It is increasingly evident that gastrointestinal parasite (GIP) control programs based on dewormers are failing because of increased dewormer resistance; thus, alternative GIP control strategies are necessary. Condensed tannins (CT) have biological effects that may aid in the control of GIP. The CT bind proteins and other molecules tightly at near-neutral pH, such as occurs in the rumen, with dissociation in the acidic pH of the abomasum, freeing them for digestion. Plant CT may have direct or indirect effects on GIP. Direct effects might be mediated through CT-nematode interactions, thereby affecting physiological functioning of GIP. Condensed tannins extracted from various forages can markedly decrease the viability of the larval stages of several nematodes in sheep and goats. Condensed tannins also may react directly by interfering with parasite egg hatching and development to infective stage larvae. Indirectly, CT can improve protein nutrition by binding to plant proteins in the rumen and preventing microbial degradation, thereby increasing amino acid flow to the duodenum. Several sheep studies have shown that improved protein nutrition decreases parasite infestation. This is assumed to be mediated by enhanced host immunity, which may be especially important with selection for immunity to GIP. Therefore, CT might counteract parasites by one or more of the aforementioned mechanisms, and mechanisms involved might differ between CT from different forage species. In conclusion, CT in forages have the potential to aid in the control of GIP.
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