Core vector control methods
Vector control is an essential component of malaria prevention. Such control targets the mosquitoes capable of transmitting malaria parasites. Vector control has been proven to successfully reduce or interrupt malaria transmission when coverage is sufficiently high. The two core, broadly applicable measures for malaria vector control are long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS).
WHO recommends that endemic countries protect all those at risk of malaria with LLINs or, where appropriate, IRS. Vector-control strategies should be devised through an integrated vector management (IVM) approach. Such an approach seeks to improve the efficacy, cost-effectiveness, ecological soundness and sustainability of controlling disease vectors. Up-to-date entomological surveillance data and intervention monitoring and evaluation information are required to inform these strategies and to identify any factors that may compromise impact. Key considerations for malaria vector control include vector abundance, behaviour and susceptibility to the insecticides used in LLINs and IRS.
Malaria control and elimination programmes should prioritize the delivery of either LLINs or IRS at high coverage and to a high standard rather than introducing the second intervention as a means of compensating for deficiencies in the implementation of the first. However, in areas with high LLIN coverage, IRS with different non-pyrethroid insecticides applied sequentially (in rotation) can be implemented to manage insecticide resistance.
Long-lasting insecticidal nets
Sleeping under a net treated with an effective insecticide can reduce contact between mosquitoes and humans by providing both a physical barrier and an insecticidal effect. Population-wide protection can result from the killing of mosquitoes on a large scale where there is high access and usage of such nets within a community.
WHO recommends the deployment of LLINs as these do not require regular re-treatment in the way that conventional insecticide-treated nets do although they do need to be replaced once their useful life has expired. Current WHO-recommended LLINs contain pyrethroid insecticides only. They are designed for a minimum lifespan of 20 standard washes or 3 years of usage under field conditions although their physical and chemical performance can vary greatly depending on the setting. This underscores the importance of monitoring LLIN durability throughout field usage.
Between 2000 and 2015 over a billion insecticide-treated nets were delivered to malaria-endemic countries. In sub-Saharan Africa, where malaria in concentrated in children aged under 5 years, the proportion of children of this age group sleeping under a net increased from under 2% in 2000 to an estimated 68% in 2015. The rapid scale-up of insecticide-treated nets has been by far the largest contributor to the impressive drops in the incidence of malaria, highlighting their effectiveness in reducing or interrupting malaria transmission when there is high population coverage and usage.
However, the proportion of other age groups sleeping under an insecticide-treated net falls below that of children under 5, and is particularly low for those aged 5-19 years. In 2013, only 29% of households that had nets had enough to protect all members of the household. For effective malaria control and elimination, it is critical that sufficient nets are available to protect all community members from malaria.
- Read the guidelines for monitoring the durability of long-lasting insecticidal mosquito nets under operational conditions
Indoor residual spraying
IRS involves spraying an effective dose of insecticide with a long residual activity, typically once or twice per year. The insecticide is sprayed on indoor walls and ceilings where malaria vectors are likely to rest after biting. To confer significant community protection, IRS needs to be implemented at a high level of coverage with sustained efficacy throughout the malaria transmission season or seasons. Globally, an estimated 116 million people (equivalent to 4% of the at-risk population) were protected by IRS in 2014.
The WHO Pesticide Evaluation Scheme (WHOPES) currently recommends for IRS 15 insecticide compounds and formulations belonging to four chemical classes. National malaria control programmes need to select insecticides for a given area on the basis of the residual efficacy of the insecticide, cost, safety and the type of surface to be sprayed and in consideration of up-to-date information on insecticide resistance.
DDT in malaria vector control
Dichlorodiphenyltrichloroethane (DDT) continues to be used for IRS in limited situations. It has a comparatively long residual efficacy, lasting more than 6 months in most settings.
The use of DDT in agriculture is banned under the Stockholm Convention on Persistent Organic Pollutants. However, countries can use DDT for malaria vector control – provided that the guidelines and recommendations of WHO and the Stockholm Convention are all met, and until locally appropriate alternatives are available for a sustainable transition from DDT. WHO updated its position statement on DDT in 2011.
Managing insecticide resistance
Resistance has been detected in all major vector species and to all classes of insecticides. Since 2010, mosquito resistance to insecticides used in public health has been identified in 60 countries around the world. However, many of the countries affected do not routinely carry out adequate testing, which means that our understanding of the scale of insecticide resistance is incomplete.
Issued in May 2012, the WHO Global plan for insecticide resistance management in malaria vectors (GPIRM) is a global plan of action for all stakeholders engaged in the fight against malaria. It also contains comprehensive technical recommendations for managing insecticide resistance in different situations – including the use of insecticide rotations for IRS.
Unfortunately, the uptake of GPIRM at national levels has been limited, in part because of critical deficiencies in finances, human resources and infrastructure. Malaria-endemic countries are urged to draw up and implement comprehensive insecticide-resistance management strategies, and to ensure timely entomological surveillance, including resistance monitoring.
- Indoor residual spraying: An operational manual for IRS for malaria transmission, control and elimination. Second edition (2015)
- WHO recommendations for achieving universal coverage with long-lasting insecticidal nets in malaria control (2013)
- Global plan for insecticide resistance management in malaria vectors (2012)
- WHO position statement on integrated vector management (2008)
- Malaria vector control and personal protection (2006)