Assessment of Entomological Parameters in Anophelines and Insecticide Susceptibility Status of Anopheles gambiae sensu lato in Selected Zones of Gambella Region, Southwestern Ethiopia

Abstract

Gambella region is characterized by high precipitation, high humidity and warm temperature. These climatological conditions are favorable for survival and distribution of malaria vectors. The region was using DDT for over four decades alternatively with malathion until it was terminated from use in 2009 and replaced by deltamethrin for IRS. Currently, deltamethrin and propoxur are in use for malaria vector control. The study was conducted in Gambella Town, Lare and Abobo districts, southwestern Ethiopia from June to October 2013. Longitudinal and cross-sectional study design was employed to assess species composition, density, parous rates, fed/gravid, gonotrophic cycle, peak hourly activity and insecticide susceptibility status in three selected sites southwestern Ethiopia. Anopheles mosquitoes were collected using CDC light trap from indoor and outdoor to assess of entomological parameters. Anopheline mosquito larvae were collected and reared to adult for insecticide susceptibility test. Data were analyzed using SPSS software package version 16.0. Mean indoor and outdoor density of adult anopheline mosquito species were compared using ANOVA and mean mosquito density pre and post spray operation was compared using T-test. P < 0.05 was considered statistically significant during the analysis. The fed/gravid ratio was determined. Overall 3,177 anopheline mosquitoes belonging to five species were collected over the 5 months study period. Of these 1,417 (44.6%) belong to An. gambiae s.l. presumably An. arabiensis followed by An. pharoensis 676 (21.3%), An. funestus group, 538 (16.9%), An. wellcomei 309 (9.7%) and An. nili 237 (7.5%). Mean probability of daily survival pre spray operation was significantly higher than mean probability of daily survival post spray operation (P < 0.05). In deltamethrin sprayed areas, mean parous rate did not show significant variation (Gambella town and Lare district). However, significantly higher reduction (more than 50%) in parous rates of anopheline mosquito species was recorded in a propoxur sprayed area (Abobo district). Mean mosquito densities did not differ significantly between pre and post-IRS operation in deltamethrin sprayed areas. In contrast, there was significant difference in mosquito density pre and post spray operation in propoxur sprayed area. The duration of gonotrophic cycle was approximately two days for all anopheline mosquito species. An. gambiae s.l., An. pharoensis and An. wellcomei showed early activity (before 22:00 hours). Moreover, An. gambiae s.l. showed resistance to DDT and deltamethrin but susceptible to bendiocarb, propoxur and primiphos-methyl in all sampling sites. The development resistance by anopheline mosquitoes against deltamethrin has been determined by assessment of entomological parameters. Resistance development of An. gambiae s.l. in susceptibility test against DDT and deltamethrin could be from the long term use of the insecticides for IRS.