koamabayili/VECTRON: Laboratory and experimental hut trial evaluation of VECTRON™ T500 for indoor residual spraying (IRS) against insecticide resistant malaria vectors in Burkina Faso

Background The recent rebound of malaria cases in some endemic areas could be due to the spread of insecticide resistance in malaria vectors. Furthermore, the insecticides recommended by WHO for vector control use are limited in number. Hence, it is essential to improve efficacy and find rotational partners for existing IRS products. VECTRON™ T500 is a novel insecticide formulation containing the active ingredient broflanilide developed by Mitsui Chemicals Agro, Inc., for IRS use to control pyrethroid resistant mosquitoes. It has a mode of action on mosquitoes completely different to other insecticides used in vector control. The aim of this study was to determine the optimum effective dose and efficacy of VECTRONTM T500, including its residual activity against susceptible and resistant strains of Anopheles in Burkina Faso. These studies were based on the WHO laboratory and experimental hut testing guidelines. Methods VECTRON™ T500 was sprayed at 50, 100 and 200 mg a.i./m² doses onto mud and concrete blocks using a Potter Spray Tower in laboratory tests. The residual activity of broflanilide was assessed after spraying through WHO cone bioassays 1 week after treatment and then monthly up to fourteen months post spraying. The efficacy of the insecticide was evaluated at 100 mg a.i./m² and 150 mg a.i./m² doses against wild free-flying mosquitoes in the experimental huts on both substrates. Actellic 300CS was applied at 1000 mg a.i/m² as a reference product. Additionally, in situ cone assays were conducted monthly, using susceptible Kisumu and resistant mosquito strains, to determine residual efficacy. Results In the laboratory, VECTRON™ T500 showed residual efficacy (≥80% mortality) on An. gambiae Kisumu strain for up to 12 months post spraying on concrete blocks and up to 14 months on mud blocks . Similar results were obtained with doses of 100 mg a.i./m² and 200 mg a.i./m² using a An. coluzzii pyrethroid -resistant strain. In experimental huts, a total of 19,552 An. gambiae s.l. were collected. Deterrence, blood-feeding inhibition and exophily obtained with VECTRON™ T500 treated huts were very low compared to the negative control huts. At doses of 100 mg a.i./m² and 150 mg a.i./m², the mortality of wild pyrethroid-resistant An. gambiae s.l. entering experimental huts ranged between 55% and 73%. Monthly wall cone bioassay mortality on VECTRON™ T500 treated mud or concrete walls remained >80% up to 9 months. Conclusions VECTRON™ T500 shows great potential as an IRS formulation for malaria vector control. It was effective against the locally dominant malaria vector population resistant to other insecticides currently used in public health. It can be added to the arsenal of IRS products for use in rotations to control malaria transmission and manage mosquito insecticide resistance.