Development of efficient tools for improved surveillance of vectors of dengue and chikungunya fever

Abstract

In the past 3 decades, arboviruses have become a major cause of re-emerging epidemic diseases in the world. Amongst the arboviruses, dengue and chikungunya fevers which are transmitted by Aedes spp have become more prevalent and have spread far beyond traditional areas of distribution - mainly in Africa and Asia - to cause severe morbidity, mortality, and economic harm. Currently control of these diseases solely depends on vector control as there is no treatment or vaccine. This calls for efficient vector surveillance tools. However, the currently available vector sampling tools are inadequate in sampling Ae. aegypti. The popularly used Centers for Disease Control (CDC) and prevention light trap, under represents Ae. aegypti abundance as they are not attracted to its incandescent light bulb. Man landing catches (MLC) technique on the other hand is ethically unacceptable as it exposes the catchers to infective bites while the BG lure baited BG sentinel trap is reported to be less effective in sampling Ae. aegypti. This lack of an adequate sampling tool has led to underestimation of the magnitude of the two diseases in endemic areas consequently leading to un predicted outbreaks that have caused high morbidities and mortalities. Therefore, developing an effective surveillance tool that would aid in timely control campaigns could be a contribution of the utmost importance. We conducted our studies in two dengue and chikungunya endemic regions of Kenya-Busia and Kilifi counties. We tested the efficacy of various colored light emitting diode (LED) traps against the (CDC) light trap in sampling Ae. aegypti using replicated, randomized field experiments and observed that the violet trap caught significantly more Ae. aegypti in Busia than the control trap. Viral testing using Flavivirus and Alphavirus universal primers showed that the Ae. aegypti in Busia were infected with insect specific flaviviruses (ISFs) and there was a preference for the violet LED trap by the ISF Infected Ae. aegypti. Replicated randomized field experiments were also used to test the efficacy of Biogents (BG) sentinel traps baited with human feet odors trapped in socks and human trunk odors trapped in T-shirts against a control trap baited with the Biogents (BG) commercial lure. We observed that the traps baited with human odors caught more Ae. aegypti than the BG lure baited trap. We also observed that some individual’s odors attracted more Ae. aegypti than others. Gas chromatography coupled with mass spectrometry (GC-MS) analysis of the human volatiles and the BG lure revealed that the BG-lure mainly emitted hexanoic acid while human volatiles had several compounds mainly aldehydes, carboxylic acids, ketones and a couple of alcohols. A further GC coupled with electroantenno- grams (GC-EAG) identified electrophysiologically active compounds from the human odors some of which were then formulated into attractant blends and tested against whole human odors and the BG lure in the field. It was observed that some single compounds like hexanoic acid were better attractants to Ae. aegypti than the BG commercial lure and some formulated blends and that some compounds when dispensed together produced antagonistic and inhibitory effects against Ae. aegypti. Additionally, we conducted population genetics studies on the Ae. aegypti samples from Busia and Kilifi using a 653-bp region of the mitochondrial DNA cytochrome oxidase I (COI) gene and observed that there was no genetic differentiation between Ae. Aegypti from the two regions suggesting that their vector competency and susceptibility to insecticides might not be different. We thus believe that our work gives greater insight into the efficient use of LED traps in sampling not only Ae. aegypti in the field but also other diurnal insects (Chapter 2). Our work also adds information on the development of attractant synthetic odor baits from host volatiles for effective sampling of Ae. aegypti in the field (Chapter 3 & 4). Additionally, our work provides vital information on the population genetic structure of mosquitoes in Busia and Kilifi which would be important in planning control measures and help immensely in understanding disease transmission risks (Chapter 5). To improve our understanding on the effect of all the biologically active compounds in human skin volatiles, we recommend further investigations on the other active compounds that might were included in our odor blends but were identified to be biologically active. This might help improve odor baits for Ae. aegypti. We also recommend further investigations on preference of the violet colored LED traps by Ae. aegypti especially the Flavivirus infected. It would also be important to determine the interaction between ISFs and arboviruses like dengue and chikungunya in a co-infected mosquito as this may potentially impact on vector competence and thus transmission.