Identification of a non-host semiochemical from tick-resistant donkeys (Equus asinus) against Amblyomma sculptum ticks

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.

Keywords: (E)-2-octenal; Allomone; Amblyomma cajennense sensu lato; Donkey; Non-host; Repellent.

Fig. 1

Typical gas chromatograph analysis from donkey (A, B, C) and horse (D, E, F) chemical extractions using protocol described in Materials and Methods. Arrows show the peak corresponding to (E)-2-octenal which is found almost exclusively in donkeys.

Fig. 2

Response of Amblyomma sculptum nymphs (n = 36) in Y-tube olfactometer assays to five compounds isolated from horses and donkeys and (E)-2-octenal tested at four different concentrations 1.0 M (A), 0.50 M (B), 0.25 M (C) and 0.125 M (D). *Significant effect of the chemical using a Chi-square test (p < 0.05) on 1 ° of freedom. Full results from the olfactometer assays, detailing counts of nymphs, Chi-squared statistics and p-values are given in Table S2 (Supplementary Information).

Fig. 3

Responses of Amblyomma sculptum nymphs (n = 36) to an attractant, ammonium hydroxide (AH), and to a mixture of an attractant and a repellent (E)-2-octenal in Y-tube olfactometer assays. *Significant effect of the chemical using a Chi-square test (p < 0.05) on 1 ° of freedom. Full results from the olfactometer assays, detailing counts of nymphs, Chi-squared statistics and p-values are given in Table S3 and S4 (Supplementary Information).