Antimicrobial-Associated Diarrhea in Three Equine Referral Practices

1. Introduction

Antimicrobial-associated diarrhea (AAD) is the most frequently observed adverse effect of antimicrobial therapy in horses. This prospective study determined the incidence of AAD in horses that developed diarrhea after antimicrobial treatment for non-diarrheic conditions and identified the antimicrobials used.

2. Materials and Methods

Horses of weanling age or older that presented with diarrhea at three equine referral clinics participated in the study. The antimicrobials used, dosage, and duration of treatment were determined. Fecal samples were tested for enteropathogens. Information on the number of horses treated with antimicrobial agents for non-gastrointestinal reason at each clinic was also determined.

3. Results

A total of 32 horses were diagnosed with presumptive AAD, an incidence of 1% (32/2,983). The AAD-diagnosed horses had an 18.8% (6/32) mortality rate. They had been collectively treated for an average of 4.2 days with 10 different agents from 8 antimicrobial classes. Of the most commonly used antimicrobial agents, the incidence of diarrhea was less than 1% (penicillin 0% [0/538], gentamicin 0.37% [2/538], trimethoprim sulfa 0.19% [1/521], and oxytetracycline 0% [0/436]). The incidence was higher with enrofloxacin 6.6% (7/106), doxycycline 2.0% (4/202), and ceftiofur 1.2% (1/83). Seven of thirty-two horses were positive for an enteropathogen. Four were positive for Clostridium difficile, and three were positive for Salmonella sp.

4. Discussion

The results indicated that agents from most antimicrobial classes are potential causes of equine AAD. Overall the incidence of equine AAD is low.

Acknowledgment

This project was funded by Pfizer Animal Health.