Seasonal stratification of anopheline abundance and species composition in southern Zambia suggests year-round, season-specific vector control approaches are needed.

BACKGROUND: Malaria control is a public health priority but in settings with seasonal residual malaria transmission, the field lacks critical knowledge regarding anopheline species composition, particularly the dynamics that contribute to persistent transmission in the dry season. This study aimed to determine relative seasonal vector species abundance and associated household level risk factors in a low transmission setting in Choma District, Zambia. METHODS: Indoor and outdoor adult vector collections were embedded in a community-based longitudinal cohort study in 60 households that were visited monthly for 2 years between 2018 and 2020. Surveys conducted at the time of trap placement collected information on animal ownership, housing structure, and the receipt of malaria interventions. Anopheline species identities were molecularly confirmed by polymerase chain reaction, and enzyme-linked immunosorbent assay was used to detect the circumsporozoite protein of Plasmodium falciparum. Generalized linear mixed effects negative binomial regression with zero-inflation models were used to describe the relationship between risk factors and the outcome of monthly anopheline counts at each household, stratified by season. RESULTS: The study collected 1532 female anophelines, 76% of which were caught outdoors. The relative abundance differed by season: in the dry season, 90% of female anophelines were caught outdoors. Anopheles arabiensis was the most common vector, but made up only 28% of outdoor collections; the remainder were understudied anophelines including Anopheles coustani, Anopheles leesoni, Anopheles rufipes, and Anopheles squamosus. The only P. falciparum-infected mosquito was a single An. squamosus that was caught outdoors. Owning more goats was associated with a 3.5 (IRR 4.47, 95% confidence interval [CI]: 2.00, 10.01) and 7.7 (IRR 8.73, 95% CI 4.40, 17.32) times increase in indoor and outdoor anopheline collections, respectively in the dry season and a 1.2 (IRR 2.18, 95% CI 1.12, 4.23) times higher risk of outdoor anophelines in the rainy season. Improved housing structure was associated with fewer indoor anophelines in the rainy season, but no change in indoor anophelines in the dry season or outdoor anophelines at any time of year. CONCLUSIONS: Vector control in this low transmission setting, therefore, needs to target anopheline mosquitoes year-round, must be expanded to target traditionally zoophillic mosquitoes, and leverage known risk factors when selecting methods of control.