Can plant secondary metabolites have a role in controlling gastrointestinal nematode parasitism in small ruminants?

Summary Plant secondary metabolites (PSM) are widely known for their antinutritional properties; their excessive consumption can detrimentally affect herbivore health and, in some cases, survival. However, a growing body of evidence is suggesting that some classes of PSM may also have some positive effects on herbivores. Such positive effects include the antiparasitic properties of certain PSM and their beneficial consequences on the herbivore's fitness. One of the better-exploited classes of PSM that have been reported with anthelmintic properties is that of the condensed tannins. The consumption of moderate concentrations of condensed tannins has resulted in reduced level of parasitism in sheep and other ruminants. In this paper we discuss the conditions that should be met in order for the positive anthelmintic properties of PSM, and condensed tannins in particular, to be beneficial for parasitised sheep. We also examine whether PSM could have a role in controlling parasitism in ruminants and point out the future research needed to achieve an efficient use of PSM for parasite control.