Since 2005, over 2 billion insecticide-treated nets (ITNs) have been distributed worldwide to prevent malaria. All of these nets were treated with only one insecticide class - pyrethroids. However, as mosquitoes in many areas are now resistant to pyrethroids, nets treated with other active ingredients are needed to control malaria.
In 2017, WHO started to recommend a new type of ITN that combines pyrethroids with piperonyl-butoxide (PBO), a chemical that enhances the potency of pyrethroids against resistant mosquitoes.
New recommendations, published today in the WHO Guidelines for malaria, cover 2 new classes of dual ingredient ITNs with different modes of action:
- Pyrethroid-chlorfenapyr nets combine a pyrethroid and a pyrrole insecticide to enhance the killing effect of the net.
- Pyrethroid-pyriproxyfen nets combine a pyrethroid with an insect growth regulator (IGR). The IGR disrupts mosquito growth and reproduction.
"These new types of nets were designed to have a greater impact against pyrethroid-resistant mosquitoes," notes Dr Jan Kolaczinski, who leads the Vector Control and Insecticide Resistance unit within the WHO Global Malaria Programme. "By including two active ingredients in an ITN, the likelihood of mosquitoes being resistant to both is greatly reduced."
WHO Guidelines for malaria
The consolidated WHO Guidelines for malaria bring together the Organization's most up-to-date recommendations for malaria in a single web-based document. Recommendations are reviewed and, where appropriate, updated using WHO's transparent and rigorous guideline development process. The guidelines are also available in the user-friendly MAGICapp platform, which always displays the updated recommendations and date of the most recent revision.
New recommendation on pyrethroid-chlorfenapyr nets vs pyrethroid-only nets
WHO is issuing a strong recommendation for the deployment of pyrethroid-chlorfenapyr ITNs vs pyrethroid-only nets to prevent malaria in adults and children in areas where mosquitoes have become resistant to pyrethroids.
The recommendation considers that, compared to pyrethroid-only nets or pyrethroid-PBO nets, pyrethroid-chlorfenapyr ITNs should have an increased killing effect against pyrethroid-resistant malaria vectors and, thus, a greater impact against malaria.
New recommendation on pyrethroid-chlorfenapyr nets vs pyrethroid-PBO nets
WHO is issuing a conditional recommendation for the deployment of pyrethroid-chlorfenapyr ITNs instead of pyrethroid-PBO nets to prevent malaria in adults and children in areas with pyrethroid resistance.
The conditionality of the recommendation is based on the judgement of the WHO Guidelines Review Group (GDG) that the balance of desirable and undesirable effects probably favours pyrethroid-chlorfenapyr nets over pyrethroid-PBO nets. The recommendation is based on evidence drawn from only one trial in Africa.
New recommendation on pyrethroid-pyriproxyfen nets vs pyrethroid-only nets
WHO is issuing a conditional recommendation for the deployment of pyrethroid-pyriproxyfen nets instead of pyrethroid-only nets to prevent malaria in adults and children in areas with pyrethroid resistance.
The conditionality of the recommendation is based on the GDG's concerns around the poor cost-effectiveness of pyrethroid-pyriproxyfen nets compared to pyrethroid-only nets; the extra resources currently required to purchase these ITNs may have a negative impact on coverage and equity.
New recommendation on pyrethroid-pyriproxyfen nets vs pyrethroid-PBO nets
WHO has issued a conditional recommendation against the deployment of pyrethroid-pyriproxyfen nets instead of pyrethroid-PBO nets.
The conditionality of the recommendation is based on the GDG's judgement that the balance of effects favours pyrethroid-PBO nets over pyrethroid-pyriproxyfen nets and that, based on current cost and efficacy data, pyrethroid-PBO nets are more cost-effective.
Assessing the quality of a body of evidence
In 2007, WHO adopted GRADE (Grading of Recommendations Assessment, Development and Evaluation) as the method for assessing the quality of a body of evidence and for determining the direction and strength of the resulting recommendations. GRADE categorizes the quality of evidence as high, moderate, low or very low. GRADE assessments are determined through the consideration of several factors, including risk of bias, inconsistency, indirectness, imprecision and publication bias. All of the recommendations published today are based on a moderate certainty of evidence.
New guidance on ITN prioritization in resource-constrained settings
Together with the new recommendations, WHO is publishing new guidance to support national malaria programmes in decisions on which nets to prioritize in resource-limited settings.
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This guidance focuses, as a first step, on ensuring coverage of ITNs for vulnerable groups - particularly pregnant women and children under the age of 5 - and then on planning for high-volume net distribution campaigns. While the guidance is based on best practices generated in Africa in recent years, it can be used as a component of prioritization processes by all countries deploying ITNs.
Threats to ITNs - a critical malaria prevention tool
Over the last 2 decades, ITNs have contributed significantly to the progress seen in reducing malaria cases worldwide. A 2015 modelling analysis published in Nature suggested that ITNs drove most of the declines in malaria seen from 2005-2015, especially in moderate-to-high transmission areas. However, progress since 2015 has slowed.
According to WHO's latest World malaria report, the emergence and wide geographic spread of pyrethroid resistance among malaria-transmitting mosquitoes is the most recognized threat to the effectiveness of ITNs.
Other threats to this key prevention tool include insufficient coverage and access; challenges relating to the physical and chemical durability of nets; and changing behaviour of mosquitoes, which appear to be biting early before people go to bed and resting outdoors, thereby evading exposure to insecticides.
