Long-term assessment of horses and ponies post exposure to monensin sodium in commercial feed.

Abstract

REASONS FOR PERFORMING STUDY: Acute monensin intoxication in equids is well described; however, the long-term effects of sublethal intoxication and ability to return to previous use are less well understood. Long-term observations may allow improved estimation of prognosis in cases of sublethal intoxication. OBJECTIVES: To assess horses and ponies exposed to sublethal amounts of monensin for evidence of chronic sequelae and ability to return to prior/intended use. METHODS: Twenty-nine horses and 8 ponies were assessed utilising serum biochemistry, treadmill exercise stress testing, electrocardiography, and pre- and post exercise echocardiography > or = 6 weeks after ingestion of monensin-contaminated feed. Animals with evidence of monensin-induced cardiomyopathy were re-examined after a period of rest of > or = 11 months. Follow-up information was obtained by owner telephone interview > or = 52 months after exposure. RESULTS: During resting echocardiography, 11 animals had reduced/low-normal left ventricular fractional shortening (FS); an increase in FS in 8 of these animals was measured > or = 11 months later. Six animals had reduced or low-normal FS during post exercise echocardiography. Two horses had ventricular premature depolarisations during exercise. Follow-up information was available for 35 animals: 21 returned to athletic/reproductive use, 13 were retired immediately and one died. Mean FS increased significantly (P < 0.001) between initial and second examination in 15 animals that underwent resting echocardiography on 2 occasions. CONCLUSIONS: Some equids exposed to sublethal doses of monensin may not develop permanent myocardial disease and a return to athletic/reproductive use is possible. POTENTIAL RELEVANCE: Exercise stress testing, echocardiography and electrocardiography may be useful for detection and monitoring of cardiac dysfunction in equids exposed to monensin and determining whether a return to athletic/reproductive use is possible.