The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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22 TheHorse.com THE HORSE October 2015 Before delving into specific causes of stress, what a "stressed" horse looks like, and what you can do to minimize stress- ors, let's take a look at stress itself and how scientists measure it. What is It? Stress, as described by the Ameri- can Psychological Association, is an "emotional experience accompanied by predictable biochemical, physiological, and behavioral changes." Psychology Today defines it as "simply a reaction to a stimulus that disturbs our physical or mental equilibrium … (to) trigger the 'fight-or-flight' response, causing hor- mones such as adrenaline and cortisol to surge through the body." Heleski says, "Most of us in applied animal behavior or animal welfare sci- ence like the definition of stress used in one of the so-called 'stress bibles,' The Biology of Animal Stress, by Moberg and Mench." In that book, Moberg writes, "I will define stress as the biological response elicited when an individual perceives a threat to its homeostasis. The threat is the 'stressor.' When the stress response truly threatens the animal's well-being, then the animal experiences 'distress.' " If you've ever been in a highly stress- ful situation, such as a car accident, then you'll recall the feeling of panic, a racing heart, and sweating that suddenly overcame you. Those physical responses to stress, along with signs such as dilated pupils, are primarily due to the reflex- like immediate activation of the sympa- thetic nervous system, the hypothalamic- pituitary-adrenal (HPA) axis, and behavioral changes. In brief, the HPA axis involves a specific region of the hypothalamus (the part of the brain that operates the autonomic nervous system—responsible for those body processes that we do not consciously direct) that secretes a slew of hormones within nanoseconds of a stressful event occurring. One of the key hormones, corticotropin-releasing factor (CRF), circulates to the pituitary gland, stimulating the release of another flood of hormones, including adrenocortico- tropic hormone (ACTH). This hormone circulates throughout the body, rapidly reaching the adrenal glands nestled near the kidneys. In turn, the adrenal glands release cortisol, the penultimate "stress hormone," which spreads throughout the body and causes the classic physiological features of stress. Assessing Horses' Stress Considering horse-to-horse variability, how do you know if your horse is actually experiencing stress? From a scientific point of view, the main ways to "quantify" stress are to measure cortisol levels in blood, saliva, or feces; changes in heart rate; and heart rate variability (HRV, the beat-to-beat variation or the difference in time between individual heart beats). Heleski and her colleague Kathalijne Visser, PhD, owner of Horsonality Con- sulting, in De Knipe, The Netherlands, de- scribed the pros and cons of each of these stress measurements during the 2014 In- ternational Society for Equitation Science conference (TheHorse.com/34684). They suggested that owners consider both the physiological measures of a horse's stress as well as behavioral changes. In a 2012 study, researchers from the University of Chester and Newcastle University, both in the U.K., determined that behavior scoring on a scale of 0 to 10, with 10 being very stressed, offers an easy-to-use objective way of assess- ing animal welfare and reduces the need for potentially invasive physiological measures, such as blood sampling, for measuring stress hormone levels. In that study, the researchers assessed behaviors such as body actions (e.g., roll- ing, standing, rearing, barging, weaving, pawing, kicking) and specific positioning of the tail, neck, ears, mouth, and head in 32 horses undergoing routine husbandry procedures. They validated the behavior scores against the horses' salivary cortisol levels. A horse with a medium stress level of 5, for example, displayed behav- iors such as scratching against stable walls, pawing at the ground, flaring his nostrils, restlessness, fidgeting, raising his tail, defecating, making repetitive head movements, and flattening his ears occasionally. What this study and others tell us is that we must learn to recognize stress- related behaviors, use those along with a variety of measures to determine how stressed our horses are, carefully assess how we manage our horses, and continue studying "stress tests" in horses, says Heleski. What Horses Find Stressful Just as a screaming child in the grocery store might not faze one person, yet cause another to reach for the antacids, what one horse might perceive as "stressful" might not necessarily seem stressful to another. For example, if two horses go on a trail ride and a bird bursts out of the bushes right in front of them, only one might spook. Other known equine stressors include: ■ Inappropriate types or timing of food (e.g., meal and forage restrictions); ■ Reproduction-related stresses; ■ Social stresses such as individual housing; ■ Medication administration; and ■ Temperature extremes. Let's take a look at a few of the com- mon stressors in horses and the most recent research on each. Feeding-related stress Researchers have linked dietary restrictions (i.e., not permitting free-choice forage consump- tion, such as grazing or constant access to hay throughout the day) to the emergence ISTOCK.COM PHOTOS Hypothalamus Pituitary Gland Adrenal Gland In both horses and humans under stress, the hypothalamus secretes hormones that circulate to the pituitary gland, triggering the release of adrenocorticotropic hormone (ACTH). ACTH makes its way to the adrenal glands located near the kidneys, which release the stress hormone cortisol that causes physical features of stress. ACTH THE HPA AXIS ALL Wo u n d U p ?