Identifying the effective concentration for spatial repellency of the dengue vector Aedes aegypti

Abstract

BackgroundCurrent efforts are underway to quantify the chemical concentration in a treated air space that elicits a spatial repellent (deterrent) response in a vector population. Such information will facilitate identifying the optimum active ingredient (AI) dosage and intervention coverage important for the development of spatial repellent tools – one of several novel strategies being evaluated for vector-borne disease control. This study reports initial findings from air sampling experiments conducted under field conditions to describe the relationship between air concentrations of repellent AIs and deterrent behavior in the dengue vector, Aedes aegypti.MethodsAir samples were taken inside and outdoors of experimental huts located in Pu Tuey Village, Kanchanaburi Province, Thailand in conjunction with mosquito behavioral evaluations. A mark-release-recapture study design using interception traps was used to measure deterrency of Ae. aegypti against 0.00625% metofluthrin coils and DDT-treated fabric (2g/m2) within separate experimental trials. Sentinel mosquito cohorts were positioned adjacent to air sampling locations to monitor knock down responses to AI within the treated air space. Air samples were analyzed using two techniques: the U.S. Environmental Protection Agency (USEPA) Compendium Method TO-10A and thermal desorption (TD).ResultsBoth the USEPA TO-10A and TD air sampling methods were able to detect and quantify volatized AIs under field conditions. Air samples indicated concentrations of both repellent chemicals below thresholds required for toxic responses (mortality) in mosquitoes. These concentrations elicited up to a 58% and 70% reduction in Ae. aegypti entry (i.e., deterrency) into treated experimental huts using metofluthrin coils and DDT-treated fabric, respectively. Minimal knock down was observed in sentinel mosquito cohorts positioned adjacent to air sampling locations during both chemical evaluations.ConclusionsThis study is the first to describe two air sampling methodologies that are appropriate for detecting and quantifying repellent chemicals within a treated air space during mosquito behavior evaluations. Results demonstrate that the quantity of AI detected by the mosquito vector, Ae. aegypti, that elicits repellency is far lower than that needed for toxicity. These findings have important implications for evaluation and optimization of new vector control tools that function through mosquito behavior modification as opposed to mortality.

Determining Airborne Concentrations of Spatial Repellent Chemicals in Mosquito Behavior Assay Systems

“…Quantifying the concentrations of airborne SR chemicals during laboratory and field mosquito behavior studies is critical to understanding the relationship between chemical exposure and mosquito behavior [26]. Such information can be used, in part, to establish entomological correlates of health outcomes related to human protection such as percent reduction in mosquito entry into a treated space, or biting rates.…”

“…Such information can be used, in part, to establish entomological correlates of health outcomes related to human protection such as percent reduction in mosquito entry into a treated space, or biting rates. This report describes important performance details for a TD GC-MS analytical method introduced previously [26], to quantify concentrations of airborne DDT in both laboratory and field mosquito assay systems.…”

“…Although our earlier reports have described air sampling outputs in conjunction with deterrent (SR) mosquito responses [26], this is the first detailed description, to our knowledge, of the conditions and performance of a method for near real-time detection of DDT in samples of air collected under field conditions. The on-site method appeared to be sufficiently sensitive to detect levels well below those that would be acutely toxic to humans (1.0 mg/m 3 ) [30] with a quantitation limit of 0.461 µg/m 3 (27 ppt) DDT during field sampling and analyses.…”

Semi-Field Evaluation of Metofluthrin-Impregnated Nets on Host-SeekingAedes aegyptiandAnopheles dirus

“…sundaicus (Rodenwaldt) and house entry behavior of Ae. aegypti (Achee et al 2012, Syafruddin et al 2014. Emanators containing 5% and 10% metofluthrin suppressed the biting rate of Ae.…”

Model-based analysis of experimental hut data elucidates multifaceted effects of a volatile chemical onAedes aegyptimosquitoes

“…The reduced repellency effect observed at high relative to low dosage may not result from reduced sensitivity of mosquitoes to this effect per se, but may instead be a result of saturation of all experimental huts with the volatile chemical. Indeed, correlations between air sampling measurements and mosquito behavior responses have been explored in previous studies using spatial repellents but with limited success due to limits of chemical detection and quantification [51][52][53]. Combining air chemistry inferences of specific active ingredients (i.e., vapor pressure, particle weight) with environmental data (i.e., air current, flow rate) into our new inference framework is therefore warranted and could enable quantification of the extent to which downstream effects result from movement of the volatile chemical or movement of exposed mosquitoes with lingering post-exposure effects.…”