Indoor residual spraying for preventing malaria in communities using insecticide‐treated nets
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 at least four 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 for three reasons: two interventions may be better than one; it may improve malaria control where mosquitoes have become resistant to the pyrethroid insecticides used in ITNs; and the combination of ITNs and IRS may also help to slow the emergence of pyrethroid resistance (where pyrethroids are no longer effective at killing mosquitoes).
Pyrethroids were the only class of insecticides approved for use in ITNs until 2018, but growing resistance to pyrethroids impairs their effectiveness. The addition of IRS could counteract this reduction in ITN effectiveness. 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.
When IRS was conducted with a non‐pyrethroid‐like insecticide, some studies and outcomes suggested an impact, but this was not consistent. Factors such as the number of people using nets did not explain the differences between studies. 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 trials 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 six trials matching our inclusion criteria, from which eight comparisons were drawn. Three trials (providing four comparisons) used a non‐pyrethroid‐like IRS throughout the study, and two trials (providing two comparisons) used a pyrethroid‐like IRS throughout. One further trial 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 six trials were conducted in sub‐Saharan Africa.
Adding non‐pyrethroid‐like IRS in communities using ITNs gave mixed results, with some trials detecting substantial effects but one trial detecting no effect. Overall, the results from the four included trials found that there may be a reduction in malaria parasite prevalence and anaemia prevalence (low‐certainty evidence). We do not know if there is an impact on the malaria incidence or on the number of infected bites received per person per year (very low‐certainty evidence).
When adding pyrethroid‐like IRS in communities using ITNs, the data from three trials indicate there is probably no effect on malaria incidence or parasite prevalence (moderate‐certainty evidence), 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 (very low‐certainty evidence).
How up to date is the review?
We searched for relevant trials up to 18 March 2019.