Abstract
Previously, 4-methylguaiacol, a major constituent of cattle anal odour, was found to have a high repellency on Rhipicephalus appendiculatus. In the present study, 10 structural analogues of the phenol were tested for repellency against R. appendiculatus in order to assess the effects of (i) absence or presence of the 4-alkyl group of varying length, (ii) inclusion of a double bond in the 4-alkyl chain, (iii) linking the two phenolic oxygen in a methylenedioxy bridge, (iv) replacement of the OCH3 with CH3 and inclusion of another CH3 at position 6, and (v) presence of an additional OCH3 group at position 6. The analogues comprised of 2-methoxyphenol (guaiacol), 4-ethyl-2-methoxyphenol, 4-propyl-2-methoxyphenol, 4-allyl-2-methoxyphenol (eugenol), 3,4-methylenedioxytoluene, 2,4-dimethylphenol, 4-ethyl-2-methylphenol, 2,4,6-trimethylphenol, 4-propyl-2,6-dimethoxy-phenol and 4-allyl-2,6-dimethoxyphenol, which were compared at different concentrations in a two-choice climbing assay set up. Each analogue showed either increased or reduced repellency compared with 4- methylguaiacol. The structural feature that was associated with the highest repellency was 4-propyl moiety in the guaiacol unit (RD75 = 0.031 for 4-propyl-2-methoxyphenol; that of 4-methylguaiacol = 0.564). Effects of blending selected analogues with high repellency were also compared. However, none of the blends showed incremental increases in repellency compared with that of 4-propyl-2-methoxyphenol. We are currently evaluating the effects of controlled release of this compound at different sites on cattle on the behavior and success of R. appendiculatus to locate their predilection for feeding sites.
Highlights
East Coast fever (ECF), caused by Theileria parva parva (Theiler, 1904), and transmitted by the brown ear tick, R. appendiculatus (Neumann, 1901), is one of the major constraints in the development of the livestock industry in eastern and southern Africa (Olwoch et al, 2008; Fry et al, 2016)
We compared the effects of different structural modifications of 4-methylguaiacol on their repellence to the brown ear tick, R. appendiculatus
In our first set of comparison with the 4-alkyl moiety of the guaiacol unit, the results showed significant increases in the repellent effect with increasing length of the carbon chain (4-propylguaiacol > 4-ethylguaiacol > 4-methylguaiacol > guaiacol). 4-Propylguaiacol was found to be the most repellent analogue among all the compounds tested in the present study
Summary
East Coast fever (ECF), caused by Theileria parva parva (Theiler, 1904), and transmitted by the brown ear tick, R. appendiculatus (Neumann, 1901), is one of the major constraints in the development of the livestock industry in eastern and southern Africa (Olwoch et al, 2008; Fry et al, 2016). Of the estimated 12.7 million heads of cattle (both indigenous and exotic), 76% are at risk to ECF (Lawrence et al, 1996). Mortality in zebu calves in ECF endemic areas and can cause up to 100% mortality in susceptible exotic and indigenous breeds (Mbogo, et al.,1995; Lawrence et al, 1996 ; Gachohi et al, 2012). Application at the cattle ears of two tick-repellent essential oils from ethno-plants growing in western Kenya were found to confuse the ticks, most of which dropped off the host animals (Wanzala et al, 2018)
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