Medicinal plants for helminth parasite control: facts and fiction.

The use of medicinal plants for the prevention and treatment of gastro-intestinal parasitism has its origin in ethnoveterinary medicine. Although until recently the majority of the evidence on the antiparasitic activity of medicinal plants was anecdotal and lacked scientific validity, there is currently an increasing number of controlled experimental studies that aim to verify and quantify such plant activity. There are indeed a large number of plants whose anthelmintic activity has been demonstrated under controlled experimentation, either through feeding the whole plant or administering plant extracts to parasitised hosts. However, contrary to traditional expectation, there are also a great number of plants with purported antiparasitic properties, which have not been reproduced under experimental conditions. In this paper, we discuss the source of such inconsistencies between ethnoveterinary wisdom and scientific experimentation. We focus on the strengths and weaknesses of the existing methodologies used in the controlled studies to determine the activity of antiparasitic plants. We discuss issues like the seasonal and environmental variability of the plant composition, and how this can affect their antiparasitic properties and highlight the importance of identifying the mechanisms of action of such plants and the target parasite species. In addition to their antiparasitic properties, medicinal plants may also have anti-nutritional properties, which can affect animal performance and behaviour. For this reason, we emphasise the need for considering additional dimensions when evaluating medicinal plants. We also question whether using similar criteria as those used for the evaluation of anthelmintics is the way forward. We propose that a holistic approach is required to evaluate the potential of medicinal plants in parasite control and maximise their benefits on parasitised hosts.