Trends of insecticide resistance monitoring in mainland Tanzania, 2004–2020

Background: Insecticide resistance is a serious threat to the continued effectiveness of insecticide-based malaria vector control measures, such as long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) of households with insecticide. We describe the trends and dynamics of insecticide resistance and its underlining mechanisms from annual resistance monitoring surveys on Anopheles gambiae s.l. populations conducted across mainland Tanzania from 2004 to 2020. Methods The World health Organization (WHO) standard protocols were used to detect susceptibility of the wild female Anopheles gambiae s.l. mosquitoes to insecticides, with mosquitoes exposed to diagnostic concentrations of permethrin, deltamethrin, lambdacyhalothrin, bendiocarb, and pirimiphos-methyl. WHO test papers at 5× and 10× the diagnostic concentrations were used to assess the intensity of resistance to pyrethroids; piperonyl butoxide (PBO) synergist tests were carried out in sites where mosquitoes were found to be resistant to pyrethroids. To estimate insecticide resistance trends from 2004 to 2020, mosquito mortalities from each site and time point were aggregated and regression analysis of mortality versus the Julian dates of bioassays was performed. Results Percentage of sites with pyrethroid resistance increased from 0% in 2004 to more than 80% in the 2020, suggesting resistance has been spreading geographically. Results indicate a strong negative association (p = 0.0001) between pyrethroids susceptibility status and survey year. The regression model shows that by 2020 over 40% of An. gambiae s.l. mosquitoes survived exposure to pyrethroids at their respective diagnostic doses. A decreasing trend of An. gambiae s.l. susceptibility to bendiocarb was observed over time, but this was not statistically significant (p = 0.8413). Anopheles gambiae s.l. exhibited high level of susceptibility to the pirimiphos-methyl in sampled sites. Conclusions An. gambiae s.l., Tanzania’s major malaria vector, is now resistant to pyrethroids across the country with resistance increasing in prevalence and intensity and has been spreading geographically between 2011 to 2020. This calls for urgent action for efficient malaria vector control tools to sustain the gains obtained in malaria control. Strengthening insecticide resistance monitoring is important for its management through evidence generation for effective malaria vector control decision making.