Indoor Residual Spraying (IRS) has long been a key method for controlling malaria control. The method involves spraying all accessible surfaces in homes to maximize mosquito exposure to insecticides. 

However, according to a review published on the Malaria Journal, emerging studies suggest that a more targeted approach – known as selective IRS – could achieve similar effectiveness at a fraction of the cost for full spraying.

The review, led by Seth Irish and Sarah Moore from Swiss TPH with affiliation to Ifakara Health Institute, alongside other colleagues from the Innovative Vector Control Consortium (IVCC) at Liverpool School of Tropical Medicine (LSTM) and the Autonomous University of Yucatan (UADY), evaluates the potential of selective IRS. 

“This narrative review summarizes previous research on the use of selective spraying for vector-borne disease control and the cost-saving implications to see whether there might be justification for the use of selective spraying for malaria control, and to determine what avenues of research might be the most impactful to maximize its efficacy,” noted the scientists.

Understanding selective IRS
Selective IRS also referred to as “targeted IRS” or “partial IRS,” aims to enhance malaria control by focusing insecticide applications on specific areas where mosquitoes are most likely to rest. 

This approach utilizes detailed knowledge of mosquito behavior to direct spraying efforts to key resting sites within homes. By focusing on areas where mosquitoes predominantly rest, spraying only those areas could reduce the amount of insecticide used while maintaining or potentially improving control effectiveness.

“Indoor residual spraying (IRS) is one of the most effective malaria control tools. However, its application has become limited to specific contexts due to the increased costs of IRS products and implementation programmes… Selective spraying has been repeatedly proposed as a solution to optimize the cost-effectiveness and minimize the logistical challenges of IRS.”

Aim of the review
The review focused on identifying studies that examined the resting behavior of Anopheles mosquitoes and those that evaluated the impact of selective spraying on entomological and malaria outcomes. Additionally, the scientists developed a cost model and conducted comparative cost analyses to evaluate the overall cost benefits of selective IRS.

According to the review, several studies have documented varying resting behaviors of mosquitoes, such as Anopheles gambiae and Anopheles funestus, across different locations and conditions. These behaviors are influenced by factors like room type, temperature, humidity, light, and surface type. Selective IRS relies on this knowledge to guide targeted spraying.

“This review aimed to assess the resting behavior of Anopheles mosquitoes. There were no clear patterns for African malaria vectors, and standardized methods for monitoring resting behavior are necessary... The existing data on selective spraying indicate that this may be a promising way of controlling malaria, but further work is necessary.”

Cost analysis of selective IRS
From cost analysis data obtained from studies carried out between 2019 to 2022, selective spraying was repeatedly proposed as a solution to optimize the cost-effectiveness and minimize the logistical challenges of IRS. 

“Selective IRS shows promise in optimizing cost-effectiveness and reducing logistical challenges. However, further studies are needed to evaluate the entomological and epidemiological impact of selective spraying with existing IRS compounds. This research is crucial for validating the method and realizing cost savings,” the researchers emphasized.

“IRS is being phased out from an increasing number of countries due to its cost despite clear evidence of effectiveness for malaria control and insecticide resistance management. Several operational studies have indicated substantial decreases in malaria prevalence using selective spraying at a fraction of the cost of full spraying,” they added.

Selective IRS as a promising strategy
As IRS programs face increasing costs, exploring and optimizing selective IRS could be key to sustaining and enhancing malaria control efforts worldwide. The approach offers a promising strategy for improving malaria control while reducing costs. However, its success depends on further research and standardization of methods for monitoring mosquito behavior.

The scientists recommend further studies to validate selective IRS and refine its application, ensuring that it can serve as an effective and cost-effective tool in the ongoing fight against malaria. This includes conducting careful calculations of the overall cost savings against the total costs of IRS programs, not just the spraying operations and insecticide costs.

Read the publication here.