Abstract
Amblyomma sculptum is a tick affecting animal and human health across Argentina, Bolivia, Paraguay and Brazil. Donkeys, Equus asinus, are known to be resistant to A. sculptum, suggesting that they can produce non-host tick semiochemicals (allomones), as already demonstrated for some other vertebrate host/pest interactions, whereas horses, Equus caballus, are considered as susceptible hosts. In this study, we tested the hypothesis that donkeys produce natural repellents against A. sculptum, by collecting sebum from donkeys and horses, collecting the odour from sebum extracts, and identifying donkey-specific volatile compounds by gas chromatography (GC) and coupled GC-mass spectrometry (GC–MS). From the complex collected blends, five main compounds were identified in both species. Hexanal, heptanal and (E)-2-decenal were found predominantly in donkey extracts, whilst ethyl octanoate and ethyl decanoate were found predominantly in horse extracts. One compound, (E)-2-octenal, was detected exclusively in donkey extracts. In Y-tube olfactometer bioassays 36 different A. sculptum nymphs were tested for each extract, compound and concentration. The dry sebum extracts and the compounds identified in both species induced neither attraction nor repellency. Only (E)-2-octenal, the donkey-specific compound, displayed repellency, with more nymphs preferring the arm containing the solvent control when the compound was presented in the test arm across four concentrations tested (p < 0.05, Chi-square test). A combination of a tick attractant (ammonia) and (E)-2-octenal at 0.25 M also resulted in preference for the control arm (p < 0.05, Chi-square test). The use of semiochemicals (allomones) identified from less-preferred hosts in tick management has been successful for repelling brown dog ticks, Rhipicephalus sanguineus sensu lato from dog hosts. These results indicate that (E)-2-octenal could be used similarly to interfere in tick host location and be developed for use in reducing A. sculptum numbers on animal and human hosts.
Highlights
The tick species complex Amblyomma cajennense (Fabricius, 1787) (Ixodida: Ixodidae) (Nava et al, 2014) is distributed across the New World, with a wide range of hosts including mammals, birds and reptiles (Barros-Battesti et al, 2006)
Gas chromatography (GC) analysis of volatile organic compound (VOC) extracts collected from horse and donkey sebum by air entrainment revealed the presence of a large number of compounds (Fig. 1)
Comparison of extracts between horses and donkeys revealed the presence of four compounds in higher amounts in donkey extracts than in horse extracts, which were tentatively identified by coupled GCmass spectrometry (GC–MS) and confirmed by gas chromatography (GC) peak enhancement as hexanal, heptanal, (E)-2-decenal and (E)-2-octenal
Summary
The tick species complex Amblyomma cajennense (Fabricius, 1787) (Ixodida: Ixodidae) (Nava et al, 2014) is distributed across the New World, with a wide range of hosts including mammals, birds and reptiles (Barros-Battesti et al, 2006). Amblyomma sculptum (Berlese, 1888), that belongs to A. cajennense complex, is distributed across Argentina, Bolivia, Paraguay and Brazil (Nava et al, 2014), and is considered the most important vector of Rickettsia rickettsii (Rickettsiales: Rickettsiaceae), the causative agent of Brazilian spotted fever (BSF) in human beings. Individuals infected with this bacterium have a high fatality rate that can reach up to 85% (Araujo et al, 2015). Given the importance of A. sculptum to human and animal health, we proposed to test this hypothesis by identifying donkey-specific odours and conducting behavioural bioassays with identified compounds, with the aim of providing new repellents for A. sculptum management
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