The Impact of Household and Community Indoor Residual Spray Coverage with Fludora Fusion in a High Malaria Transmission Setting in Northern Zambia.

Zambia's National Malaria Elimination Program transitioned to Fludora Fusion in 2019 for annual indoor residual spraying (IRS) in Nchelenge District, an area with holoendemic malaria transmission. Previously, IRS was associated with reductions in parasite prevalence during the rainy season only, presumably because of insufficient residual insecticide longevity. This study assessed the impact of transitioning from Actellic 300CS to long-acting Fludora Fusion using active surveillance data from 2014 through 2021. A difference-in-differences analysis estimated changes in rainy season parasite prevalence associated with living in a sprayed house, comparing insecticides. The change in the 2020 to 2021 dry season parasite prevalence associated with living in a house sprayed with Fludora Fusion was also estimated. Indoor residual spraying with Fludora Fusion was not associated with decreased rainy season parasite prevalence compared with IRS with Actellic 300CS (ratio of prevalence ratios [PRs], 1.09; 95% CI, 0.89-1.33). Moreover, living in a house sprayed with either insecticide was not associated with decreased malaria risk (Actellic 300CS: PR, 0.97; 95% CI, 0.86-1.10; Fludora Fusion: rainy season PR, 1.06; 95% CI, 0.89-1.25; dry season PR, 1.21; 95% CI, 0.99-1.48). In contrast, each 10% increase in community IRS coverage was associated with a 4% to 5% reduction in parasite prevalence (rainy season: PR, 0.95; 95% CI, 0.92-0.97; dry season: PR, 0.96; 95% CI, 0.94-0.99), suggesting a community-level protective effect, and corroborating the importance of high-intervention coverage.

Expanded geographic distribution and host preference of Anopheles gibbinsi (Anopheles species 6) in northern Zambia.

BACKGROUND: Nchelenge District in northern Zambia suffers from holoendemic malaria transmission despite a decade of yearly indoor residual spraying (IRS) and insecticide-treated net (ITN) distributions. One hypothesis for this lack of impact is that some vectors in the area may forage in the early evening or outdoors. Anopheles gibbinsi specimens were identified in early evening mosquito collections performed in this study area, and further insight was gleaned into this taxon, including characterizing its genetic identity, feeding preferences, and potential role as a malaria vector. METHODS: Mosquitoes were collected in July and August 2019 by CDC light traps in Nchelenge District in indoor sitting rooms, outdoor gathering spaces, and animal pens from 16:00-22:00. Host detection by PCR, COI and ITS2 PCR, and circumsporozoite (CSP) ELISA were performed on all samples morphologically identified as An. gibbinsi, and a subset of specimens were selected for COI and ITS2 sequencing. To determine risk factors for increased abundance of An. gibbinsi, a negative binomial generalized linear mixed-effects model was performed with household-level variables of interest. RESULTS: Comparison of COI and ITS2 An. gibbinsi reference sequences to the NCBI database revealed > 99% identity to "Anopheles sp. 6" from Kenya. More than 97% of specimens were morphologically and molecularly consistent with An. gibbinsi. Specimens were primarily collected in animal pen traps (59.2%), followed by traps outdoors near where humans gather (24.3%), and traps set indoors (16.5%). Host DNA detection revealed a high propensity for goats, but 5% of specimens with detected host DNA had fed on humans. No specimens were positive for Plasmodium falciparum sporozoites. Animal pens and inland households > 3 km from Lake Mweru were both associated with increased An. gibbinsi abundance. CONCLUSIONS: This is the first report of An. gibbinsi in Nchelenge District, Zambia. This study provided a species identity for unknown "An. sp. 6" in the NCBI database, which has been implicated in malaria transmission in Kenya. Composite data suggest that this species is largely zoophilic and exophilic, but comes into contact with humans and the malaria parasites they carry. This species should continue to be monitored in Zambia and neighbouring countries as a potential malaria vector.