Photo: Megan Arszman

Many horse owners will agree that keeping their charges safe from disease risk is a top priority. But could a healthy horse in one's own backyard be a disease risk for other horses? At the 2011 American College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo., Angelika Schoster, DVM, a DVSc candidate at the University of Guelph's Ontario Veterinary College discussed novel epidemiologic features of several bacteria often related to diarrhea and colitis (inflammation of the colon) in horses. Schoster explained that some healthy horses might also carry these bacteria and act as potential reservoirs to infect others with gastrointestinal ailments caused by the organisms.

Schoster described a study in which researchers evaluated healthy horses for several enteric (intestinal) pathogens known to cause colitis, diarrhea, and other gastrointestinal ailments in equines: Clostridium difficile, Clostridium perfringens, Salmonella, and Escherichia coli. In addition, they examined the antibiotic sensitivity of commensal E. coli (nonpathogenic), as it can be a good indicator of antimicrobial sensitivity in the common pathogens.

Twenty five horses from five farms were examined monthly over the course of a year. None had any known gastrointestinal disease, nor had they received any medications at the time of enrollment into the study.

Researchers did not detect C. perfringens or Salmonella sp. in any of the horses. However, they did detect C. difficile in 44% of the horses and on 60% of the farms, and the bacteria detected was a toxigenic (produces a toxin) form of C. difficile. Schoster noted that researchers believe the toxigenic strain ribotype 078, which is similar to a strain becoming increasingly important in human enteric disease, is becoming more prevalent in horses. The study results suggest that C. difficile shedding occurred only for short durations. Simply put, while it's not uncommon for healthy horses to carry bacteria, it's relatively unlikely that they will actually infect other horses.

E. coli was isolated in 77.3% of case horses, and 13.4% of the isolates collected were resistant to one or more antibiotics, with 6.8% being multidrug resistant (i.e., resistant to three or more antibiotics). Trimethoprim-sulfa drugs had the highest resistance rate (72%) followed by tetracycline with 22% resistance. Other drugs that demonstrated resistance problems included ampicillin, cefoxin, chloramphenicol, streptomycin, and kanamycin.

Schoster concluded that the while many healthy horses might carry disease-causing bacteria in their gastrointestinal tracts, study results suggest there's little risk of these horses falling ill, infecting others, or perpetuating antimicrobial-resistant bacteria: "The low prevalence of commensal E. coli antimicrobial resistance suggests that healthy horses on pleasure farms are unlikely to be a major reservoir of resistance in enteric bacteria."

Of course, consult a veterinarian if health concerns do arise, as certain bacteria can cause serious problems for susceptible horses. Additionally, since antimicrobial-resistant bacteria are a growing problem, if an antimicrobial is warranted ensure that it's used properly so as to not contribute to resistance.