Impact of virginiamycin on the development of intestinal structure and functional capacity in growing pigs

Antibiotics have been used in animal feed to promote weight gain, improve feed efficiency, and control disease since the 1950s. Concerns about antibiotic resistance have increased since the widespread use of in-feed antibiotics began, but a clear connection between antibiotic use in animals and adverse human health effects has not been established. Understanding the mechanisms behind antibiotic growth promoters is an important step in determining the safety of in-feed antibiotics. Our study tested the hypothesis that diets with virginiamycin would increase intestinal surface area and nutrient processing capacity compared with diets without virginiamycin. Littermate barrows with similar body weight (n=72; 23.0±1.3 kg; 9 weeks of age) were randomized to pens and pens were randomized to one of three treatment groups (0, 11, or 27.5 mg virginiamycin/kg corn-soybean diet). Within each pen, pigs were further randomized to a time point (7 or 14 days). Intestinal samples were obtained for assessment of gross morphology, histomorphology, immunohistochemistry, biochemical measurements, disaccharidase activity, and nutrient and ion transport. Analysis of variance was used to statistically analyze the data using the Mixed Model of SAS fitted with a split plot design (treatment as the whole plot and time as the subplot). Preplanned contrasts between virginiamycin and control groups were completed. Pigs fed diets supplemented with virginiamycin gained an average of 10% more weight per day with about 12% better feed conversion compared with control. Virginiamycin impacted intestinal functional capacity to a greater extent than structural indices. Glutamine and arginine transport were increased 2-fold in the jejunum of virginiamycin fed pigs compared with control (p=0.010, p=0.046, respectively). Ileal carbachol induced chloride secretion was lower at day seven, but it increased over time with virginiamycin resulting in increased secretion at day fourteen with virginiamycin compared with control (p=0.017). Virginiamycin directly impacted proximal gut nutrient transport with a more delayed response in the distal gut by providing protection against loss in nutrient transport and secretory capacity. Increased nutrient transport was observed without increases in absorptive surface area indicating that increased functional capacity may be a mechanism by which antibiotic promote growth. Increasing our knowledge on the mechanisms behind antibiotic growth promoters will help direct government policy on antibiotic use in animals