Abstract
Airborne spatial repellency (SR) is characterized and distinguished from other chemical actions including contact locomotor excitation and toxicity. The use of volatile spatial repellents is a potential new intervention class for combatting mosquito-borne pathogen transmission; therefore, continuing investigations on the actions of these chemicals that modify mosquito host‐seeking behavior (i.e., bite prevention) is needed. The objective of this study is to characterize the key behavioral avoidance actions of transfluthrin (TFT) to advance spatial repellent development into practical products. Behavioral avoidance responses were observed for adult laboratory strains of Aedes aegypti, Anopheles minimus and An. dirus, and two field populations of An. harrisoni and Ae. aegypti, respectively. Established TFT sublethal (LC50 and LC75), lethal concentrations (LC99) and discriminating concentrations (DCs) were selected corresponding to each mosquito test species. Spatial repellency and contact excitation (‘irritancy’) responses on adult mosquitoes to TFT were assessed using an excito-repellency assay system. At LC50, TFT exhibited strong avoidance with An. minimus (60.1% escape) and An. dirus (80% escape) laboratory strains, showing between 12 and 16x greater escape response than Ae. aegypti (5% escape). Repellency responses for field collected Ae. aegypti and An. harrisoni were 54.9 and 47.1% escape, respectively. After adjusting the initial contact escape response (a measure of combined irritancy and repellency) to estimate only escape due to contact, the LC50 and LC99 showed moderate escape irritancy with laboratory Ae. aegypti (41.4% escape) and no contact activity against the field population. Adjustment showed only weak contact activity (16.1% escape) in laboratory An. minimus at LC50. Spatial repellency is the predominant mode of action of TFT among colonized and field mosquitoes used in this study. Established baseline (susceptible) dose-response curves assist in optimizing SR products for mosquito control and pathogen transmission prevention.
Highlights
More than 4 billion people, primarily residing in tropical and sub-tropical countries, are at risk of infection from mosquito-borne diseases
Mosquitoes exposed to TFT had significantly (P < 0.05) greater percent escape in both noncontact and contact trials compared to the controls, except for Ae. aegypti in noncontact at LC50 (P = 0.0807) (S1 Table)
The percent escape in contact tests was inverse between the lower concentrations (0.00852% to 0.01274%), and LC99 (0.03412%) and the discriminating concentrations (DCs) (0.06824%) due to the higher knockdown (KD) effect inside the chamber (25.8–47.8% vs. 85.7–94.7% KD for LC50-LC75 and LC99-DC, respectively) compared to those that escaped (4.8–24% KD)
Summary
More than 4 billion people, primarily residing in tropical and sub-tropical countries, are at risk of infection from mosquito-borne diseases. An estimated 390 million dengue infections occur and around 200 million malaria cases are reported annually [1, 2]. Dengue and other Aedes-borne viruses (yellow fever, chikungunya, Zika) are projected to expand and pose a greater risk, both geographically and demographically, in the decades ahead [1]. While malaria programs have recently made substantial progress in reducing disease burden worldwide, the fight is far from over. In Thailand, 85,849 dengue infections were reported in 2018, while malaria contributed 6,625 cases, of which 82% were Plasmodium vivax infections, a latent relapse form of parasite complicating elimination goals in the country [3]. Even though malaria cases decreased 2.2-fold in 2018 from 2017, the actual number of cases are underreported [3]
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