Characterisation of cattle anal odour constituents associated with the repellency of Rhipicephalus appendiculatus

Adults of the Brown Ear Tick (Rhipicephalus appendiculatus) have a predilection for feeding inside and around the ears of cattle and other hosts. A previous study has shown that the tick locates the host ears by 'push-pull' deployment of a repellent blend emitted at the anal region and an attractant blend emitted at the ears. Interestingly, the two odours play reverse roles with Rhipicephalus evertsi, which prefer to feed around the anal region. The present study was undertaken to characterize the major constituents of the cattle anal odour and to evaluate their repellence to R. appendiculatus. The anal odour was trapped with reverse-phase C₁₈-bonded silica, Porapak Q and Super Q placed in an oven bag attached at the anal region of the cattle for 6 h. The adsorbents were then removed and extracted with dichloromethane, and the extracted compounds analyzed by linked gas chromatography-mass spectrometry (GC-MS). The major constituents of the odour were o-xylene, 4-hydroxy-4-methyl-2-pentanone, 4-methyl-2-methoxyphenol, ethylbenzene, 2,6,6-trimethyl-[1S(1α,β,5α)]bicycloheptanes, 5-ethoxydihydro-2(3H)-furanone, 3-methylene-2-pentanone, 5-methyl-2-phenyl-1H-indole, and 3-pentanone. The repellency of the available compounds (o-xylene, 4-hydroxy-4-methyl-2-pentanone, 4-methyl-2-methoxyphenol, ethyl benzene, 3-methylene-2-pentanone, and 3-pentanone) and blends was evaluated using a dual choice tick climbing assay at different doses. The anal odour showed repellence with RD₇₅ of 0.39. Of the compounds tested, 4-methyl-2-methoxyphenol was found to be most repellent (RD₇₅ = 0.56) and 3-pentanone least repellent (RD₇₅ = 622.7). The blend of the six constituents showed RD₇₅ of 0.34, comparable to that of the crude anal odour blend. A series of subtractive bioassays with one constituent of the 6-component blend missing was also carried out. Subtraction of 3-methylpentanone gave the most repellent blend (RD₇₅ = 0.097), whereas subtraction of 4-methylguaiacol gave the least repellent blend (RD₇₅ = 160.7) consistent with the high individual activity of this phenol. The study lays down useful groundwork for on-host deployment of controlled-release of a selected repellent or blend to disrupt the tick's ability to locate its preferred feeding site.