**4. Pathophysiology of equine stress**

From the outset of stress research, it has been observed that not all stress reactions and the consequences are equal (despite the stereotypical neuroendocrine changes), and Selye introduced the terms "distress" (pathological) and "eustress" (physiological). Physiological stress accompanies long-term positive biological adaptation of the animal (for example, physical training, increases in horse muscle mass). The acute or chronic stress of severe intensity, which is not resolved by the adaptation of the animal, is considered distress. However, the clear contours and boundaries between eustress and distress, acute or chronic stress are difficult to define in equine practice, support for its existence is ambiguous, and there are still efforts in this area to clearly differentiate between normal homeostatic and pathophysiological responses. In other words, the chronic stress phenotype is not clearly defined [92]. Further complicating toward a clearer understanding is that equine stress responses are context-dependent and may reflect differences in the environment, timing (e.g., time of day, season), history of previous stressors, and huge among-individual variations [168]. There is widespread recognition that the effects of stress on the equine body are associated with the characteristics of the stress factor (the strength of the action, its duration, and frequency, predictability, controllability, avoidability), the breed and age, as well as some features of the stressful experience (previous contact with the same or other stress stimuli) [80, 112]. Thus, young horses and foals are much more stress reactive with more pathophysiological responses in comparison to adult animals.

The importance of acknowledging the protective, as well as the potentially damaging effects of stress reaction, has led to the introduction of different terms, for example, allostasis, allostatic load, or allostatic overload [169]. In response to a threat, allostasis maintains stability through adaptive dynamic activation of neuroendocrine, autonomic, cardiovascular, and immune systems, principally aiming to produce the new optimal

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

level of various physiological parameters. For example, physical exercise in horses leads to increased heart rate and blood pressure to provide optimal oxygen concentration to the vital organs. By focusing on neuroendocrine responses, allostasis involves a feed-forward mechanism rather than the negative feedback mechanisms used in homeostasis, with a continuous re-evaluation of need and continuous readjustment of all parameters toward new set points [170]. Using this theory allows us to explain the positive effects of stress on the animal body, for example, an increase in resistance to adverse factors and survival in extreme conditions. As mentioned above, the allostatic mechanism in horses must quickly disconnect once the threat has passed, as it impossible to keep a high level for a long time, due to depletion of reserve capabilities. Straining of the body supporting homeostasis under frequent or prolonged stress is called allostatic loading. It is also defined as the wear and tear associated with chronic hyperactivity or inadequate responses [171]. As allostatic load is characterized by an unstable functioning of the body, if it lasts for a long time or often, it causes the appearance of pathological changes (due to the cumulative effect), which are called allostatic overload.

Usually, under chronic stress, pathological changes are noticed simultaneously in many organs and tissues, primarily stress hormones cause the production of reactive oxygen species (ROS) or free radicals. It especially produces catecholamines, which as phenolic compounds easily undergo oxidation via a one-electron pathway involving several toxic products, such as semiquinones, quinones, and ROS, independently of the oxidation promotors. Almost any chronic stress through ROS has been shown to be responsible for the depletion of several free radical detoxifying enzymes, such as glutathione peroxidase, catalase, and superoxide dismutase [172]. It results in oxidative overloading, which has been implicated in the pathogenesis of different stress-associated pathologies, as well as in the occurrence of various mutations [173]. It is known that mutations accumulate as a result of DNA damage and imperfect DNA repair mechanisms. In animals (including horses) the accumulation of mutations is limited in two primary ways: through p53-mediated programmed cell death and cellular senescence mediated by telomeres at the end of chromosomes [174]. Telomeres shorten at every cell division and cells stop dividing once the shortest telomere reaches a critical length [175]. Cellular stress shortens the length of telomeres, and therefore it indirectly records the history of stress exposure. As a result, as biomarkers of cellular aging, telomere length and telomerase activity, have been considered for investigating the effects of chronic stress in human medicine [176]. This aspect of stress has not been adequately researched in horses to date.

There are numerous equine pathologies that are directly or indirectly associated with stress factors. According to our clinical experience, due to stress exposure, the most commonly noticed diseases in horses are—gastric ulcers, proximal enteritis, acute colitis, and pleuropneumonia [177–179]. It is important to note that for the occurrence of these diseases along with stress, other pathogenomonic factors have also had a significant impact. For example, for the occurrence of paralytic ileus or colitis, a high concentration of endotoxins in the blood also plays a very important role [177, 178]. Noxious gases (e.g., NH, NO, and CO) in the transport environment may be partially responsible for transport-related equine pleuropneumonia [179]. It is known that equine transport causes strong psychological stress reactions as it often combines the effects of neophobia, claustrophobia, social separation, and balancing. During equine transport, the physiological and endocrine stress changes prepare the horse's body for the "fight or flight" reaction, however, these actions do not actually follow, and therefore the mobilized energy is not used. Without a doubt in horses this leads to more often pathophysiological consequences, in comparison to stress physical overload, the neuroendocrine changes are accompanied by locomotion thus providing an outlet for the mobilized energy. The reason for this phenomenon is poorly understood. It is possible, that physical activity in horses, as well as in humans, leads to the release of higher concentrations of endogenous opioids and other protective mediators against the destructive effects of stress hormones. It is also possible that the mobilized, but "unused" energy will lead to higher production of ROS.

Interestingly, of the four most common stress pathologies in the clinic in horses, three are associated with the gastrointestinal tract. The reasons for this appearance are multiple. Primarily, stress has strong adverse effects affecting the normal function of the GI tract, for example on the absorption process, mucus and stomach acid secretion, functioning of ion channels, and *peristalsis* [178, 180]. Stress induces increased intestinal permeability, allowing bacterial antigens (including LPS) to cross the epithelial barrier which activates a mucosal immune response, which in turn alters the composition of the microbiome and leads to enhanced activation of the neuroendocrine HPA axis. In other words, the equine intestinal microbiota also has been implicated in a variety of stress-pathophysiological responsiveness, but also in healthy horses, it clearly modulates the function of the immune and neuroendocrine systems, as well as various metabolic processes. The routes of communication between equine microbiota and CNS are slowly being unraveled, and include the microbial metabolites such as short-chain fatty acids, the vagus nerve, intestinal hormone signaling, and tryptophan metabolism.

Whether the horse will get sick or die from these and other stress-associated diseases depend primarily on immune systems and individual resistance. The individual characteristics of equine stress resistance are determined by the type of the nervous system, lability, or dominance of the parasympathetic or sympathetic nervous systems [168]. Based on our experience, horses with a high parasympathetic tone are much less likely to die from the stress-associated disease than animals with a high sympathetic tone (neurotic horses). Equine neuroticism is also linked to a low pain threshold, indicating such horses were more likely to be stressed by pain [181]. The effect of stress is also dependent on the initial level of hormones, which in turn depends on various factors, including the phase of the light cycle [92], but to date, there has not been a detailed investigation as to which period of the day horses are more tolerant toward stress-induced disease.
