The toxicity and repellent properties of plant extracts used in ethnoveterinary medicine to control ticks

Abstract

Six plant species (Aloe ferox, Aloe marlothii, Clerodendrum glabrum, Jatropha curcas, Ricinus communis and Strychnos madagascariensis) used by communities in the northern parts of South Africa to control ticks on livestock were selected from the database of the Onderstepoort Veterinary Institute. The leaves of these species were extracted with organic solvents ranging from polar to non-polar (methanol, acetone and dichloromethane). Infusions (soap-water-paraffin) and decoctions traditionally used were also prepared. The tick repelling and toxicity activities of the extracts were evaluated against the livestock tick, Rhipicephalus appendiculatus. Initial results of the tick-climbing repellency bioassay illustrated that a 30% concentration of crude acetone extracts of A. ferox, A. marlothii and C. glabrum; DCM extracts of R. communis and A. marlothii and MeOH extract of J. curcas repelled the ticks to a statistically significant level. Of all the aqueous extracts only the decoction of S. madagascariensis repelled ticks significantly. At lower concentration of 10%, only acetone extracts of C. glabrum were effective. Three types of bioassays were conducted for toxicity i.e., dipping, topical and dry extract applications. Organic extracts were not effective. Infusions of A. ferox and S. madagascariensis. had strong topical application toxicity at a concentration of 35.5 % while the same concentration of infusions of A. ferox and J. curcas had a strong dipping toxicity effect. Both A. ferox and S. madagascariensis extracts were still toxic at the concentration of 30% in topical and dipping bioassays. In general acetone was a good extractant with lower yields but higher percentage repellency. Polar aqueous extracts had high yields but lower percentage repellencies. It may be that volatiles present in organic extracts are repellents and non-volatiles from aqueous extracts are toxic. Some of the fractions of C. glabrum had a higher tick repellency activity than the commercial tick control agents used as positive controls. Unfortunately the activity decreased after 2.5 hours probably due to a volatility of the biologically active compound. Attempts were made to isolate the repellent compound from the acetone extract of C. Glabrum. The process gave very good results up to a late stage of the bioassay guided fractionation. At that stage the repellent activity was lost. When two fractions were combined the repellent activity was present again. This is strong evidence for the existence of a synergistic effect. Further attempt to isolate the compounds at this stage and characterize them by nuclear magnetic resonance spectroscopy failed probably through decomposition of the compounds. This aspect can probably be followed up in more detail. The results substantiated the rationale behind the use of these plant extracts to control ticks. Even a simple acetone extract of leaves of some species has excellent tick repelling activities. If the duration of this effect could be increased it could very well develop into a commercially useful product. The tick repelling activity of S. madagascariensis decoctions and tick killing activity of A. ferox soap infusion holds great promise for the control of ticks by rural farmers. In vivo trials under controlled conditions should provide useful results.