Abstract
BackgroundVector 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 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 10 sprayed and 4 unsprayed sites to determine the impact of IRS on entomological indices related to malaria transmission risk.MethodsWHO 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 (unbaited) as a proxy measure for indoor and outdoor biting rate and time of biting. Prokopack aspirators were used indoors to capture resting malaria vectors. A sub-sample of Anopheles was tested by PCR to determine species identity and ELISA for sporozoite rate.ResultsAnnual 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 Anopheles arabiensis (58.1%, n = 4,403 IRS sites, 58%, n = 2,441 unsprayed sites). There was a greater proportion of Anopheles funestus sensu stricto in unsprayed sites (20.4%, n = 858) than in sprayed sites (7.9%, n = 595) and fewer Anopheles parensis (2%, n = 85 unsprayed, 7.8%, n = 591 sprayed). Biting peaks of Anopheles gambiae sensu lato (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.ConclusionThis study contributes to the understanding of malaria vector species composition, behaviour and transmission risk following IRS around Lake Victoria and can be used to guide malaria vector control strategies in Tanzania.
Highlights
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
Residual efficacy of Actellic® 300CS Cone bioassays were conducted on interior wall surfaces according to World Health Organization (WHO) protocols to determine the quality of spray within 14 days of application and the duration of residual efficacy, which was monitored monthly until mortality was lower than 80% for two consecutive months [14]
Anopheles gambiae Kisumu strain were known to be fully susceptible to pirimiphos-methyl and were reared in the National Institute for Medical Research Mwanza insectary at 27 ± 1 °C, and 60–80% relative humidity before being transported to the field in cool boxes for the assays
Summary
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 sub-Saharan Africa, recent gains in malaria control have been mostly accomplished through a substantial boost in vector control using long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS). These tools have significantly contributed to a 50% reduction of Plasmodium falciparum infection prevalence in endemic countries between 2000 and 2015 [1]. In mainland Tanzania, IRS implementation funded by the US President’s Malaria Initiative (PMI) was launched in 2007 in Muleba and Karagwe districts, located in Kagera Region. Thereafter, IRS activities with pyrethroid insecticides were progressively expanded to other districts in the Lake Victoria basin, including the remaining five districts of Kagera Region in 2009 and, in 2010 and 2011 to all 18 districts of Kagera, Mwanza, and Mara, covering 1.1 million structures and targeting nearly 6.3 million people [6]
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