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

Reasons for peforming 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 >= 6 weeks after ingestion of monensin-contaminated feed. Animals with evidence of monensin-induced cardiomyopathy were re-examined after a period of rest of >= 11 months. Follow-up information was obtained by owner telephone interview >= 52 months after exposure. Results: During resting echocardiography, It animals had reduced/low-normal left ventricullar fractional shortening (FS), an increase in FS in 8 of these animals was measured >= 11 months la(er. 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 under-went 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 possibl