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London School of Hygiene & Tropical Medicine Malaria Centre

Malaria research in vector studies

Indoor Residual Spraying (IRS) and Long-lasting Insecticide Treated Nets (LLIN): Integration of methods and insecticide mode of actions for control of African malaria vector mosquitoes.

LSHTM investigators:
Fredros Okumu & Sarah Moore.
External collaborators:
John Grieco & Nicole Achee.
Funding body:

Insecticide treated nets (ITNs) and indoor residual spraying (IRS) are the preferred techniques for malaria vector control in Africa, where their application has already contributed to significant reductions in the burden of the disease.

Even though both methods are commonly used together in the same households, evidence of greater health benefits due to these combinations as opposed to use of either ITNs or IRS alone has been minimal and inconclusive.

The main aim of this research was therefore to contribute to this essential evidence, by way of experimental hut studies and mathematical simulations. We investigated whether there would be any added protective advantages when any of three selected long lasting insecticidal nets (LLINs) are combined with any of three selected IRS chemicals, as opposed to using any of the treatments alone. Data generated from the experimental hut studies was then inputed into an optimised deterministic mathematical model, simulating a typical malaria endemic village.

Both the field studies and the simulations showed that any synergies or redundancies resulting from LLIN/IRS combinations are primarily a function of modes of action of active ingredients used in the two interventions. Where LLINs are already present, addition of IRS would be redundant unless the IRS chemical is highly toxic, but where IRS is the pre-existing intervention, these combinations always confer improved protection. Therefore, IRS households should always be supplemented with nets, preferably LLINs, which not only protect house occupants against mosquito bites, but also kill additional mosquitoes. Finally, where resources are limited, priority should be given to providing everybody with LLINs and ensuring that these nets are consistently and appropriately used, rather than trying to implement both LLINs and IRS in the same community at the same time.