Diarrhea-associated pathogens, lactobacilli and cellulolytic bacteria in equine feces: Responses to antibiotic challenge

Antibiotics are important to equine medicine, but antibiotic-associated diarrhea (AAD) can lead to poor performance and even mortality. AAD is attributed to disruption of the hindgut microbiota, which permits proliferation of pathogenic microbes. The goal of this study was to evaluate the effects of common antibiotics on cellulolytic bacteria, lactobacilli, and AAD-associated pathogens in the feces of healthy horses. Fifteen horses were assigned to three treatment groups (blocked by age and sex): control (no antibiotics), trimethoprim–sulfadiazine (PO), or ceftiofur (IM). Fecal samples (n = 8 per horse) were taken during dietary adaptation (3 weeks), antibiotic challenge (1 week), and withdrawal (1 week). Bacteria were enumerated by serial dilution and viable count. Cellulolytic bacteria decreased by >99% during administration of either antibiotic (P < 0.0001) and were still less than controls at the end of the withdrawal period (P < 0.0001). Fecal samples from horses challenged with ceftiofur had 75% fewer lactobacilli than those from control horses at the end of the antibiotic challenge period (P < 0.05). Antibiotic challenged horses also shed more salmonella than control horses (P < 0.05). Antibiotics had no effect on the number of Clostridium perfringens isolates. There was no detectable Clostridium difficile during adaptation or in any control horse. C. difficile increased (P < 0.0001) to approximately 104 cfu/g when horses were challenged with antibiotics, and were still detectable 1 week after withdrawal. These results indicate that antibiotics can disrupt the normal gastrointestinal microbiota and allow proliferation of Salmonella spp. and C. difficile.