Stress has been defined as a state in which an animal "is required to make abnormal or extreme adjustments in its physiology or behaviour in order to cope with adverse aspects of its environment and management". The most common stressors for horses are transport, exercise, lameness, and environmental temperature and humidity changes. Considerable interindividual animal variation occurs in the endocrine response observed subsequent to exposure to these stressors, particularly transport and lameness, and indicates considerable variation in individual animals' psychological responses to stress. Resting, exercise, and recovery heart rates have also been shown to be useful indicators of fatigue or lameness. Both exercise and transport have been demonstrated to have considerable effects on pulmonary defence mechanisms and other immunological functions.
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