The development of a dexamethasone challenge model for evaluating feed additives in sheep.

OBJECTIVE To develop an accessible ruminant immune challenge model for rapid in vivo assessments of feed additives. ANIMALS 60 hair-breed ram lambs. METHODS Sheep were randomly assigned to 1 of 4 treatments: treatment 1, not immunosuppressed, control fed (n = 12); treatment 2, immunosuppressed, supplemented with a yeast and botanical extract (n = 18); treatment 3, immunosuppressed, supplemented with a blend of natural aluminosilicates and yeast components (n = 18); and treatment 4, immunosuppressed, control fed (n = 12). Twice-daily injections of dexamethasone (Dex; 0.1 mg/kg bodyweight, SC) were used to induce immunosuppression throughout the study (from September 25, 2020, to November 2, 2020). All sheep were immunized with keyhole limpet hemocyanin (KLH) on days 0 and 14 and injected with heataggregated KLH, ID, to induce a skin induration on day 15. Measurements included body weight (BW), average daily gain (ADG), CBC, and skin induration diameter. RESULTS Dex treatment resulted in reduced BW and ADG that was not mitigated by either feed additive. Dex reduced lymphocyte percentage, RBC count, hemoglobin, hematocrit, and skin induration diameter and increased concentrations of granulocytes and granulocyte percentage. Effects on hematocrit, hemoglobin, RBC, and skin induration diameter were mitigated with the addition of feed additives. CLINICAL RELEVANCE The described model is a tool to evaluate the ability of feed additives to mitigate the immunosuppressive effects of Dex. [ABSTRACT FROM AUTHOR]