In horses, ionophore toxicosis typically presents in what sequence of clinical signs?

• A. GI signs only
• B. Neurological signs only
• C. GI/renal -> neuro -> cardio/pulmonary
• D. Cardio/pulmonary signs only

Answer: (C)

Ionophore toxins disrupt ion transport in skeletal and cardiac muscle, causing rapid energy failure and muscle cell death. In horses, this leads to a predictable progression: first the GI tract shows signs from irritation and decreased intake (colic-like pain, diarrhea, anorexia) and there can be secondary renal involvement from dehydration and myoglobinuric effects. As muscle damage progresses and systemic energy depletion worsens, neuro effects appear due to impaired muscle function and electrolyte disturbances (weakness, ataxia, depression, tremors). Eventually the heart bears the consequences of myocardial injury, leading to cardiopulmonary signs such as tachycardia, arrhythmias, reduced contractility, and dyspnea or pulmonary edema. This sequence—GI/renal signs first, followed by neuro signs, then cardio/pulmonary signs—best fits the typical course of ionophore toxicosis in horses.