Stress has been defined as a state in which an animal ''is required to
make abnormal or extreme adjustments in its physiology or behaviour i
n order to cope with adverse aspects of its environment and management
''. The most common stressors for horses are transport, exercise, lame
ness, and environmental temperature and humidity changes. Considerable
interindividual animal variation occurs in the endocrine response obs
erved subsequent to exposure to these stressors, particularly transpor
t and lameness, and indicates considerable variation in individual ani
mals' psychological responses to stress. Resting, exercise, and recove
ry heart rates have also been shown to be useful indicators of fatigue
or lameness. Both exercise and transport have been demonstrated to ha
ve considerable effects on pulmonary defence mechanisms and other immu
nological functions.