Acaricidal characteristics of ethnoveterinary plants used for tick control in southern Africa

Abstract

Ticks and tick borne diseases remain a huge threat to livestock productivity the world over. While several efforts have been made to control ticks, current control measures are still not adequate. Conventionally, tick control programmes are heavily reliant on the use of synthetic chemical acaricides while the impact of other less frequently used control methods has not been fully established. Unfortunately, heavy chemical use has led to a number challenges that include: unsustainable high costs of acaricides, development of tick resistance, environmental pollution, contamination of animal products with chemical residues and many other topical issues. Ethnoveterinary plants are however an alternative but possibly effective, environmentally benign and safe option that can complement and in some cases substitute synthetic chemical acaricides. In this study, plant species identified in Zimbabwe and found elsewhere in southern Africa were characterised for anti-tick properties with the aim of developing an ethnobotanical product for use. The initial step involved the identification of plants through an ethnobotanical survey carried out in 4 arid and semi-arid districts of Zimbabwe, namely Muzarabani, Chiredzi, Matobo and Kadoma. These areas were purposively selected on the basis of high cattle production and high likelihood of use of traditional practices in primary animal health care. More than 51 plant species were recorded and a ranking according to frequency of mention showed that Cissus quadrangularis, Aloe sp., Lippia javanica and Psydrax livida were the most popular plants mentioned by farmers. The most common method for preparation was crushing and soaking in water before spraying the animals. Despite the farmers acknowledging that they had access challenges to the normal government-provided dipping services and having knowledge on traditional practices of tick control, the actual use of these practices was low. It was concluded that farmers and other knowledgeable people do have plants they know that have anti-tick properties, thus providing a good basis for the development of ethno-based tick control products. In order to confirm farmer claims of efficacy of the plant extracts and to find ways of increasing that efficacy, three in vitro screening experiments were done using the modified Shaw Larval Immersion Test on Rhipicephalus (Boophilus) decoloratus tick larvae. Different extraction methods were used in the screening: crude water extracts, acetone extracts and solvent – solvent fractions of acetone extracts of Maerua edulis. Results showed that contrary to the high activity reported by farmers in the surveys, water extracts were not toxic to the tick larvae. Perhaps the high activity reported by farmers, if confirmed may be associated with the repellence of volatile emissions from the plants. The addition of a liquid soap as a surfactant however increased the efficacy of the M. edulis tuber aqueous extract to activity levels comparable with those of an amitraz-based commercial acaricide, which was the positive control. The use of the organic solvent acetone as an extractant markedly increased the efficacy of 13 of the plant species under study, particularly M. edulis, Monadenium lugardae and Kleinia species. The chloroform and hexane fractions from M. edulis exhibited very high activity, possibly indicating that less-polar compounds are responsible for the observed activity. Thus, the use of water as a sole extractant is limited in terms of extracting compounds active against ticks, but organic solvents and acetone in particular increase the efficacy of the extracts. In the case of M. edulis less polar extracts and fractions were most active against the ticks. Because Maerua edulis consistently showed good activity in all prior testing, it was further tested using low-cost optimisation strategies like the use of hot water, a surfactant and a different organic solvent (methanol). Hot water extraction and use of a surfactant increased efficacy of the crude extracts of the M. edulis leaves against ticks to satisfactory levels compared to cold water extracts. There was no significant difference between the positive control and methanol-extracted M. edulis. It is, however, the use of ordinary soap that may bring relief to rural farmers who are generally unable to have access to organic solvents. From the observation that the hexane and chloroform extracts of the M. edulis leaf and roots were very active against the ticks, cytotoxicity of the extracts on African Green monkey kidney (Vero) cells and bovine dermal cells was determined to shed some preliminary insights on safety aspects of the plant. Neither extract had high toxicity against these cell lines. The LC50 was greater than 20 _g/ml which is considered as a maximum threshold for indicating toxicity of plant extracts. After confirmation that non-polar fractions of M. edulis were active against ticks and that cytotoxicity results showed that the extracts are relatively non-toxic to animal cell lines, attempts to isolate and identify the active compounds in the chloroform fractions of M. edulis were made without much success. Using column chromatography, an impure compound was isolated in the chloroform fraction but the amount was too low for characterisation by NMR. When the compound was analysed using Gas Chromatography-Mass Spectrometry, a number of chemicals in the isolate were avident but which did not have the pre requisite high similarities with the compound library to be considered. Because of the low quantities no further work was done to further purify and test the compounds against the ticks. For purposes of confirming laboratory activity under field conditions, M. edulis, C. quadrangularis and Aloe vera crude water extracts combined with a surfactant (liquid soap), were tested on Mashona cattle at Henderson Research Station (Zimbabwe) over 7 weeks during the period of peak tick infestation. Only M. edulis tuber extracts with a surfactant were as effective as the amitraz-based positive control. There was no significant difference in activity between the other plant extracts and the negative control. It can be concluded that there is scope to use M. edulis tubers extracted with locally available surfactant as a tick control product. This whole study therefore shows that ticks can be controlled using locally available plant materials provided they are prepared and applied properly. While the overall aim of the study of producing a working plant based tick control product was not met, there is sufficient data from the study to justify developing crude formulations from M. edulis that can be used to control ticks.