Monensin poisoning in horses - an international incident.

Several hundred Michigan horses were accidentally exposed to varying levels of monensin. Severity of effects was proportional to the level of feed contamination; sudden death resulted on at least two premises. Acute signs of cardiovascular impairment occurred on one premises having received feed containing over 200 grams of monensin per tonne. Gross and histological postmortem lesions consisted of acute myocardial necrosis. Although only circumstantially confirmed, investigations led to the suspicion that the source of poisoning was a ration formulation error in a feedmill in southwestern Ontario. Concern over possible undetected heart damage in exposed horses led to clinical monitoring on one farm over a period of several months. Electrocardiographic and serum enzyme monitoring were used soon after the incident to implicate exposure in some horses; they were poor prognostic indicators. Applicable legislation, the cooperative role of government departments, and legal implications relative to potential prosecution and lawsuits arising from sale of contaminated feed between Canada and the USA are summarized.