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Background  A longitudinal  Anopheles gambiae  s.l. insecticide resistance monitoring programme was established in four sentinel sites in Chad 2008–2010. When this programme ended, only sporadic bioassays were performed in a small number of sites.    Methods  WHO diagnostic dose assays were used to measure the prevalence of insecticide resistance to 0.1% bendiocarb, 4% DDT, 0.05% deltamethrin, 1% fenitrothion, and 0.75% permethrin in the main malaria vectors at the beginning and end of the malaria transmission season for three years 2008–2010, with subsequent collections in 2011 and 2014. Species and molecular identification of  An .  gambiae  M and S forms and  kdr  genotyping was performed using PCR-RLFP; circumsporozoite status was assessed using ELISA.    Results  Between 2008 and 2010, significant changes in insecticide resistance profiles to deltamethrin and permethrin were seen in 2 of the sites. No significant changes were seen in resistance to DDT in any site during the study period. Testing performed after the period of routine monitoring had ended showed dramatic increases to DDT and pyrethroid resistance in 3 sites. No resistance to organophosphate or carbamate insecticides was detected.  An .  arabiensis  was the predominate member of the  An .  gambiae  complex in all 4 sites; adult collections showed temporal variation in species composition in only 1 site.  Kdr  analysis identified both 1014F and 1014S alleles in  An .  gambiae  S only. Circumsporozoite analysis showed the highest vector infection rates were present in Donia, a site with extensive use of agricultural insecticides.    Conclusions  During the monitoring gap of four years, significant changes occurred in resistance prevalence in 3 of the 4 sites (p = <0.001), endangering the efficacy of currently implemented malaria control interventions. Significant changes in insecticide resistance profiles and a lack of  kdr  resistance alleles in adult populations highlight the urgent need for comprehensive entomological monitoring to be implemented and sustained in country.

Spatial and Temporal Trends in Insecticide Resistance among Malaria Vectors in Chad Highlight the Importance of Continual Monitoring