Determining discriminating concentrations of insecticides for monitoring resistance in mosquitoes: report of a multi-centre laboratory study and WHO expert consultations

Overview

WHO conducted a multi-centre study in 2017–2021 involving 23 laboratories throughout the world to establish and validate discriminating concentrations (DCs) of insecticides in order to monitor the resistance to insecticides of the main Anopheles and Aedes spp. mosquito vectors of human diseases. A total of 18 insecticides in eight classes and one synergist, piperonyl butoxide (PBO), were tested in susceptible strains of the five main Anopheles spp., Anopheles gambiae s.s, An. funestus, An. minimus, An. stephensi and An. albimanus; and two Aedes spp., Aedes aegypti and Ae. albopictus.

The key outcomes of the study are:

• formation of an international network of 23 laboratories in five WHO regions that have susceptible colonized mosquito species for testing 18 insecticides and PBO according to standardized methods and test procedures; „ establishment and validation of 17 new insecticide DCs for Aedes spp. in either WHO filter paper bioassays or bottle bioassays;
• establishment and validation of 13 new DCs for Anopheles spp. in WHO filter paper or bottle bioassays (Tables 3 and 4);
• development and validation of a new standard bottle assay procedure, “the WHO bottle bioassay”, for testing compounds with modes of action that are not suitable for impregnation on filter paper;
• development of comprehensive standard operating procedures (SOPs) for impregnation of filter papers with insecticides and conduct of WHO tube tests and WHO bottle bioassays for testing the susceptibility of mosquitoes to insecticides;
• development and validation of an SOP for testing the susceptibility of adult mosquitoes to the sterilizing properties of pyriproxyfen, a juvenile hormone analogue that acts as an insect growth regulator;
• development of a provisional SOP for determining the optimal concentration of PBO for synergist-insecticide bioassays with Aedes spp. mosquitoes;
• development of a centralized database of bioassay records for > 400 000 mosquitoes of concentration–response tests and assessments of the source of variation in mosquito mortality under different test conditions; and
• identification and recommendation of measures to improve laboratory testing procedures and to guide the selection of DCs of insecticides commonly used for vector control