198 - Effect of Saccharomyces Cerevisiae Fermentate on Immune Cell Function Following Prolonged Head Elevation in 2-year-old Horses in Training

Horses are often restricted from lowering their heads while being transported, which prevents nasal drainage and triggers upper respiratory tract inflammation. Based on positive outcomes in other species, we hypothesized Saccharomyces cerevisiae fermentate (SCF) would modify this immune response in horses. Two-year-old Quarter Horses (mean ± SEM; initial age 22 ± 0.3 mo and BW 439 ± 3 kg) were randomly assigned to receive SCF (Diamond V, Cedar Rapids, IA; 21 g/d; n = 10) or no supplement (CON; n = 10) added to their diet (60% hay, 40% concentrate) for 60 d. Horses were exercised 4 d/wk for 30-45 min/d at light to moderate intensity. On d 57 horses were tethered with their heads elevated 35 cm above wither height for 12 h to mimic long-distance transport. Whole blood samples were obtained before and up to 72 h after stress induction to evaluate immune cell function. Data were compared using mixed model ANOVA with repeated measures. Serum cortisol (P < 0.01) and blood leukocytes (P < 0.05) were greater after head elevation. Lymphocyte proliferation in response to lipopolysaccharide was lower (P < 0.01) following head elevation but did not differ by treatment. Lymphocyte proliferation in response to concanavalin A exhibited a time × treatment effect (P = 0.05) where it decreased in CON horses after head elevation (P < 0.05) but was unchanged in SCF horses. Neutrophil phagocytosis of Streptococcus equi (a respiratory pathogen) was temporarily reduced (P < 0.05) after head elevation in both treatments. A time × treatment effect (P = 0.05) was observed for phagocytosis-induced oxidative burst, where it increased in SCF (P < 0.01) but did not change in CON horses. These data indicate SCF modified peripheral immune cell activity following a localized mucosal stressor. Whether these responses improve resistance to opportunistic pathogens following transport needs to be determined.