Structure-Activity Studies And On-Host Evaluation Of Controlled-Release Formulations Of Optimised Potent Repellents Against Rhipicephalus Appendiculatus

ABSTRACT

East Coast Fever (ECF), commonly known as theileriosis, is a tick-borne disease caused by a protozoan organism known as Theileria parva. It has been a major limitation in livestock production and food security in several developing countries. ECF is transmitted predominantly by the brown ear tick, Rhipicephalus appendiculatus. In Africa, it is estimated that about 1.1 million cattle suffer from ECF resulting to losses of about US dollars 168 million. In Kenya, approximately 50-80% of the national cattle population of about 10 million animals is vulnerable to tick infestations, and out of this number, 1% die of ECF each year. Use of semiochemicals represents a prospective ecofriendly strategy for the control and management of the vector. Previously, on-host behavioral observations showed preference of R. appendiculatus adults to feed primarily inside and around the ear of their hosts. Combination of a repulsive blend from the anal region and an attractive blend at the ear was revealed to play natural “push” and “pull” roles, respectively, to guide the ticks to their predilection feeding sites. In a preliminary study, use of a crude repulsive blend obtained from the anal region at the ears was found to confuse the ticks, most of which dropped off the cattle. In a follow up study, 4- methylguaiacol was found to be the major component of cattle anal odour, and also most repellent to R. appendiculatus. Three sets of studies were then carried out. First, structure-activity of 10 analogues of 4-methylguaiacol and that of selected blends was performed at different doses in a dual-choice climbing assay set up. Each analogue showed either higher or lower 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-propylguaiacol; that of 4-methylguaiacol, RD75 = 0.564). However, blending selected analogues with high repellency showed no incremental increases in repellency compared with that of 4-propyl-2-methoxyphenol. Second, encapsulation of 4-propylguaiacol in β-cyclodextrin, ethyl cellulose and PVP, respectively, was carried out, and the inclusion of 4-propylguaiacol in the resulting complexes was confirmed by FT-IR, XRD and SEM. The rates and duration of release of the repellent from 0.2 g of each inclusion complex were then compared at 38-40 0C every 3 hours for 24 hrs. The β-cyclodextrin complex was found to last longer than the others with a release rate of 0.396 mg per hr. Third, on-host behavior effects of individual ticks (male or female) placed at one of six different body locations (i.e. forehead, shoulders, dewlap, escutcheon, hind leg and front leg) of Friesian steers, with no prior exposure to ticks, and tagged with β-cyclodextrin, ethyl cellulose or PVP inclusion complexes at one or both ears, were monitored. Tagging the treatment on one ear and both ears showed that β-cyclodextrin complex gave the best repellency with Mean ± SE of 0.64 ± 0.05 for tagging the treatment on one ear and Mean ± SE 0.35 ± 0.05 for tagging on both ears for ten days. Tagging the treatment on both ears gave 100% protection for seven days. This makes the complex a superior means of delivering the repellent and on-host use than the other complexes. The tagging on both ears was more effective than tagging on one ear. The most appropriate dosage was 0.5 mg with Mean ± SE 0.35 ± 0.05 when the treatment was on both ears. The β-cyclodextrin complex was found to repel the ticks for a longer time. This push approach exhibited by the β-cyclodextrin complex in small doses for a long time provides a potential tool to protect the cattle against the brown ear tick.