Antibiotic-Associated Diarrhea

CHAPTER 96 Antibiotic-Associated Diarrhea



Samuel L. Jones


As the use of antibiotics in veterinary medicine has increased, so has the incidence of antibiotic-associated complications, including diarrhea. Antibiotic-associated diarrhea can occur in any age or breed of horse. Factors that may increase the risk of antibiotic-associated diarrhea include increasing age, underlying gastrointestinal disease (e.g., colic), a rapid change in diet, a change in the horse’s environment (e.g., moving to a hospital from stable or pasture), the type and route of administration of the antibiotic, and the microbial sensitivity to the administered antibiotic.


Antibiotic-associated diarrhea is a potentially severe enteric disease of horses that can have a high associated mortality rate. In a retrospective study performed by Cohen and Woods, the strongest factor associated with death among horses with acute diarrhea was prior administration of an antibiotic drug. Although any antibiotic has the potential to induce diarrhea in horses, those associated with a higher risk include tetracyclines, erythromycin, penicillins, clindamycin, lincomycin, and trimethoprim-sulfonamide combinations. There is also evidence that geographic differences in intestinal flora and susceptibility patterns can influence the incidence of antibiotic-associated diarrheas.


Antibiotic drugs increase the risk of diarrhea predominantly by altering the resident gastrointestinal flora, increasing the proliferation of pathogens already present in small numbers, or increasing the ability of pathogens in the environment to be transmitted from horse to horse and to colonize the intestinal tract. The normal flora protects the host from pathogenic bacteria by colonization resistance. In addition obligate anaerobes produce short-chain fatty acids (SCFAs) and other metabolites that inhibit the growth of potential pathogens. Antibiotics such as macrolides, lincosamides, tetracyclines and β-lactams target obligate anaerobes, the group of bacteria most critical to maintaining colonization resistance. This may explain why these antibiotics are often associated with diarrhea. Enterohepatic circulation is another factor that increases the likelihood that an antibiotic will induce diarrhea. Tetracyclines and erythromycins that are excreted via the bile achieve high, prolonged intraluminal concentrations that increase their effects on normal intestinal flora.


Intestinal colonization with Clostridium perfringens, Clostridium difficile, and Salmonella spp. has been linked with antibiotic-associated diarrhea in horses. Tetracycline administration, for example, has been associated with an increase in the number of C. perfringens and Salmonella spp. in feces of horses as well as with reactivation of salmonellosis. There is strong evidence suggesting that C. difficile may be the most important pathogen associated with antibiotic-induced diarrhea. A significant number of adult horses and foals with diarrhea shed C. difficile organisms and cytotoxins in their feces, suggesting a causative link between C. difficile infection and enterocolitis. All the horses in a reported hospital outbreak of C. difficile colitis were being treated with antibiotics, suggesting that antibiotic use had a role in the pathogenesis. A larger study in Sweden revealed that, of horses that developed acute colitis during antibiotic treatment, 18 of 43 (42%) had positive results for C. difficile by fecal culture and 12 of these (28%) had positive results of a fecal cytotoxin B test. In contrast, C. difficile was isolated from only a small number of diarrheic mature horses (4 of 72 [6%]) with no history of antibiotic treatment, and was not isolated at all from 273 healthy mature horses or 65 horses with colic. Studies of mares in Sweden that developed acute colitis during treatment of their foals with erythromycin and rifampin revealed a higher incidence of C. difficile organisms in the feces of affected mares. C. difficile was not cultured from the fecal material of healthy mares in this study. In a more recent study, also performed in Sweden, mares were inoculated with C. difficile organisms. Significantly more C. difficile organisms were cultured from fecal material collected from mares treated with penicillin before inoculation than from those that did not receive the antibiotic. Inoculation of clindamycin-treated foals with vegetativeC. difficile cells or spores resulted in a high rate of gastrointestinal signs, including diarrhea, fulfilling Koch’s postulates for C. difficile–induced diarrhea and strengthening the link between antibiotic administration and C. difficile–induced colitis.


Both C. difficile and Salmonella spp. are important nosocomial pathogens in many species, suggesting a link between antibiotic administration and environmental exposure in the hospital setting up the mechanism of intestinal colonization and, ultimately, development of diarrhea. Cross-infection between patients, use of antibiotics, and environmental contamination are important risk factors associated with human hospital outbreaks of C. difficile colitis and salmonellosis in horses. Taken together, these studies suggest that if the normal intestinal flora becomes disrupted by antibiotic treatment, there is an increased risk that C. difficile

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Antibiotic-Associated Diarrhea

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