It might be difficult to imagine that a few tiny organisms can proliferate and cause an infection so serious it could result in the death or euthanasia of the host horse. For instance, one of the most dangerous organisms that can sicken horses is the bacterium Clostridium difficile. Scott Weese, DVM, DVSc, Dipl. ACVIM, associate professor at the University of Guelph's Ontario Veterinary College, discussed some of the recent advancements in understand and combating C. difficile at the 2011 American College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo.

"Clostridium difficile is an organism known to be a common cause of intestinal disease in both humans and animals," Weese explained.

As with humans, C. difficile causes colitis (an inflammation of the colon) and severe diarrhea in horses. This organism can persist as a spore in a dormant state for many years (i.e., not cause an active infection), all the while remaining resistance to antimicrobial drugs and disinfectants. Weese noted that recent mutations in some strains of C. difficile have increased its virulence (the ability to cause disease) through amplified expression of toxin genes.

Weese noted that researchers recently learned that C. difficile is present in the gastrointestinal tract of some healthy individuals. Not only does this create a carrier state, but it also increases the risk of a horse developing an overwhelming C. difficile infection due to a trigger such as antibiotic therapy