title: Micro-encapsulated pirimiphos-methyl shows high insecticidal efficacy and long residual activity against pyrethroid-resistant malaria vectors in central Côte d'Ivoire.
abstract: BACKGROUND
  The wide-scale implementation of insecticide-treated nets and indoor residual spraying (IRS) has contributed to a considerable decrease of malaria morbidity and mortality in sub-Saharan Africa over the last decade. Due to increasing resistance in Anopheles gambiae sensu lato mosquitoes to dichlorodiphenyl trichloroethane (DDT) and pyrethroids, alternative insecticide formulations for IRS with long-lasting residual activity are required to sustain the gains obtained in most malaria-endemic countries.
  
  
  METHODS
  Three experimental capsule suspension (CS) formulations of the organophosphate pirimiphos-methyl were evaluated together with Actellic 50 EC, an emulsifiable concentrate (EC) of pirimiphos-methyl, and the pyrethroid ICON 10 CS, a lambda-cyhalothrin CS formulation, in an experimental hut trial. The formulations were tested on two types of surfaces: mud and cement. The study with a 12-month follow-up was carried out in Bouaké, central Côte d'Ivoire, where An. gambiae mosquitoes show high levels of resistance against pyrethroids, DDT and carbamates. Residual activity was also tested in cone bioassays with the susceptible An. gambiae KISUMU strain.
  
  
  RESULTS
  One of the CS formulations of pirimiphos-methyl, CS BM, outperformed all other formulations tested. On cement surfaces, the odds ratios of overall insecticidal effect on An. gambiae s.l. of pirimiphos-methyl CS BM compared to Actellic 50 EC were 1.4 (95% confidence interval (CI): 1.2-1.7) for the first three months, 5.6 (95% CI: 4.4-7.2) for the second three months, and 3.6 (95% CI: 3.0-4.4) for the last six months of follow-up. On mud surfaces, the respective odds ratios were 2.5 (95% CI: 1.9-3.3), 3.5 (95% CI: 2.7-4.5), and 1.7 (95% CI: 1.4-2.2). On cement, the residual activity of pirimiphos-methyl CS BM measured using cone tests was similar to that of lambda-cyhalothrin and for both treatments, mortality of susceptible Kisumu laboratory strain was not significantly below the World Health Organization pre-set threshold of 80% for 30 weeks after spraying. Residual activity was shorter on mud surfaces, mortality falling below 80% on both pirimiphos-methyl CS BM and lambda-cyhalothrin treated surfaces at 25 weeks post-treatment.
  
  
  CONCLUSIONS
  CS formulations of pirimiphos-methyl are promising alternatives for IRS, as they demonstrate prolonged insecticidal effect and residual activity against malaria mosquitoes.
authors.forename: Emile S
authors.initials: ES
authors.lastname: Tchicaya
authors.forename: Christian
authors.initials: C
authors.lastname: Nsanzabana
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Smith
authors.forename: Jennifer
authors.initials: J
authors.lastname: Donzé
authors.forename: Maiti Laserna
authors.initials: ML
authors.lastname: de Hipsl
authors.forename: Yao
authors.initials: Y
authors.lastname: Tano
authors.forename: Pie
authors.initials: P
authors.lastname: Müller
authors.affiliation: Department of Epidemiology and Public Health, Swiss Tropical and Public Health Institute, CH-4002 Basel, Switzerland. olivier.briet@unibas.ch.
authors.forename: Olivier J T
authors.initials: OJ
authors.lastname: Briët
authors.forename: Jürg
authors.initials: J
authors.lastname: Utzinger
authors.forename: Benjamin G
authors.initials: BG
authors.lastname: Koudou
mesh_terms: D000818:Animals; D000852:Anopheles / Q000187:drug effects* / Q000502:physiology; D007560:Cote d'Ivoire; D007303:Insect Vectors / Q000187:drug effects*; D007306:Insecticides / Q000494:pharmacology*; D009946:Organothiophosphorus Compounds / Q000494:pharmacology*; D011722:Pyrethrins / Q000494:pharmacology; D016019:Survival Analysis
chemical_list: D007306:Insecticides; D009946:Organothiophosphorus Compounds; D011722:Pyrethrins; C014153:pirimiphos methyl