Antimicrobial-Associated Diarrhea in Equine Referral Practices (AAEP 2010)

Veterinarians commonly treat many equine infections with antimicrobial drugs to achieve resolution. However, in some cases, the antimicrobials themselves induce gastrointestinal disturbances with subsequent diarrhea. "This unfortunate side effect of antibiotics may prolong the time of hospitalization, increase the cost of treatment, and increase the risk of mortality," explained Bonnie Barr, VMD, Dipl. ACVIM, of Rood & Riddle Equine Hospital in Lexington, Ky. She presented her study in which she examined antimicrobial-associated diarrhea cases (AAD) from three referral practices (in Florida, Kentucky, and New Jersey), at the 2010 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Baltimore, Md.

In the study she and colleagues reviewed nonhospitalized horses older than weanling age that developed diarrhea during treatment with antibiotics for nongastrointestinal problems. Subsequently, these 32 cases were admitted to a referral hospital. Fecal samples were tested for enteropathogens such as Clostridium difficile, Clostridium perfringens, and Salmonella spp. Ages of the horses, mostly Thoroughbreds, ranged from 4 months to 28 years. Of these horses, 18 had received a single antimicrobial drug, while 14 had received a combination of two antibiotics. The target of the original antibiotic treatment in most cases was respiratory problems.

Barr pointed out one noteworthy finding from the AAD submission data to three referral practices: There was a regional difference in AAD incidence. She suggested that this might be attributable to differences in diet and management that affect intestinal flora and the presence of intestinal pathogens. The equine referral hospital in New Jersey had 2.8% incidence of AAD, as compared to Kentucky (0.7% incidence) and Florida (0.3% incidence).

In comparison, when Barr reviewed records of 5,251 horses from the three practices combined on antimicrobial therapy she identified an overall 0.6% incidence of AAD. The most commonly administered antimicrobial was oxytetracycline (1,243 cases), with no associated diarrhea cases. She expected enrofloxacin to be not as likely to cause diarrhea due to its poor anaerobic activity, but it actually resulted in eight cases of diarrhea. Use of single antimicrobials such as trimethoprim-sulfa, doxycycline, ceftiofur, or oxytetracycline was associated with low numbers of AAD cases despite previously reported figures of a higher incidence.

According to Barr, combination treatments, with their broad-spectrum activity, are more likely to disrupt intestinal flora and subsequently increase AAD incidence.

Twenty-two percent of horses presenting with AAD tested positive for an enteropathogen, namely C. difficile or Salmonella. Of these, 19% died, confirming that antibiotics can result in disruption of the normal flora and, if severe enough, have a negative outcome.

Barr summarized that veterinarians must use antimicrobials at the recommended dose, with considerations regarding efficacy, ability to reach the infection site, route of administration, whether the drug is labeled for equine use, and the potential for side effects. Veterinarians should only use these drugs off-label when no approved or commonly accepted drug or dose is available. Barr's study confirmed that almost all antimicrobials can be associated with diarrhea, but the overall incidence is low.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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