Update: Indoor residual spraying for preventing malaria in communities using insecticide‐treated nets

17 Jan 2022

What was the aim of this review?

Indoor residual spraying (IRS) is the regular application of chemical insecticides to household walls. The insecticide lasts for several months, killing mosquitoes that land on them. Insecticide‐treated nets (ITNs) are bed nets treated with insecticides, preventing mosquitoes from biting people and reducing the mosquito population. Both interventions help to control malaria by reducing the number of people being bitten by mosquitoes infected with malaria. Implementing IRS in communities that are using ITNs may be better for malaria control than using ITNs alone simply because two interventions may be better than one; but also because it may improve malaria control where mosquitoes have become resistant to the pyrethroid insecticides used in ITNs. Pyrethroids were the only class of insecticides approved for use in ITNs until 2018, but growing resistance of mosquitoes to pyrethroids impairs their effectiveness. The addition of IRS could counteract this reduction in ITN effectiveness and may help to slow the emergence of pyrethroid resistance. We could expect that IRS insecticides that have a different way of working to pyrethroids ('non‐pyrethroid‐like') could restore effectiveness better than those that have the same way of working ('pyrethroid‐like'). The aim of this review was to summarize the impact of pyrethroid‐like or non‐pyrethroid‐like IRS on malaria, when implemented in communities that are using ITNs.

Key messages

The addition of IRS using a non‐pyrethroid‐like insecticide was associated with reduced malaria prevalence. Malaria incidence may also be reduced on average, but this effect was absent in two studies, and consequently there remains some uncertainty over whether the intervention will be effective in all settings.

When a pyrethroid‐like insecticide was used for IRS, data were limited but there was no additional effect demonstrated.

What was studied in the review?

We searched for studies that evaluated the impact on malaria transmission when IRS, using a World Health Organization (WHO)‐recommended dosage, was implemented in communities that were using either ready‐treated ITN products or standard nets treated with insecticide at a WHO‐recommended dose. We considered effects on both human health outcomes and on mosquito populations.

What were the main results of the review?

In total, we identified 10 studies matching our inclusion criteria, from which we made 12 comparisons. Seven studies (providing eight comparisons) used a non‐pyrethroid‐like IRS throughout the study. Each of these were conducted in areas where the vectors were described as resistant or highly resistant to pyrethroids. Two studies (providing two comparisons) used a pyrethroid‐like IRS throughout. One further study used a pyrethroid‐like IRS in the first study year and switched to a non‐pyrethroid‐like IRS in the subsequent years, therefore providing two different comparisons. All studies were conducted in sub‐Saharan Africa.

Adding non‐pyrethroid‐like IRS in communities using ITNs appeared to improve malaria outcomes in most settings. Overall, the results from the eight included studies found lower malaria parasite prevalence, while there may be a reduction in malaria incidence and anaemia prevalence. We do not know if there is an impact on the number of infected bites received per person per year.

When adding pyrethroid‐like IRS in communities using ITNs, the data from three studies indicate there is probably no effect on malaria incidence or parasite prevalence, and there may be little or no effect on the prevalence of anaemia. Data on the number of infected bites received per person per year were too limited to draw a conclusion.

How up to date is the review?

We searched for relevant studies up to 8 November 2021.