Selection of tannins by sheep in response to gastrointestinal nematode infection

Herbivores learn to select compounds that attenuate the aversive effects of plant secondary metabolites (PSM), but can they increase intake of PSM they typically avoid when these PSM provide medicinal effects? We hypothesized that herbivores learn to increase intake of PSM-containing feeds when experiencing a gastrointestinal parasitic infection. Ten lambs with natural gastrointestinal parasitic burdens (PB) and 10 nonparasitized lambs (NP) were offered a choice of alfalfa (Medicago sativa) and alfalfa mixed with 10% quebracho tannin (Schinopsis quebracho-colorado; alfalfa:tannins) before and after they were conditioned with the postingestive effects of tannins. Preference for alfalfa:tannins did not differ between groups before experiencing the postingestive effects of tannins (P = 0.85) or when parasite loads were terminated due to the administration of ivermectin (P = 0.18). In contrast, when tested with a parasite burden, lambs in PB consumed more alfalfa:tannins (P = 0.08), showed greater preference for alfalfa:tannins (P = 0.07), and consumed less alfalfa than lambs in NP (P = 0.06). Ingestion of tannins by lambs in PB was followed by reduced fecal egg counts (FEC; P = 0.006), and there was a direct proportional relationship between preference for alfalfa:tannins and FEC (P = 0.07). In summary, parasitized lambs increased their intake of alfalfa:tannins when they experienced a parasite burden, which suggests they self-medicated with tannins against parasites. Self-selection of PSM has implications for the quest for alternatives to chemoprophylaxis in the treatment and well-being of parasitized wild and domestic animals grazing in pasturelands and in confinement.