An entomological survey in the Sudanese Guinean environmental transition zone after indoor residual spraying, Chad

Introduction: malaria is a major public health issue in Africa. In Chad in 2019, with 955,243 confirmed cases and 2,955 deaths, malaria is the main cause of consultations. A longitudinal entomological study was conducted in Moissala Health District. Its objective was to assess the impact of indoor residual spraying with 80% bendiocarb wettable powder on malaria transmission. Methods: two areas were defined for the study: Dembo, located in the sprayed area, Moissala, in the untreated area. Two sampling methods were used: pyrethrum spray catches and human landing catches. Results: sixteen sessions of human landing catches totalling 32 man-nights were conducted and 160 rooms/site were sprayed. Two anopheles were captured in Dembo and 547 in Moissala. In Moissala, An coluzzii, An funestusand An rufipeswere captured in the rooms and on human bait. An colluzziiand An funestuswere captured in pyrethrum spray catches in Dembo. The anophelian human landing catches density was zero in Dembo while it was 8.38 bites/man/night in outdoor and 10.06 bits/man/night in indoor in Moissala. Only An coluzziiwas found infected in human landing catches and sporozoite index of was 7.46% (10/134) in outdoor and 7.45% (12/161) in indoor in Moissala. Malaria transmission was estimated at 0.53 infected bites/man/night in outdoor and 0.75 infected bites/man/night in indoor i.e. 229.95 infected bites/man/year in outdoor and 273.75 infected bites/man/year. In pyrethrum spray catches, An coluzziiand An rufipeswere the two species found infected in Moissala with sporozoite indices of 6.70% (23/343) and 20% (2/10) respectively. However, in Dembo, neither of the two captured mosquitoes was found infected. Conclusion: the indoor residual spraying campaign in the eastern zone of Moissala has led to the collapse of vectors' density and aggressiveness. However, its evaluation over a short period of time is not sufficient to assess the impact of malaria transmission in this constant and endemic malaria zone.