Monensin [coccidiostat] toxicosis in broiler chickens

Chickens treated at seven weeks of age with 150, 200, or 250 mg of crystalline monensin sodium/kg body weight had signs of toxicosis, including extreme weakness, anorexia, paralysis and death. Gross lesions included emaciation, generalized congestion, myocardial enlargement and pallor, and hydropericardium. Samples of ventricular myocardium, superficial pectoralis (white fibers), anterior latissimus dorsi (intermediate fibers), and sartorius muscles (mixed red, intermediate, and white fibers) were examined by light microscope. Intermyofibrillar vacuolation, histochemically positive for neutral fat, was severe in the myocardium and red muscle fibers and moderate in the intermediate muscle fibers. Myofiber necrosis was limited to the red muscle fibers. Mitochondrial degeneration was apparent in myocardial sections from several chickens. Interstitial infiltration by macrophages and heterophils was common in the myocardium and aerobic skeletal muscle.