Secondary plant compounds have shown bioactivity against multi-drug resistant Haemonchus contortus in small ruminants. This study screened 51 strains of birdsfoot trefoil (BFT, Lotus corniculatus) crude aqueous extracts (BFT-AqE) for anti-parasitic activity in vitro against egg hatching, and of those 51 strains, 13 were selected for further testing of motility of first (L1) and third stage (L3) larvae, and exsheathment of L3. Proanthocyanidin content ranged between 1.4 and 63.8 mg PAC g−1 powder across the 51 BFT strains. When tested against egg hatching, 21 of the 51 aqueous extracts had an EC50 of 1–2 mg powder mL−1, 70% of the strains were >90% efficacious at 6 mg powder mL−1 and 11 of the strains were 100% efficacious at 3 mg powder mL−1 BFT-AqE. Across the 13 strains tested against L3, efficacy ranged from 0 to 75% exsheathment inhibition, and 17 to 92% L3 motility inhibition at a concentration of 25 mg powder mL−1 BFT-AqE. There was no correlation between the PAC content of BFT powders and the anti-parasitic activity of aqueous extracts, therefore other secondary compounds may have contributed to the observed anti-parasitic effects. Further testing of BFT using bioactivity-driven fractionation and screening of BFT populations for the identified anti-parasitic compounds is needed.

The mode of action of bioactive plants on gastrointestinal nematodes remains obscure. Previous in vitro studies showed that exsheathment was significantly disturbed after contact with tannin-rich extracts. However, the role of important factors (extract concentration, parasite species) has not been assessed and no information is available on the occurrence in vivo. These questions represent the objectives of this study. The model incorporated the parasites Haemonchus contortus and Trichostrongylus colubriformis with sainfoin as the bioactive plant. A set of in vitro assays was performed, measuring the changes observed, after 3 h of contact with increasing concentrations of sainfoin, on the rate of artificial exsheathment. The results indicated that sainfoin extracts interfered with exsheathment in a dose-dependent manner and the process overall was similar for both nematodes. The restoration of control values observed after adding PEG to extracts confirms a major role for tannins. A second study was performed in vivo on rumen-cannulated sheep fed with different proportions of sainfoin in the diet to verify these in vitro results. The consumption of a higher proportion of sainfoin was indeed associated with significant delays in Haemonchus exsheathment. Overall, the results confirmed that interference with the early step of nematode infection might be one of the modes of action that contributes to the anthelmintic properties of tanniniferous plants.