Entomological indicators of malaria transmission, and insecticide susceptibility status of Anopheles arabiensis in Sille village, south-west Ethiopia

Abstract

In Ethiopia, vector control is the principal strategy to reduce the burden of malaria. The entomological indicators of malaria transmission such as density, sporozoite rate and entomological inoculation rate (EIR) are the major parameters to assess the impact of vector control interventions. The susceptibility of the malaria vectors also determines the effectiveness of insecticide based vector control tools. Hence, the main aim of the study was to assess the species composition, sporozoite rate and EIR, and insecticide susceptibility status of malaria vectors. A total of 33 houses (18 for Centre for Disease Prevention and Control (CDC) light traps and 15 for exit traps) were randomly selected to sample Anopheles mosquitoes from October 2015 to May 2016. The Plasmodium circum-sporozoite proteins (CSPs) of An. arabiensis and An. pharoensis were determined using Enzyme-Linked Immuno-Sorbent Assay (ELISA) technique. Five Anopheles species were identified by CDC Light traps and exit traps. Anopheles arabiensis (80.2%) was the predominant species, followed by An. pharoensis(18.5%). Anopheles pretoriensis, An. tenebrosus and An. rhodesiensis were documented in small numbers. A total of 1056 Anophelesmosquitoes were tested for CSPs. Of which nine (eight An. arabiensis andone An. pharoensis) were positive for CSPs with the overall CSP rate of 0.85% (95% CI: 0.3-1.4). Four Anopheles were positive for P.vivax_210 and 5 were positive for P. falciparum. The P. falciparum CSP rate of An. arabiensis was 0.46% (95% CI: 0.13-1.2) and it was 0.54% (95% CI: 0.01-2.9) for An. pharoensis. The EIR of An. arabiensis was 10.1 infectious bites per/person (ib/p)/eight months. The highest EIR of An. arabiensis was found in February with 4.78 ib/p/month, following the highest density of An. arabiensis in January 2016.Anopheles arabiensis was resistant to dieldrin (4%), and deltamethrin (0.05%) with mortality rates of 57% and 71%, respectively. Permethrin (0.75%) and malathion (0.8%) showed possible resistance with mortality rates 90.4% and 92.5%, respectively. Anopheles arabiensiswas fully susceptible to propoxur and bendiocarb.Based on the EIR of An. arabiensis, malaria is endemic in the Sille village, and the main malaria transmission was occurred from December 2016-February 2016. Hence, supplementary vector control interventions are required as the vector developed resistance to the insecticides used for impregnation of bed nets.