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Claviceps purpurea is a fungus of many grasses and several cereal grains, especially rye, oats and wheat. The sclerotium of the fungus is a dark, elongated body and often can be seen on cereal grain heads and in processed grains. The fungus produces three major alkaloids that cause ergotism. The primary lesions caused by the alkaloids include arteriolar vasoconstriction and endothelial cell injury that often leads to thrombosis. When present in low levels, the alkaloids can result in reduced growth rates. Larger amounts lead to ischemic necrosis followed by a dry, gangrenous sloughing of parts of extremities, especially tails, ears and hooves. Symptoms of ergotism are exacerbated by cold weather. In pregnant sows, ergotism can inhibit mammary development, reduce litter size, reduce birth weights, and cause a profound post-farrowing agalactia. The agalactia is believed to be related to inhibition of prolactin secretion.

Diagnosis of ergotism is based on lesions coupled with the gross or microscopic identification of significant numbers of ergot sclerotia in grains or the ground feed. Doubtful results may be verified by laboratory confirmation of significant amounts of alkaloids in the feed.