While treating a foal with erythromycin, which organism puts the mare at risk for developing enterocolitis?

The correct answer is Clostridium difficile, which is known to be associated with antibiotic-induced enterocolitis in various species, including horses. When a mare is treated with antibiotics like erythromycin, there is a significant alteration in her normal gastrointestinal flora. This disturbance allows opportunistic pathogens, such as Clostridium difficile, to proliferate. Clostridium difficile produces toxins that can lead to severe inflammation of the intestine, manifesting as enterocolitis. The risk is particularly heightened in cases where the natural gut bacteria are suppressed or disrupted due to antibiotic treatment.

Other organisms listed do not have the same established link to antibiotic-associated enterocolitis in mares. For instance, while Clostridium novyi is an important pathogen in certain conditions, it is not primarily associated with antibiotic-induced enterocolitis. Escherichia coli is a common gut inhabitant and typically not a primary concern in this context, while Lawsonia intracellularis is linked with different clinical presentations, particularly in young equines, and is not directly related to the impact of erythromycin on the mare's gut flora. Thus, Clostridium difficile stands out as the key organism of concern following erythromycin treatment in a mare.