Bio-efficacy of insecticides used for vector control in Busia and Tororo districts, Eastern Uganda

Abstract

High malaria transmission rates in Sub-Saharan Africa (SSA) are attributed to the continuous presence of effective and competent Plasmodium vectors, Anopheles gambiae complex and the Anopheles funestus group. These vectors play a key role in transmitting the most dangerous malaria parasite species Plasmodium falciparum. The key vector control interventions in Africa including Uganda, rely heavily on utilisation of long-lasting insecticidal nets (LLINs) and insecticide-based indoor residual spraying (IRS). This study assessed the bio-efficacy of LLINs and IRS as vector control interventions used in Busia and Tororo districts. Samples of Anopheles mosquito larvae were collected from various breeding grounds before rearing them in an insectary. Additionally, a nine-year period malaria cases data set (2012-2020) was obtained from the health management database to depict malaria trends. Emerged adults were dissected under a stereoscopic microscope and identified using standardized morphological keys after respective bioassays on used nets and sprayed walls. A One-Way Analysis of Variance (ANOVA) was used to test for significant differences in the mean composition of mosquito species, mean variations, mortalities by net and wall type. Results showed that mean mortalities of Anopheles mosquitoes to used brands of nets from Busia (9.86±11.35) and Tororo (9.64 ±11.12) varied insignificantly (t=0.119, p=0.906). A disaggregated analysis for each net used revealed that, the DAWA plus 2.0 registered a highest mean mortality of mosquitos, followed by PermaNet 3.0, Olyset and PermaNet 2.0 respectively. The average number of Anopheles mosquitoes that died after 2 weeks (6.86±4.07) and 1 month of chemical spray (6.69±4.01) varied insignificantly (t=0.175, P=0.861). Meanwhile, disaggregated results by time of spray (T0 and T1) and exposure periods, showed that plastered painted wall, had higher mortalities followed by Brick Plain and Mud/Wattle wall substrates, respectively. Considering the malaria cases registered on annual basis, Mann- Kendal test revealed a drastic decline of malaria cases over the nine year period (2012 -2020) in Tororo district with Sen’s slope of -22, while for Busia district it revealed an increase of malaria cases (Sen’s slope +28). Equally, these trends reveal varied spatial patterns over the two districts. Both the districts revealed a high prevalence of malaria between May-November in respect to bimodal rainfall pattern, matching to the perennial transmission setting of Uganda. This study has shown that vector behaviour, biology and physiology need consistent monitoring and surveillance for further entomological characterisation.