Field evaluation of a volatile pyrethroid spatial repellent and etofenprox-treated clothing for outdoor protection against forest malaria vectors in Cambodia
Authors: Vajda Élodie A , Ross Amanda , Doum Dyna , Fairbanks Emma , Chitnis Nakul , Hii Jeffrey , Moore Sarah J , Richardson Jason , Macdonald Michael , Sovannaroth Siv , Kimheng Pen , McIver David J , Tatarsky Allison , Lobo Neil F 
Published: February 2024

Cambodia’s goal to eliminate malaria by 2025 is challenged by persisting transmission in the country’s forest and forest fringe areas. People living in, or traveling to the forest, are exposed to malaria vector bites during the day due toAnophelesdaytime biting; and during the night, due to low bed net use and open sleeping structures. Volatile pyrethroid spatial repellents (VPSRs), and insecticide treated clothing (ITC) may help address these gaps in protection. In this field study the authors evaluated the outdoor application of one passive, transfluthrin-based VPSR, four etofenprox-ITCs paired with a picaridin topical repellent, and a combination of VPSR and ITC against wildAnopheleslanding in Cambodia. Mathematical modeling was also used to predict the reduction of vectorial capacity of these interventions.

A 7×7 Latin-square (6 interventions and one control) was conducted over 49 collection nights in seven temporary, open structures in a forest in Mondulkiri Province, Cambodia. Pairs of participants conducted human landing catches (HLCs) from 18h00 to 06h00, with each collector conducting collections for six hours. A randomly selected subset of collectedAnopheleswere identified to species using molecular methods. The rate ratio of each intervention compared to the control onAnopheleslandings was estimated using a mixed-effect negative binomial regression with intervention, structure, and collector-pair as fixed-effects, and with collection date and structure-night as random effects. The modeling assessment aims to predict the relative reduction in vectoral capacity. Initial calculations involved establishing a “baseline scenario” without intervention, utilizing biometric parameters forAnopheles dirus. Various scenarios accounting for intervention coverage and adherence were then considered. The study aims to update parameters using field study estimates for wildAnopheles, incorporating multiple semi-field estimates for interventions and accounting for the variability and uncertainty in parameter values.

Of the total 8,294Anophelesspecimens collected, 15% (n=1,242) of specimens were confirmed to species or species group via PCR. Fifteen species were confirmed;Anopheles dirusForm A was predominant (n=429), followed byAnopheles maculatus(n=189), andAnopheles minimus(n=60). All six interventions reducedAnopheleslanding substantially; protective efficacies ranged between 61% (95% confidence interval (CI): 48 – 71%) (etofenprox-ITC, washed) and 95% (95% CI: 93 – 96%) (combined VPSR and unwashed etofenprox-ITC). Finally, the modelling assessment demonstrates significant reductions in vectoral capacity, with the highest impact observed for the combined ITC and VPSR as well as the VPSR used alone, although effectiveness decreases with intervention aging, and variability exists in the magnitude of predicted reductions due to differences in experimental conditions.

These transfluthrin-based VPSR and etofenprox ITC interventions have the potential to reduce outdoor and daytimeAnophelesbiting by providing substantial protection againstAnopheleslanding. One or more of these tools may play a valuable role in the push for elimination in Cambodia and the Greater Mekong Subregion if programs can achieve effective coverage.