Sheep skin odor improves trap captures of mosquito vectors of Rift Valley fever

Abstract

In recent years, the East African region has seen an increase in arboviral diseases transmitted by blood-feeding arthropods. Effective surveillance to monitor and reduce incidence of these infections requires the use of appropriate vector sampling tools. Here, trapped skin volatiles on fur from sheep, a known preferred host of mosquito vectors of Rift Valley fever virus (RVFV), were used with a standard CDC light trap to improve catches of mosquito vectors. We tested the standard CDC light trap alone (L), and baited with (a) CO(2) (LC), (b) animal volatiles (LF), and (c) CO(2) plus animal volatiles (LCF) in two highly endemic areas for RVF in Kenya (Marigat and Ijara districts) from March-June and September-December 2010. The incidence rate ratios (IRR) that mosquito species chose traps baited with treatments (LCF, LC and LF) instead of the control (L) were estimated. Marigat was dominated by secondary vectors and host-seeking mosquitoes were 3-4 times more likely to enter LC and LCF traps [IRR = 3.1 and IRR = 3.8 respectively] than the L only trap. The LCF trap captured a greater number of mosquitoes than the LC trap (IRR = 1.23) although the difference was not significant. Analogous results were observed at Ijara, where species were dominated by key primary and primary RVFV vectors, with 1.6-, 6.5-, and 8.5-fold increases in trap captures recorded in LF, LC and LCF baited traps respectively, relative to the control. These catches all differed significantly from those trapped in L only. Further, there was a significant increase in trap captures in LCF compared to LC (IRR = 1.63). Mosquito species composition and trap counts differed between the RVF sites. However, within each site, catches differed in abundance only and no species preferences were noted in the different baited-traps. Identifying the attractive components present in these natural odors should lead to development of an effective odor-bait trapping system for population density-monitoring and result in improved RVF surveillance especially during the inter-epidemic period.

Figure 1. Mean mosquito captures in 10 replicate trials per treatment at the two districts in Kenya.

A) Ijara district; B) Marigat district. Bars followed by similar letters are not significantly different at P = 0.05. L, light only; LF, light+sheep odor; LC, light+CO₂; LCF, light+CO₂+sheep odor.

Figure 2. Mean mosquito captures/trap/night for different RVFV vector groups in 10 replicate trials/district in Kenya.

A) Key primary vectors; B) Primary vectors; C) Secondary vectors. Bars followed by similar letters are not significantly different at P = 0.05. L, light only; LF, light+sheep odor; LC, light+CO₂; LCF, light+CO₂+sheep odor.