one of the nets made before 2013 did.
Looking specifically into a net called PermaNet 2.0, in the new paper the team found that there was a high polymer fluorine content present in the pre-2013 nets, but from 2013 there was none.
“There are product specifications that can be found on the WHO websites, but it appears that over recent years we have learned the hard way that these regulations were not very stringent,” says Karl.
“This would have included things like total insecticide content … but it did not include specifications for the coating formulations.”
After the first paper came out, some framework and guideline revisions have begun at the WHO to make sure this doesn’t happen again, but for now, Karl says the newer, substandard bed nets are still being used.
It’s hard to tell whether the numbers of malaria cases have continued going up since 2020 due to routine surveillance being relatively unreliable. However, the National Health Surveillance System has shown a small increase year on year, and the problem is unlikely to go away on its own.
“It’s a slow increase. It’s not a huge wave or something like that. But when we when you look at the case numbers, it’s basically been increasing slowly since 2014, up to now,” he says.
A recent report from the WHO has shown that 15 countries have not distributed the majority of their bed nets, and only 75% of nets worldwide have been distributed.
All this is not to say that only nets have caused this problem.
Karl notes population growth, solar panels in remote communities allowing people to be outside at night more, and changing mosquito behaviours as other potential causes. But having high quality insecticidal netting would certainly help.