Negative Cross Resistance Mediated by Co-Treated Bed Nets: A Potential Means of Restoring Pyrethroid- Susceptibility to Malaria Vectors Michael T. White 1 , Dickson Lwetoijera 2 , John Marshall 1 , Geoffrey Caron-Lormier 3 , David A. Bohan 4 , Ian Denholm 5 , Gregor J. Devine 6 * 1 MRC Centre for Outbreak Analysis and Modelling, Imperial College, London, United Kingdom, 2 Ifakara Health Institute, Ifakara, Tanzania, 3 University of Nottingham, Sutton Bonington, Leicestershire, United Kingdom, 4 INRA, UMR 1347 Agroe ´cologie, Po ˆ le ECOLDUR, Dijon, France, 5 University of Hertfordshire, Hatfield, Hertfordshire, United Kingdom, 6 QIMR Berghofer Medical Research Institute, Brisbane, Australia Abstract Insecticide-treated nets and indoor residual spray programs for malaria control are entirely dependent on pyrethroid insecticides. The ubiquitous exposure of Anopheles mosquitoes to this chemistry has selected for resistance in a number of populations. This threatens the sustainability of our most effective interventions but no operationally practicable way of resolving the problem currently exists. One innovative solution involves the co-application of a powerful chemosterilant (pyriproxyfen or PPF) to bed nets that are usually treated only with pyrethroids. Resistant mosquitoes that are unaffected by the pyrethroid component of a PPF/pyrethroid co-treatment remain vulnerable to PPF. There is a differential impact of PPF on pyrethroid-resistant and susceptible mosquitoes that is modulated by the mosquito’s behavioural response at co-treated surfaces. This imposes a specific fitness cost on pyrethroid-resistant phenotypes and can reverse selection. The concept is demonstrated using a mathematical model. Citation: White MT, Lwetoijera D, Marshall J, Caron-Lormier G, Bohan DA, et al. (2014) Negative Cross Resistance Mediated by Co-Treated Bed Nets: A Potential Means of Restoring Pyrethroid-Susceptibility to Malaria Vectors. PLoS ONE 9(5): e95640. doi:10.1371/journal.pone.0095640 Editor: Rick E. Paul, Institut Pasteur, France Received January 28, 2014; Accepted March 28, 2014; Published May 1, 2014 Copyright: ß 2014 White et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was partly funded by the Bill and Melinda Gates Foundation (grant ID OPP52644). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No additional external funding was received for this study. Competing Interests: The authors have declared that no competing interests exist. * E-mail: greg.devine@qimrberghofer.edu.au Introduction A recent surge in effort and funding has led to the expansion of insecticide treated bed net (ITN) and indoor residual spray (IRS) programs in many parts of Africa and dramatic decreases in malaria transmission. Although four insecticide classes (carba- mates, organophosphates, pyrethroids and the organochlorine DDT) are currently approved for IRS, the vast majority of spraying programs utilise synthetic pyrethroids. This is also the only insecticide class approved for use on ITNs [1]. The ubiquitous presence of pyrethroids in public health and the agricultural sector has resulted in strong selection pressure for mutations that confer resistance to pyrethroids in insect vectors of disease. In the absence of remedial measures, the impacts of this on malaria transmission can be severe [2,3]. Pyrethroid resistance is widely reported in African malaria vectors [4] but there is little that can be done in response. There are few novel insecticidal products nearing commercialisation and the reassessment of old and previously resisted chemistries in new guises is now commonplace. A novel, resistance-beating combi- nation of safe compounds with World Health Organisation (WHO) approval is therefore a timely and exciting proposition. We propose a mechanism to delay or reverse selection for pyrethroid resistance through a phenomenon called negative cross resistance (NCR) in which organisms resistant to one compound of a binary mixture are hyper-susceptible to the other. This imposes a fitness cost on the resistant genotype that can decrease the frequency of resistant alleles. This is distinct from the conventional use of binary mixtures and rotations where there is no hyper- sensitivity and whose role in resistance management is severely limited if the target pest has already developed resistance to either compound [5]. NCR has long been discussed by agricultural [6,7] and public health entomologists [8] but it has largely eluded attempts at practical implementation. It remains an intriguing alternative to the ‘‘treadmill’’ approach of resistance management (the sequen- tial replacement of one chemical class by another, as insects evolve a succession of protective mechanisms). In our model, we exploit a potent chemosterilant (pyriproxyfen or PPF) and the differential behaviour of pyrethroid-resistant and susceptible mosquitoes at pyrethroid-treated surfaces. The model draws on the impacts of pyrethroids on susceptible and resistant insects and on recent proofs that PPF exposure dramatically reduces egg viability in Anopheles gambiae [9,10]. Assumptions Our thesis requires unequivocal differences in the mortality and behaviour of pyrethroid-resistant and susceptible Anopheles mos- quitoes when exposed to binary treatments of PPF and pyrethroids. Host-seeking or resting mosquitoes are more likely to be irritated, repelled or killed by co-treated surfaces if they are pyrethroid-susceptible. Conversely, pyrethroid-resistant insects are PLOS ONE | www.plosone.org 1 May 2014 | Volume 9 | Issue 5 | e95640