Malaria vector species composition and entomological indices following indoor residual spraying in regions bordering Lake Victoria, Tanzania

Abstract

Background: Vector control through long lasting insecticidal nets 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 Lake Victoria basin since 2007. Due to pyrethroid resistance in malaria vectors, use of pyrethroids for IRS was phased out and from 2014 to 2017 pirimiphos-methyl (Actellic® 300CS) was sprayed in regions of Kagera, Geita, Mwanza and Mara. Entomological surveillance was conducted in ten sprayed and four unsprayed sites to determine the impact of IRS on entomological indices related to malaria transmission risk. Methods: WHO Cone bioassays were conducted monthly on interior house walls to determine residual efficacy of pirimiphos-methyl CS. Indoor CDC light traps with or without bottle rotator were hung next to protected sleepers indoors and also set outdoors (un-baited) as a proxy measure for indoor and outdoor biting rate and time of biting. Prokopack aspirators were used indoors to capture potentially resting malaria vector. A sub-sample of Anopheles were tested by PCR to determine species identity and ELISA for sporozoite rate. Results: Annual IRS with Actellic® 300CS from 2015 to 2017 was effective on sprayed walls for a mean of 7 months in cone bioassay. PCR of 2016 and 2017 samples showed vector populations were predominantly An. arabiensis (58.1%, n=4,403 IRS sites, 58%, n=2,441 unsprayed sites). There was a greater proportion of An. funestus s.s. in unsprayed sites (20.4%, n=858) than sprayed sites (7.9%, n=595) and fewer An. parensis (2%, n=85 unsprayed, 7.8%, n=591 sprayed). Biting peaks of An. gambiae s.l. followed periods of rainfall occurring between October and April, but were generally lower in sprayed sites than unsprayed. In most sprayed sites, An. gambiae s.l. indoor densities increased between January and February, i.e. 10-12 months after IRS. The predominant species An. arabiensis had a sporozoite rate in 2017 of 2.0% (95% CI: 1.4-2.9) in unsprayed sites compared to 0.8% (95% CI: 0.5-1.3) in sprayed sites (p=0.003). Sporozoite rates were also lower for An. funestus collected in sprayed sites. Conclusion: This study contributes to the understanding of malaria vector species composition, behavior and transmission risk following IRS around Lake Victoria and can be used to guide malaria vector control strategies in Tanzania.

Increasing role of pyrethroid-resistant Anopheles funestus in malaria transmission in the Lake Zone, Tanzania: implications for the evaluation of novel vector control products

“…Around Lake Victoria, species abundance and transmission intensity vary quite considerably spatially and temporally [28], with implications for the deployment of effective malaria vector control interventions. These heterogeneities likely reflect differences in climatic conditions such as rainfall and ecological settings, which support the breeding of particular vector species [29].…”

“…In some countries, Anopheles funestus sensu stricto (s.s.) has historically played a significant role in malaria transmission [14][15][16][17] largely due to its predominantly anthropophilic and endophilic tendencies [18], intense pyrethroid resistance [19][20][21][22][23][24] and greater daily survival probabilities (higher parity rates) [25,26]. In other areas, notably south-east Tanzania [25,27], far northwest Tanzania [28] and parts of Kenya [13], this species is rapidly replacing An. gambiae s.s. and An.…”

“…Malaria prevalence around Lake Victoria remains amongst the highest in Tanzania [30], despite high community-level coverage with LLINs, and periodic IRS campaigns [6,28].…”