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.
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.
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.
READ MORE: OFFICIAL LINK (WHO, 14 March 2023)
WHO Guidelines for malaria
WHO regularly convenes the world’s leading malaria experts to review available evidence and advise on recommendations to control and eliminate malaria. The consolidated Guidelines for malaria, launched in February 2021, bring together – for the first time – all of the Organization’s current recommendations for malaria in one user-friendly online resource. Through the MAGICapp platform you will find:
- All WHO evidence-based recommendations for malaria prevention (vector control and preventive chemotherapies) and case management (diagnosis and treatment) and elimination.
- Links to other resources, such as guidance and information on the strategic use of information to drive impact; surveillance, monitoring and evaluation; operational manuals, handbooks, and frameworks; and a glossary of terms and definitions.
These consolidated guidelines replace 2 guideline documents on the WHO website: the Guidelines for the treatment of malaria, third edition and the Guidelines for malaria vector control. As new evidence becomes available, the recommendations will be reviewed and updated, where appropriate, using WHO’s transparent and rigorous guideline development process.
Clear, evidence-informed recommendations are designed to guide countries as they develop national malaria strategic plans; they support decisions around “what to do”. WHO also develops implementation guidance such as operational and field manuals to advise countries on “how to” deliver the recommended tools and strategies.
The consolidation of WHO’s malaria guidelines is one of a number of actions the Organization has undertaken in recent years to make its guidance more accessible to end users in malaria-endemic countries. The overall aim is to deliver timely, high quality recommendations through processes that are more transparent, consistent, efficient and predictable.