Research published in NatureCommunications shows that insecticide-treated mosquito nets, the mainstay in the global fight against malaria, are no longer providing the protection they once had – and scientists say this is of serious concern in tropical and subtropical countries around the world gives.
Long-lasting insecticidal nets (LLINs) are believed to have saved 6.8 million lives between 2000 and 2015.
“While an untreated net will keep mosquitoes from biting you while you sleep – which provides valuable protection – these nets are treated with a long-lasting insecticide that actually kills mosquitoes that come in contact with them,” said Dr. Stephan Karl, a malaria researcher from James Cook University’s Australian Institute for Tropical Health and Medicine and the Papua New Guinea Institute for Medical Research.
“LLINs provide protection at the community level by massively reducing the total number of mosquitoes. In other words, people who do not use these networks directly benefit from their presence in the communities, ”said Dr. Karl.
The introduction of LLINs in Papua New Guinea in 2006 resulted in a significant decrease in malaria cases, but the infection rate has since increased again – from less than 1% in 2013-2014 to 7.1% in 2016-2017.
“The nets are really a front line defense – in Papua New Guinea they are the only tools currently used in the national campaign against the mosquitoes that can transmit malaria,” said co-author Dr. Moses Laman, associate director at the PNG Institute of Medical Research.
“Malaria kills around half a million people worldwide each year, so any evidence that the networks are not working is a matter of great concern.”
When researchers investigated Papua New Guinea, Australia and the UK, their search took an unexpected turn.
“It was early assumed that the rise in cases was due to a lack of antimalarial drugs,” said co-author Tim Freeman, country manager of Rotarians Against Malaria. “But after the drug supply was restored, the cases continued to rise.” Rotarians Against Malaria are supporting the National Ministry of Health of Papua New Guinea with the net distribution across the country.
Other possible explanations were examined. Have mosquitoes built resistance to the insecticide or avoided the insecticide by eating more animals and humans outdoors? Were people bitten more often because having better access to electricity allowed them to stay up later?
“We can safely rule out insecticide resistance because our studies have shown time and time again that the malaria mosquitoes in PNG are currently not insecticide-resistant,” said Dr. Karl.
“Any of the remaining factors could, to some extent, be contributing to an increase in the rate of infection, but the rapid rise in cases – to levels near pre-control – indicated that we still lacked a major cause.”
The mosquito nets themselves were not an obvious culprit as their insecticide levels are regularly checked during pre-delivery inspections.
The LLINs used in Papua New Guinea are all manufactured by a single company according to the specifications set by the World Health Organization. The model is widespread – in 2014 it had the largest market share of LLNIs in the world.
When researchers tested the performance of the nets in knocking off and killing mosquitoes, the problem was uncovered.
“With new nets in place by 2012, the rate of mosquitoes killed was always close to 100%,” said Dr. Karl.
“Using the same standard tests on new nets made from 2013 to 2019, the kill rate dropped to an average of 40%, with some of the nets killing mosquitoes at all.
“That is an alarming loss of effectiveness for critical protective equipment.
“This also calls into question the regulations and standards that govern the quality of LLINs around the world, if such networks are still considered acceptable.”
All of the nets tested appeared to have the same amount of insecticides, which begs the question of how nets with the same insecticide levels could be less fatal to mosquitoes.
The authors agree that the answer most likely lies in changes in how the nets are made. “We hope to work with the manufacturer to conduct further research,” said Dr. Karl.
In the meantime, researchers are calling for LLINs to be tested for their ability to kill mosquitoes – not just for their insecticide content.
They have informed the World Health Organization and the manufacturer of their results.
The research is published in the current issue of Nature Communications.
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