Efficacy of a volatile pyrethroid spatial emanator (SE) in reducing Anopheles host-seeking in outdoor kitchens in Southern Zambia.

Abstract

In southern Zambia, malaria transmission is low, with outdoor biting Anopheles mosquitoes playing a significant role in malaria transmission. Locals cook in outdoor, open-walled kitchen shelters, exposing them to these outdoor biting vectors and malaria. Volatile pyrethroid spatial emanators (SE) operate through a mode of action which could provide local protection around these kitchens. In this study, SE devices containing the pyrethroid transfluthrin were deployed to local kitchens, where human landing collection (HLC) was utilized to determine differences in host-landing rates associated with protection. Forty-one households from two villages were enrolled in the study in clusters of five (or six) and randomly assigned a treatment by cluster. Local mosquito collectors were recruited and trained to conduct HLCs from 18:00-06:00 once per week for 15 weeks. SE and placebo devices were replaced monthly. Following collection, mosquitoes were returned to the lab for morphological identification. Results were analyzed in R with negative binomial generalized linear mixed models (GLMM) considering all-night and per-hour capture. A total of 3021 mosquitoes were collected during the follow-up period, with Anopheles species composing roughly half of all specimens. Anopheles activity was lower in SE protected kitchens based on all night and hourly comparisons. Mosquito activity was highest in the middle of the night, and SE was not significantly associated with protection between 18:00-21:00. SE usage appeared to reduce mosquito host-seeking activity by approximately 65-70%, with this efficacy appearing to diminish gradually over time to approximately 20-25% four weeks after opening, at which point they were replaced. Culicine mosquito behavior was not significantly impacted by the SE. The SE device provided protection to individuals within protected kitchen structures overnight and during most hours of the night. The number of mosquitoes was lowest during the early collection hours between 18:00-21:00, a period in which the SE devices did not significantly impact mosquito host-seeking behavior. This result has implications for this use-case and should be further explored.