Learn how insecticide-treated nets work, what pyrethroid resistance means for malaria control, and how newer dual-ingredient nets help restore protection.
Understanding Insecticide-Treated Nets
Insecticide-treated nets are a core tool in malaria control. They protect in two ways: they act as a physical barrier that stops mosquitoes from biting, and they deliver an insecticide that kills or repels mosquitoes. The World Health Organization describes these nets as a core intervention, along with indoor residual spraying, and says they are highly effective in preventing infection and reducing transmission. When a mosquito lands on the net, it picks up a lethal dose of insecticide. This not only protects the person sleeping under the net but also helps the wider community. Mosquitoes that contact the net are less likely to survive long enough to transmit malaria. Treating nets with insecticide makes them far more protective than untreated nets. They have been distributed on a massive scale, with more than two billion nets delivered since 2005, according to estimates cited by the WHO.
The Role of Pyrethroids in Bed Nets
For decades, most insecticide-treated nets have used a class of chemicals called pyrethroids. These are valued because they are safe for humans and highly effective against mosquitoes. However, years of widespread use have led to the evolution of pyrethroid resistance in some mosquito populations. Resistance means that a mosquito can survive exposure to the insecticide, so the net no longer kills it. This reduces the net's protective effect, both for the individual and the community. A 2016 study in eLife found that as pyrethroid resistance increases, mosquitoes are less likely to be deterred from entering a hut with a net, and washing erodes efficacy faster when resistance is high. Even low levels of resistance can increase malaria incidence by reducing mosquito mortality and community protection. The authors predicted that in a setting with 10 percent slide prevalence, 30 percent resistance would add hundreds of cases per thousand people each year.
What Is Pyrethroid Resistance?
Pyrethroid resistance is a genetic change in mosquitoes that makes them less susceptible to the insecticide. It is a natural evolutionary response to repeated exposure. When a net treated with pyrethroid is used, most mosquitoes die, but a few with resistance mutations survive and reproduce. Over time, the proportion of resistant mosquitoes in the population grows. This is a serious problem for malaria control, as the WHO notes that emerging resistance to insecticides among Anopheles mosquitoes threatens progress. Resistance can be measured in various ways, such as the percentage of mosquitoes that survive a standard insecticide exposure. The eLife study used a threshold of over 60 percent survival to define a highly resistant population, above which a person under a new net loses substantial personal protection. Even before that point, community protection is eroded. Understanding resistance levels is crucial for choosing the right type of net.
How Dual-Ingredient Nets Improve Protection
To combat resistance, manufacturers have developed nets with two active ingredients. The WHO recommends two new classes of dual-ingredient nets. Pyrethroid-chlorfenapyr nets combine a pyrethroid with a pyrrole insecticide, which kills mosquitoes through a different mode of action. Pyrethroid-pyriproxyfen nets combine a pyrethroid with an insect growth regulator that disrupts mosquito development and reproduction. Having two active ingredients greatly reduces the chance that mosquitoes are resistant to both, according to a WHO official. In areas where mosquitoes are resistant to pyrethroids, WHO strongly recommends pyrethroid-chlorfenapyr nets over pyrethroid-only nets, because they provide a better killing effect. The recommendation for pyrethroid-pyriproxyfen nets is conditional, because of concerns about cost-effectiveness and potential harm to coverage and equity. These new nets represent an important tool in the fight against malaria.
Pyrethroid-PBO Nets: A Targeted Solution
Pyrethroid-PBO nets are a type of dual-ingredient net that has been recommended since 2017. They combine pyrethroid with piperonyl butoxide (PBO), a chemical that inhibits the enzymes mosquitoes use to detoxify pyrethroids, thereby restoring their susceptibility. The WHO issued conditions for deployment of these nets based on evidence from epidemiological trials, including a study in Tanzania. In areas with high pyrethroid resistance, switching from standard pyrethroid-only nets to PBO nets can avert many additional cases of malaria. The eLife study estimated that with 80 percent resistance, switching to PBO nets would avert about 500 additional cases per 1,000 people per year. However, the authors noted that recommending PBO nets over standard nets requires cost-effectiveness and affordability data. PBO nets are now widely available and are considered an important tool in areas with resistance.
Why Physical Barriers Still Matter
While insecticide is important, the physical barrier provided by a bed net is essential. Even if a mosquito is resistant to the insecticide, the net still prevents it from biting the person sleeping underneath. This personal protection remains substantial until mosquito populations become highly resistant, according to the eLife study. A net that is intact and correctly used can reduce mosquito bites significantly, regardless of insecticide resistance. However, if the net has holes or tears, mosquitoes can find their way through. Therefore, it is crucial to use a net that is in good condition and to ensure it is properly tucked in. The physical barrier also protects the wider community by reducing the number of mosquitoes that feed on humans and potentially transmit the parasite. Physical barriers are a real protection when they are correctly specified, intact, and correctly used.
Choosing the Right Net in a Changing World
When choosing a bed net, it is important to consider the local level of pyrethroid resistance. In areas with no or low resistance, standard pyrethroid-only nets are still effective. In areas with moderate to high resistance, dual-ingredient nets such as pyrethroid-PBO or pyrethroid-chlorfenapyr nets offer better protection. The WHO recommends that countries use data on mosquito resistance to guide their net choices. This is part of a broader strategy to manage insecticide resistance. As the threat of malaria continues, with an estimated 282 million cases worldwide in 2024, according to the WHO, it is vital to use the most effective tools available. Newer nets are becoming more widely available and represent an important step forward. However, no single tool is enough; nets should be combined with other measures such as indoor residual spraying and prompt access to diagnosis and treatment.
Sources
- WHO publishes recommendations on two new types of insecticide-treated nets: World Health Organization
- Malaria: World Health Organization
- The impact of pyrethroid resistance on the efficacy and effectiveness of bednets for malaria control in Africa: eLife Sciences Publications
- Conditions for deployment of mosquito nets treated with a pyrethroid and piperonyl butoxide: recommendations: World Health Organization