Use of plants in novel approaches to control of gastrointestinal helminths in livestock with emphasis on small ruminants

Summary

Helminth infections are a major cause for reduced productivity in livestock, particularly those owned by the poor worldwide. Phytomedicine has been used for eons by farmers and traditional healers to treat parasitism and improve performance of livestock, and many modern commercial medicines are derived from plants. However, scientific evidence on the anti-parasitic efficacy of most plant products is limited regardless of their wide ethnoveterinary usage. Scientific validation of the anti-parasitic effects and possible side effects of plant products in ruminants is necessary prior to their adoption as a novel method for parasite control.
A variety of methods has been explored to validate the anthelmintic properties of such plant remedies, both in vivo and in vitro. In vitro assays are mainly performed with the free-living rather than the parasitic stages of nematodes. Concentrations of potentially active substances used in vitro do not always correspond to in vivo bioavailability. Therefore, in vitro assays should always be accompanied with in vivo studies when used to validate the anthelmintic properties of plant remedies.
In vivo controlled studies have showed that plant remedies have in most instances caused reductions in the level of parasitism less pronounced than that of anthelmintic drugs, and only rarely has reductions similar to that caused by synthetic anthelmintics been achieved. Whether it is necessary or not to achieve the latter in order for plant remedies to have a role in the control of parasitism depends on determination of biologically important levels of reduction of parasitism. This is required prior to the wide-scale use of plant products for parasite control. Similarly, standardisation of validation studies in reference to the numbers of animals required for in vivo studies to measure direct anthelmintic effects of a plant needs to be done.
Although in many cases the active compounds in the herbal remedies have not been identified yet, plant enzymes, such as cysteine proteinases, or secondary metabolites, such as alkaloids, glycosides and tannins have shown dose-dependent anti-parasitic properties. However, as some of the active compounds may also have anti-nutritional effects against the parasitised herbivores, such as reduced food intake and performance, it is essential to validate the anti-parasitic effects of plant products in relation to their potential anti-nutritional and other side effects. A concerted effort on isolation, development, and validation of the effects of these herbal remedies will have to be undertaken before their wider acceptance.