Abstract
Eleven Thoroughbred colts were fed tylosin (0.88g per head) mixed with feed once (for 6 colts) or twice (for 5 colts). They began to show apathy and anorexia 1-2 days after the first administration, and diarrhea one day later. Then 3 adulthorses were fed the residual medicated feed. They exhibited the same signs. Four colts died 2-3 days after the first administration. Autopsy revealed mild lesions in the gastrointestinal tract. Microscopically, there were no changes of infection. No pathogenic bacteria were isolated from any organ or the intestinal contents.
