PS6-5. Effect of Dietary Aid on Stress and Markers of Intestinal Barrier Integrity in Young, Exercising Horses Undergoing Prophylactic Antibiotic Therapy.

Abstract Antibiotic (AB) therapy can disrupt the gastrointestinal environment, resulting in systemic stress, inflammation, and impaired intestinal barrier integrity. The objective of this study was to determine if supplementing a dietary aid to support the microbiome, buffer gastric acid, and provide trace mineral fortification would modulate systemic responses to such stressors. To test this, twenty 2-year-old stock-type horses were used in a completely randomized design, stratified by age, body weight (BW), BCS, and sex into one of two dietary treatments: concentrate containing no dietary aids (CON; n = 10), or the same concentrate containing Digestive Shield: a blend of pre-, pro-, and post-biotics, marine calcite, and organic trace minerals (DS; n = 10; Cargill, Inc.). Concentrate was offered at 0.75% BW/d, as-fed. On d0, horses began dietary treatments and a progressive exercise program (5 d/wk, 30 min/d) to mimic early performance training. On d44, horses began a 5-d course of oral sulfadiazine trimethoprim (AB; 24 mg/kg BW), administered every 12h. Samples were collected on d0, 14, 28, 42, 49, 56, and 63, with additional fecal samples on d44 and 46 during AB challenge. Blood samples were analyzed for cortisol and 16S rRNA abundance, and fecal samples for microbial sequencing and IgA concentration. Illumina shotgun sequencing was used to characterize the fecal microbiome. Data were analyzed via PROC MIXED in SAS with fixed effects of treatment, time, and their interaction. Significance was declared when P ≤ 0.05 and a tendency when 0.05