Optimization of long-lasting insecticidal bed nets for resistance management: a modeling study and user-friendly app

Abstract

Abstract Background: Up until the present, pyrethroid-treated bed nets have been a key tool for vector control in the fight against malaria. A global system that sets standards and facilitates procurement has successfully driven down the price of bed nets to enable more bed nets to be distributed. As a result of their mass rollout, malaria cases have been significantly reduced, but pyrethroid resistance is now widespread. Going forward, new insecticides have been and continue to be developed for use on bed nets, but it is unclear how to best deploy them for maximum impact. Methods: Here, an app for the optimization of bed nets based on their insecticide loading and deployment lifespan is presented. Underlying the app are simple models that incorporate the chemical and physical properties of bed nets, and the genetic and ecological properties of resistance evolution in mosquitoes. The app numerically searches across a massive number of these simple models with variable loading and lifespan to find their optima under different optimality criteria that place constraints on the options for vector control. Results: Optimization can lead to substantial gains in the average control of the mosquito population throughout the simulation, and these gains are comparatively greater with lower budgets for the purchasing of bed nets. Whilst optimizing a bed net with a new insecticide within the constraints of the incentives of the existing system of standards and procurement leads to substantially greater control than not optimizing the bed net, optimizing the bed net without constraints leads to yet substantially greater control. The most important factor in this optimization is coverage, which depends on the price per bed net. With this in mind, it is unsurprising that a pyrethroid is a preferred partner for a new insecticide because it is cost-effective in the balance of being less expensive than the new insecticide but also less effective due to pre-existing resistance. Surprisingly, a pyrethroid is shown to be an effective partner for a new insecticide because of its contribution to resistance management in delaying the onset of resistance to the new insecticide. Conclusions: This study highlights the importance of optimizing mixtures to maximize their impact in vector control. Further, it suggests that there are challenges in the roll-out of optimal bed nets because of the constraints imposed by the global system of standards and procurement, which currently fails to adequately incentivize important considerations in bed net design for new insecticides, like resistance management.