**2. Equine stress factors**

There is wide consensus, that horses, by the nature of their use and management, are more likely to be exposed to different stressor factors compared to other

#### *Equine Stress: Neuroendocrine Physiology and Pathophysiology DOI: http://dx.doi.org/10.5772/intechopen.105045*

domesticated animals. There are numerous equine stress factors, these can roughly be divided into internal and external, acute and chronic, physical and psychological, but more often horses are exposed to numerous stress factors at the same time. From a pathophysiological and clinical point of view, strangulation intestinal obstruction causes the strongest equine stress response, as it is accompanied by several stress factors at the same time. Firstly, there is severe abdominal pain (through tissue damage and intestinal distension) [10]. Secondly, there is the occurrence of intestinal dysbacteriosis with the release of LPS, which triggers an increased concentration of proinflammatory cytokine and the *development of severe endotoxic shock* [11]. The treatment of such horses in equine clinics and the associated changes in environmental conditions (other stables, unknown people, and other horses) significantly enhance the stressful response [12]. If such horses undergo abdominal surgery and general anesthesia, this will increase stress exposure placed on the animals [13]. Numerous other equine stress factors also exist, for example for used horses in sports and competition there are associated transport conditions, the novelty of their surroundings, exposure to a noisy public, and physical overload [14, 15]. Physical exercise is a stress condition solicited in the organism creating a new dynamic equilibrium that requires adaptive responses. Exercise-induced stress is often proportional to the horse's competition level [16]. Water or food deprivation (after intestinal surgery), metabolic disorders by various equine diseases (acidosis, hypovolemia, electrolyte imbalance, and hypoglycemia), and inflammation also cause a stress response [17, 18]. Temporarily limited but very intensive psychological loads are exposed to foals at weaning [19, 20]. The lack of activity, for example constantly staying in a stall, stabling, and isolation without social contact with other horses, causes most horses to undergo chronic stress responses [21]. It should be taken into account that different horses may show stress in specific ways, and some horses respond better to stressful situations than others. The adaptive response of each horse to stress is determined by a multiplicity of genetic, environmental, and developmental factors. Equine stress response in horses is also dependent on the animal's perception of the situation. It is considered in human and equine medicine that the crucial factor that determines if a psychological stressor has a negative, neutral, or even positive outcome, is whether the central nervous system (CNS) perceives to be in control of the situation or not [22]. Certainly, it has an important role has the experience of the horse. Frequently, most horses appear to fear novel situations, and these are perceived as being threatening. In addition to all of the above, nervous riders or veterinary personnel may cause a horse to behave more reactively because they present as ambiguous stimuli [23]. Undoubtedly, nervous people also transmit their fear to horses, which enhances the equine stress response. It is well known that horses recognize angry human faces and interpret them as negative [24].
