Insecticide resistance intensity and efficacy of synergists with pyrethroids in Anopheles gambiae (Diptera: Culicidae) from Southern Togo.

BACKGROUND: This study was designed to provide insecticide resistance data for decision-making in terms of resistance management plans in Togo. METHODS: The susceptibility status of Anopheles gambiae sensu lato (s.l.) to insecticides used in public health was assessed using the WHO tube test protocol. Pyrethroid resistance intensity bioassays were performed following the CDC bottle test protocol. The activity of detoxification enzymes was tested using the synergists piperonyl butoxide, S.S.S-tributlyphosphorotrithioate and ethacrinic acid. Species-specific identification of An. gambiae s.l. and kdr mutation genotyping were performed using PCR techniques. RESULTS: Local populations of An. gambiae s.l. showed full susceptibility to pirimiphos methyl at Lomé, Kovié, Anié, and Kpèlè Toutou. At Baguida, mortality was 90%, indicating possible resistance to pirimiphos methyl. Resistance was recorded to DDT, bendiocarb, and propoxur at all sites. A high intensity of pyrethroid resistance was recorded and the detoxification enzymes contributing to resistance were oxidases, esterases, and glutathione-s-transferases based on the synergist tests. Anopheles gambiae sensu stricto (s.s.) and Anopheles coluzzii were the main species identified. High kdr L1014F and low kdr L1014S allele frequencies were detected at all localities. CONCLUSION: This study suggests the need to reinforce current insecticide-based malaria control interventions (IRS and LLINs) with complementary tools.

Resistance intensity status of Anopheles gambiae s.l. species at KOLOKOPE, eastern plateau Togo: A potential site to assess new vector control tools.

According to WHO recommendations, the deployment of the next generation of Long-Lasting Insecticidal Nets (LLINs) for malaria vector control requires appropriate investigations on the insecticide resistance profile of the vector. Most of the next generation of LLINs are impregnated with a combination of pyrethroid insecticides and piperonyl butoxide (PBO), a synergist with an additional impact on the increase in the mortality rate of Anopheles gambiae s.l. (Diptera: Culicidae). Kolokopé is a cotton-growing area in the central region of Togo characterized by an intensive use of agricultural pesticides and insecticides where there is a phase II experimental hut station. For the characterization of the site, WHO susceptibility tests using diagnostic doses of ten insecticides, PBO synergist assays and intensity assays of three pyrethroids (5x and 10x) were conducted on adult female mosquitoes obtained from larvae collected around the site. Anopheles gambiae s.l. from Kolokopé showed high resistance to pyrethroids and DDT, but to a lesser extent to carbamates and organophosphates. Likewise, high intensity of resistance to pyrethroid was observed with less than 40% mortality at 10x deltamethrin, 52 and 29% mortality at 10x permethrin and 10x alphacypermethrin, respectively. Also, PBO treatment resulted in increased mortality which was higher than the mortality rate at 10x doses of pyrethroids. The high pyrethroid intensity resistance recorded at Kolokopé could be mainly due to the selection pressure on An. gambiae s.l. caused by the excessive use of insecticide in agriculture. These results can be used to assess the next generation of LLINs either in experimental hut or at a community trial.

Insecticide resistance profiles of Anopheles gambiae s.l. in Togo and genetic mechanisms involved, during 3-year survey: is there any need for resistance management?

BACKGROUND: Malaria, one of the world's greatest public health challenges, is an endemic disease with stable transmission in Togo. Combating malaria requires an effective vector control. This study provides temporal data on insecticide resistance status in the major malaria vector Anopheles gambiae sensu lato (s.l.) from Togo. METHODS: Two to 5 days old females of An. gambiae s.l., originating from three localities (Baguida, Kovié, Kolokopé) were subjected to insecticide-impregnated papers during 3 years (2012, 2013, 2016) as follows: organochlorides (4% DDT), pyrethroids (0.05% deltamethrin, 0.75% permethrin, 0.05% lambdacyhalothrin), carbamates (0.4% bendiocarb and 0.1% propoxur), and organophosphates (5% malathion, 0.4% chlorpyrifos methyl, 1% fenitrothion) following the WHO standard protocol. Dead and surviving mosquitoes were stored separately in Eppendorf tubes containing silica gel for DNA extraction, species identification, and kdr and ace-1 genotyping. RESULTS: Knockdown times (KDT50 and KDT95) were high in An. gambiae s.l. The lowest KDTs were recorded at Baguida in 2013 for deltamethrin (KDT50 = 24.7, CI [22.4-27.12] and KDT95 = 90.78, CI [76.35-113.49]). No KDTs were recorded for DDT and in some instances for permethrin. In general, An. gambiae s.l. was resistant to most of the four classes of insecticides during the survey periods regardless of locality and year, except to chlorpyrifos methyl. In some instances, mosquitoes were fully susceptible to fenitrothion (Kolokopé: 100% and Kovié: 98.05%, CI [95.82-100.26]) and malathion (100% at both Kolokopé and Kovié) in 2013, and malathion only (Kolokopé; 100%) in 2016. Anopheles coluzzii, An. gambiae and Anopheles arabiensis were the three sibling species identified at the three localities with some hybrids at Baguida (2013), and Kovié (2012 and 2016), respectively. Anopheles gambiae was relatively dominant (61.6%). The kdr 1014F allele frequency was > 0.9 in most of the cases, except at Kolokopé (f (1014F) = 0.63, CI [0.55-0.71]) in 2013. The kdr 1014S allele frequency was below 0.02. The highest ace-1 frequencies were identified in An. gambiae at Baguida (2012: 0.52, CI [0.34-0.69] and 2013: 0.66, CI [0.46-0.86]). CONCLUSION: The resistance status is worrying in Togo and should be considered in future malaria vector resistance management programmes by decision-makers.