First Report on Intensity Bioassays for Pyrethroid Resistance in Anopheles culicifacies s.l in District Dindori of Madhya Pradesh State and Districts Kanker and Bastar of Chhattisgarh State, India

BACKGROUND: The major malaria vector in India is Anopheles culicifacies, and indoor residual spraying (IRS) and distribution of long-lasting insecticidal nets (LLINs) are the two main vector control measures in the national program. This species has shown resistance to dichlorodiphenyltrichloroethane (DDT), malathion, and synthetic pyrethroids (SP). The study was carried out in three districts, that is, Dindori of Madhya Pradesh and Kanker and Bastar from Chhattisgarh state to know the range of resistance phenotypes and to assess the strength of resistance in An. culicifacies. METHODS: An. culicifacies collected from the field was tested to determine the susceptibility status to the discriminating concentration (DC) of different insecticides, that is, DDT 4.0%, malathion 5.0%, alphacypermethrin 0.05%, and deltamethrin 0.05% following the World Health Organization (WHO) procedures. Further, intensity bioassays of the resistant An. culicifacies to 1X discriminating concentration (DC) of alphacypermethrin and deltamethrin were conducted by exposing 5X and 10X concentrations of 1X DC. Results are interpreted as per the WHO criterion. RESULTS: The overall result of susceptibility status in An. culicifacies in districts Dindori, Kanker, and Bastar revealed confirmed resistance to DDT, malathion, alphacypermethrin, and deltamethrin registering mortality of 15% (10–20%), 70% (65–75%), 78.6% (77–82.5%), and 87% (84.8–91.3%), respectively. Further, in district Dindori and Baster, the intensity bioassay test at 5X DC of alphacypermethrin and deltamethrin revealed 99% and 100%, respectively, while in district Kanker, the species registered moderate resistance with 92.5% and 95% mortality, respectively, in 5X DC of alphacypermethrin and deltamethrin. However, in 10X DC, the An. culicifacies was susceptible to both pyrethroids (100%) in district Kanker. CONCLUSION: The results of the intensity bioassay tests with SP-resistant An. culicifacies suggested no change of insecticide is required in the ongoing intervention. However, regular monitoring of insecticide susceptibility and intensity bioassays in malaria vectors in view of continued use of these interventions may increase resistance and for implementing effective vector management strategies.