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Background  The development and spread of insecticide resistance among malaria vectors, is a threat to the continued effectiveness of interventions to control and eliminate the disease. The status of insecticide resistance among malaria vector populations at two sites in Kome, southern Chad, was evaluated to inform decisions on vector control.    Methods  Mosquito larvae were collected from temporary rain-filled and semi-permanent breeding places at two sites and reared in a laboratory. Emerging  Anopheles gambiae  ( senso lato ) ( s.l .) adults were morphologically identified, sorted and evaluated for susceptibility to WHOPES recommended insecticides. Standardized biomolecular and biochemical methods were used to determine sibling species and molecular forms: knockdown resistant alleles ( kdr-w ) for pyrethroids and DDT; acetylcholinesterase-1 resistant alleles for organophosphate and carbamates; biochemical resistance through measurement of the levels of non-specific esterase (α and β), oxidase and glutathione-s-transferases activities.    Results   Anopheles gambiae  ( s.l. ) was the main vector group in the two study sites and comprised of  Anopheles gambiae  ( senso stricto ) ( s.s. ) and  An. arabiensis , respectively, at 71 and 29 % in Site A, and 60 and 40 % at Site B.  Anopheles gambiae  ( s.s. ) was composed of M ( Anopheles coluzzii ) and S [nominotypical  An. gambiae  ( s.s. )] molecular forms.  Anopheles coluzzii  accounted for over 98 % of the sub-group. There was extensive phenotypic resistance to pyrethroids, DDT and carbamates, but full susceptibility to organophosphates. Population-wide frequency of knockdown resistant allele in  An. gambiae  ( s.l. ) was 43 homozygous (RR), 19 heterozygous (RS) and 38 % homozygous susceptible (SS). When segregated by species and molecular forms,  An. coluzzii  had the highest  kdr-w  frequency of 37.4 homozygous resistant alleles, and 17.5 % heterozygous, with 8.3 % homozygote susceptible alleles.  An. gambiae  ( s.s. ) had 1 % homozygous resistant allele. Levels of esterase, oxidase and glutathione-s-transferases were not significantly different compared to fully susceptible laboratory raised  An. gambiae  ( s.s. ) Kisumu reference, although few individuals showed significant elevation of esterases (> 0.04 μg/protein), indicating a likely start of biochemical enzyme resistance.    Conclusions  There is an urgent need for action to stop and reverse significant insecticide resistance in the area. A comprehensive entomological surveillance and monitoring program is needed to understand the full extent of resistance to enable realistic insecticide resistance management strategy, and also to track future changes in the vector populations.

parasites and vectors

Species composition and insecticide resistance status of Anopheles gambiae (s.l.) (Culicidae) in Kome, southern Chad and the implications for malaria control