Definition of Stress
Horses demonstrate extreme behavioural flexibility in adapting to a wide range of management and conditions over a lifetime. Housing, nutrition, social groupings, handling, training, breeding and transportation of horses are widely diverse in terms of the natural behaviour of the horse (McDonnell, 2003). Although transporting horses by road is a common activity, from the horse’s perspective it is an unnatural activity that is potentially stressful both physically and psychologically (Smith et al.,1994a).
Everyone knows what stress is, however, it is not easily defined. One of the most useful definitions of stress is ‘when adverse effects in the environment or management system force an animal to make abnormal or extreme adjustments of physiology or behaviour in order to avoid physiological malfunctioning, thus, assisting the animal in coping with its environment’ (Stull, 1997; Leadon, 1999).
The duration and intensity of a stressor, immediate environmental constraints, previous experiences, and physiological status will determine the reactive response of an animal to a challenge in its immediate environment, and incorporates several interacting mechanisms including physiological, biochemical, immunological, anatomical and behavioural parameters (Stull, 1997; Leadon, 1999).
The degree of stress can adversely affect the horse’s capacity to grow, reproduce, train, and maintain health, thus identifying and minimising stressful situations could protect the performance and economical potential of the horse (Baucus et al., 1990; Stull, 1997; Broom et al., 2002). Measuring various adaptive responses can indicate the extent to which horses have been challenged by a journey and provide an assessment of the rate and duration of recovery (Leadon, 1999; Broom et al., 2002).
Physiological Response to Stress
In both humans and animals, physiological parameters that have been utilised in studying the stress response include measuring levels of hormones released from the brain or other organs, fluctuations in leukocyte parameters, and changes in heart rate. Physiological responses to stress have been investigated more than behavioural profiles, probably due to the relative ease of sampling, measurement, and objectivity of physiological parameters. In addition, much research is carried out in food animals on species having a ‘market value’ that may be affected by meat quality, which can be negatively affected during stressful conditions or experiences.
A rapid heart rate is not just a consequence of increased activity; the cardiovascular state of an animal is determined via a mixture of reflex effects and psychogenic responses. Psychogenic responses are often triggered by sensory stimuli and involve neural pathways of the midbrain and forebrain, including the limbic system and cerebral cortex. The defence-alarm reaction is an important psychogenic response for prey species and is an emotional response to a threatening situation, physical injury or trauma involving increased sympathetic activity and decreased parasympathetic activity (Stephenson, 2002).
The perception of the stressor initiates an immediate neuroendocrine response via the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis plays a major role within the neuroendocrine system, having a primary function in the reaction to stress by balancing hormonal release from the adrenal medulla (adrenaline) and adrenal cortex (corticosteroids). Upon activation of the HPA axis, corticotrophic-releasing factor (CRF) is produced by the hypothalamus, stimulating the anterior pituitary gland to produce adrenocorticotrophic hormone (ACTH) and β–endorphin.