Mike: Hello, I'm Mike Clarke, podcast editor for the Cochrane Library. Malaria is arguably the most important parasitic disease in the world and the Cochrane Infectious Diseases Group has produced more than 60 relevant reviews. These were added to in August 2023 with a new review of the effects of topical repellents. We asked lead author, Juan Carlos Gabaldon from the Barcelona Institute for Global Health in Spain to tell us about the findings in this podcast.

Juan: Malaria affects over 240 million people, and causes more than 600,000 deaths every year, mostly among children in Sub-Saharan Africa. It’s caused by parasites of the genus Plasmodium, which are transmitted to humans through the bite of some species of mosquitos from the genus Anopheles. The control of malaria is mostly based on the distribution of insecticide-treated nets, which protect people from being bitten while they sleep, and the use of indoor residual spraying, in which insecticides are applied on walls inside houses. While these interventions are very effective at reducing the overall burden of malaria, they preferentially target mosquitos that feed indoors, late at night, and from humans, allowing species with different behaviours to survive, and keep transmitting the disease, albeit at a lower intensity.
Topical repellents are substances that can be applied directly to the skin of people to prevent mosquito bites. They have been suggested as potential complementary interventions to aid in malaria elimination efforts, particularly by protecting people who have a higher risk of getting bitten outdoors, and therefore, to whom the nets and indoor spraying might offer incomplete protection. However, unlike insecticides, repellents don’t kill mosquitos, and their efficacy is largely dependent on their adequate use. Mosquitos could, therefore, be diverted from protected to unprotected individuals, making the actual effect on malaria transmission dynamics hard to predict.
We investigated whether the use of topical repellents reduced malaria caused by Plasmodium falciparum, the species most commonly associated with severe disease, and the most common in Africa. We measured the effect on disease incidence, which is the number of new cases in a period of time and prevalence, the number of cases at a specific timepoint. We focused on the effect on people less likely to be protected by the indoor interventions, such as forest workers, refugees, the military, and other groups with higher risk of being bitten outdoors. We also evaluated the adherence of people to the appropriate use of repellents and any adverse events.
We analysed 8 trials, which included a total of more than 60,000 people in nine countries of South America, South East Asia, and Africa. We found that topical repellents may slightly reduce both incidence and prevalence of P. falciparum malaria in settings where no other interventions are widely available. However, they probably make little or no difference if insecticide-treated nets or indoor residual spraying are already being used. Based on the small number of mild adverse effects, we consider topical repellents to be safe. We also found that adherence to their correct use is highly variable, and hard to measure in most trial settings.
In summary, while there is insufficient evidence to conclude that topical repellents can help reduce malaria when other interventions are already in place, our results suggest that they might produce a modest benefit in specific settings with poor access to insecticide-treated nets or indoor residual spraying, for instance, for people living in refugee camps, or in the context of humanitarian crisis. However, further research about their use in these settings is needed to fully understand how beneficial they actually are.

Mike: If you’d like to learn more about the effects of topical repellents on malaria and watch for future updates of the review if those evidence gaps are filled by new research, just go online to Cochrane Library dot com and search ‘topical repellents for malaria’ to access it for free.