An improved odor bait for monitoring populations of Aedes aegypti-vectors of dengue and chikungunya viruses in Kenya

Abstract

Background: Effective surveillance and estimation of the biting fraction of Aedes aegypti is critical for accurate determination of the extent of virus transmission during outbreaks and inter-epidemic periods of dengue and chikungunya fever. Here, we describe the development and use of synthetic human odor baits for improved sampling of adult Ae. aegypti, in two dengue and chikungunya fevers endemic areas in Kenya; Kilifi and Busia counties.

Methods: We collected volatiles from the feet and trunks of two female and two male volunteers aged between 25 and 45 years. We used coupled gas chromatography- electroantennographic detection (GC/EAD) analysis to screen for antennally-active components from the volatiles and coupled GC-mass spectrometry (GC/MS) to identify the EAD-active components. Using randomized replicated designs, we compared the efficacies of Biogents (BG) sentinel traps baited with carbon dioxide plus either single or blends of the identified compounds against the BG sentinel trap baited with carbon dioxide plus the BG commercial lure in trapping Ae. aegypti. The daily mosquito counts in the different traps were subjected to negative binomial regression following the generalized linear models procedures.

Results: A total of ten major EAD-active components identified by GC/MS as mainly aldehydes and carboxylic acids, were consistently isolated from the human feet and trunk volatiles from at least two volunteers. Field assays with synthetic chemicals of the shared EAD-active components identified from the feet and trunk gave varying results. Ae. aegypti were more attracted to carbon dioxide baited BG sentinel traps combined with blends of aldehydes than to similar traps combined with blends of carboxylic acids. When we assessed the efficacy of hexanoic acid detected in odors of the BG commercial lure and volunteers plus carbon dioxide, trap captures of Ae. aegypti doubled over the trap baited with the commercial BG lure. However, dispensing aldehydes and carboxylic acids together in blends, reduced trap captures of Ae. aegypti by ~45%-50%.

Conclusions: Our results provide evidence for roles of carboxylic acids and aldehydes in Ae. aegypti host attraction and also show that of the carboxylic acids, hexanoic acid is a more effective lure for the vector than the BG commercial lure.

Figure 1

The study sites; Kilifi district in the coast and Busia district in western Kenya.

Figure 2

Representative GC/EAD profiles showing EAD- active components identified from; the feet- Panel 1 and trunk - Panel 2, of volunteers. Panel A- GC/EAD responses from F1 generation Ae. aegypti from Rabai, Kilifi. Panel B – GC/EAD responses from inbred 66^th generation Ae. aegypti from Rabai, Kilifi. EAD-active components; 1- propionic acid, 2- hexanal, 3- methyl butyric acid, 4- heptanal, 5- hexanoic acid, 6- octanal, 7- nonanal, 8- decanal, 9- undecanal, 10- 6,10-dimethyl-5,9-undecadien-2-one (geranyl acetone).

Figure 3

The mean number ± S.E of Aedes aegypti captured by the various BG sentinel traps baited with different baits in Busia and Kilifi County. Blend 1; Acids - propionic + 3-methylbutyric acid, Blend 2; Aldehydes - nonanal + octanal, Blend 3; Blend 1 + Blend 2, Blend 4; Blend 2 + hexanoic acid. The different panels show comparisons at the two locations; Panel A - Experiment 1 in Busia, Panel B - Experiment 1in Kilifi and Panel C - Experiment 2 in Kilifi. Error bars indicate standard error of the mean.