الفهرس 
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Abstract
A total number of 200 one day old broiler chicks were used to study the toxic effect of monensin. Four groups (Gps. 1, 2, 3, 4) were used, the first three groups were fed on ration containing 220, 440, 660 ppm monenSIn, respectively for 7 weeks and Gp. 4 was kept as a control. Clinical signs, serum enzymatic activities (LDH, ASH and CK) and histopathologic results were dose- and time dependent particularly of skeletal and cardiac myopathy of the groups fed on the medium and high doses of monensin.
Clinically, there were anorexia, depression and sometimes leg weakness and recumbency. Stunting growth with severe emaciation and dehydration were the main signs observed in the last 3 weeks of the experiment in case of birds of Gps. 2 and 3.
Necropsy findings revealed severe emaciation, skeletal muscles atrophy and flabby heart. These gross lesions were mainly observed on the last two weeks only in Gp. 3.
The dead cases of Gps. 2 and 3 showed myocardial streaking with lack of myocardial tone. The activity of serum enzymes indicative for myopathy (i.e. LDH, AST and CK) were significantly increased in case of Gps. 2 and 3.
Histopathologically, moderate and severe myopathy were observed in the birds of Gps. 2 and 3, respectively. The main skeletal muscle lesions of Gp. 2 appeared in the form of intermyofibrillar edema with round cells and sometimes heterophiles aggregation and multifocal myodegeneration and necrosis without mineralization or vacuolation. The same lesions were observed in Gp. 3 but were more severe and diffuse.
Cardiac lesions were observed either in the sacrificed or the dead cases in Gps 2 and 3. However, the cardiac lesions were more severe in the dead cases than in the sacrificed ones, these lesions were in the form of hyalinization, myonecrosis with inter myofibrillar round cell aggregation and fragmentation of the myofibers.
The hepatic and renal changes were generally mild except of few degenerative and inflammatory changes occurred in some cases of Gps. 2 and 3.
No significant gross or microscopic lesions could be seen In the spleen, brain, sciatic nerve and intestine.