The notion of stress was introduced into human medicine by Dr. Hans Selye in 1925. He defined it as follows: “Aggression of the organism by a physical, psychic or emotional agent causing an imbalance which must be compensated by an adaptation work; agent which aggresses; nervous tension, constraint of the organism in response to a shock (sudden event, trauma, strong sensation, noise, overwork)“. Concerning animal welfare, there is no universally recognized definition.
The horse is a naturally stressed animal, its survival initially depending on its ability to flee. Its innate anxiety multiplies the potential sources of stress, even though each horse feels and reacts differently to stress. It can be beneficial to the animal, but it can also have pathological effects.
In this article, we will look at the main indicators for measuring stress, as well as the physiological and behavioral consequences of stress. Finally, we will look at the different methods for stress management in order to improve the well-being of the horse.
Stress indicators in horses
The environment, the rider, the competition, the feed, the isolation… all have an impact on a horse’s stress level. Stress analysis is a complexe task because it can have cumulative effects and affects mental and physical condition of the horse, involving both psychological and physiological aspects.
The horse responds to stress by activating the hypothalamic-pituitary-adrenal (HPA) axis, causing a subsequent increase in stress hormones, such as adrenaline and cortisol, usually coupled with a flee or fight reaction.
Some of the key indicators for measuring stress levels are:
Behavioral parameters
Confronted with a new situation, the stressed horse will act in an unusual and often aggressive way: ears pinned back, head raised, explosive reactions, …
Two main types of adaptation to stress have been identified in horses: “proactive” coping, which consists of actively trying to find a solution to the problem; and “reactive” coping, which consists of passively accepting the situation (Ijichi et al, 2013). A behavioral stress score, validated by physiological and biochemical data, was created to assess stress in the stabled horse. It contains only behaviors and a qualitative description related to the proactive coping style (Young et al, 2012).
This table highlights the most visible behavioral changes in stressed horses. It is important to note, however, that the absence of a reaction does not necessarily indicate the absence of stress.
Eye temperature
It increases when the horse is confronted with a stressful situation (Valera et al, 2012; McGreevy et al, 2012), often accompanied by an increase in the number of eye blinks (Hale et al, 2011). Like eye temperature, body surface (skin) temperature also increases when the horse is stressed.
Cortisol levels
Several studies have found a link between a horse’s stress and its cortisol level, a steroid secreted by the adrenal gland. Changes in cortisol levels are regularly used to assess stress, as cortisol increases in stressful situations. The graphs below show the changes in cortisol levels in a healthy horse on a typical day. It is obvious that the level of cortisol, and thus stress, decreases throughout the day, especially once the work is completed.
To be measured, a blood or saliva sample must be taken at the time of the stressful event, or within a very short period of time. This limitation, in addition to the time required to analyze the results, limits the use of this indicator.
Heart rate and respiratory rate
These increase as the horse’s stress level increases, while heart rate variability decreases. Connected heart rate monitoring systems are valuable tools for detecting stress. A sudden and significant increase in heart rate during a steady work session indicates a stressful event. It will then be possible to take appropriate action to limit the negative effects on the horse’s health and performance.
Physiological and behavioral impacts of stress in horses
Regular exposure to stressful situations can have long-term negative effects on the horse’s health and impact its nervous system, which regulates vital functions such as breathing and blood circulation…
Physiological impacts
In horses, stress has a significant impact on the digestive system. Increased gastric acid production will attack the mucous membranes, causing significant inflammation and even lesions that can lead to gastric ulcers. Digestional dysfunction can also cause abdominal pain, which can lead to significant weight loss, fatigue, or colic. Stress can also cause severe sweating and diarrhea, resulting in severe dehydration. Several cases have also revealed stress-related reproductive disorders.
Physical impacts
Stress in horses can greatly increase the risk of injury. A stressed horse can be unpredictable and dangerous to those around him and to himself. Natural instincts will take over and the horse will run away, often reacting in an unreasonable manner. As a result, more injuries occur in stressed horses, usually to the limbs or the head.
Performance impacts
In some cases, stress can improve a horse’s performance by increasing its adrenaline level and allowing it to pay close attention to its surroundings. However, stressful situations can have an immediate impact on a horse’s sporting performance, not only by affecting its ability to concentrate but also causing physical pain: stiffness, muscle contractions, etc.
Behavioral impacts
Stress frequently leads to behavioral issues. Cribbing, windsucking, head shaking are among the most common behavioral disorders in horses. They frequently affect horses who are anxious or who struggle with solitude, boredom, or isolation.
How to manage stress to improve horse well-being?
Measuring and managing stress is essential for the well-being and safety of the horse. Several techniques can be used to positively impact a horse’s stress level.
Nutrition and living conditions
Feeding can be a significant source of stress in horses, and adapting their diets can help alleviate it. Consistent access to forage and forage diversity (Thorne et al, 2005), whenever possible, can reduce frustration and promote serotonin production (Alberghina et al, 2010). Stressed horses will also prefer a fixed feeding time, with a clear signal, used for no other reason (Bassett and Buchanan-Smith, 2007).
In addition to an appropriate diet, dietary supplements based on magnesium, tryptophan, B vitamins, homeopathy and/or herbal treatments (lemon balm, chamomile, hawthorn, hops,…) can be good ways to reduce stress.
Finally, increasing the frequency of outdoor work and outings to the paddock, as well as developing the horse’s living environment to avoid boredom as much as possible, can reduce the horse’s stress level (ball, toy, teddy bear, etc.).
Relaxation techniques
Several relaxation techniques are known to reduce a horse’s stress, such as massage (McBride et al, 2004), friction on the horse’s withers (Feh and de Mazieres, 1993; Normando et al, 2003; Payne et al, 2015), or simply grooming. As a preventive action, it is also possible to use pheromones in gels, to be spread on the nostrils before a stressful event.
Manipulation techniques
When a stressed horse is handled, it is important to adapt your behavior to build trust and, ideally, to calm the animal. Synchronize your actions with the animal’s body language, make your movements predictable, establish routines…. Adapting your daily work, both walking and riding, can create important positive changes.
Conclusion
Stress is an inevitable part of an animal’s life in a domestic environment. Although it is only a normal physiological response, it is nowadays found in many horses and can be a source of risk to the horse’s health and performance. In order to maintain its physical integrity, it is therefore important to limit it as much as possible. However, it appears necessary to be able to detect and analyze stress episodes in advance in order to prevent the emergence of disorders and/or pathologies.
Mots-clés : stress, equine veterinarians, medical-sports follow-up of the horse, heart rate, monitoring
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