• May occur with any antimicrobial drug but most often associated with:

Diarrhea has also been reported in mares whose young foals were receiving erythromycin for treatment of Rhodococcus equi infection, presumably because of exposure to erythromycin in the foals’ feces.

• Changing from one class of antimicrobial to another or concurrent hospitalization, surgery, or gastrointestinal disease may increase the likelihood of development of antimicrobial-associated diarrhea.

• Recent or ongoing antimicrobial administration

• Inappetence, colic, fever and diarrhea are most common, as for other causes of colitis (see “Colitis/Diarrhea, Acute” in this section).

• Antimicrobial-associated diarrhea presumably results from alteration in the normal colonic bacterial flora, allowing overgrowth of or colonization with a pathogenic bacterial species.

• Overgrowth of toxin-producing Clostridium difficile strains has been documented in some horses with antimicrobial-associated colitis, although salmonellosis or diarrhea caused by an overgrowth of an unidentified pathogen may also occur.

• Infection with a pathogenic bacterial species results in damage to and disruption of the colonic mucosal barrier as for other causes of diarrhea (see “Colitis/Diarrhea, Acute” and “Diarrhea, Clostridial” in this section), resulting in fluid, electrolyte, and protein losses and diarrhea as well as signs of endotoxemia and colic.

• Clinical signs are typically, but not always, seen within 1 to 7 days of initiating antimicrobial therapy.