Vector control through long-lasting insecticidal nets (LLINs) and focal indoor residual spraying (IRS) is a major component of the Tanzania national malaria control strategy. In mainland Tanzania, IRS has been conducted annually around the Lake Victoria basin since 2007. Due to pyrethroid resistance in malaria vectors, the use of pyrethroids for IRS was phased out and from 2014 to 2017 pirimiphos-methyl (Actellic® 300CS) was sprayed in the regions of Kagera, Geita, Mwanza, and Mara. Entomological surveillance was conducted in 10 sprayed and 4 unsprayed sites to determine the impact of IRS on entomological indices related to malaria transmission risk.

The study found that annual IRS with Actellic® 300CS from 2015 to 2017 was effective on sprayed walls for a mean of seven months in cone bioassay. Biting peaks of Anopheles gambiae sensu lato (s.l.) were generally lower in sprayed sites than unsprayed. The predominant species An. arabiensis had a higher sporozoite rate in 2017 in unsprayed sites than sprayed sites. Sporozoite rates were also lower for An. funestus collected in sprayed sites.

This study contributes to the understanding of malaria vector species composition, behavior, and transmission risk and can be used to guide malaria vector control strategies in Tanzania.