Physiological stress can be an important equine welfare issue
Life is stressful. We two-legged mammals owe our nail-biting and tense shoulders to a slew of triggers—piles of bills, crying children, and too few hours in the day, to name a few. Our horses experience stress, as well. They might not balance a checkbook, produce progeny that wail endlessly, or live their lives to the tune of an Outlook calendar, but their hearts pound and bodies suffer all the same. Our job as their keepers is to understand what horses “stress out” over, recognize when they’re stressed, and more importantly, what to do about it.
Historically, horses were prey animals that lived in herds grazing for the majority of their days, so it is not surprising that today’s domesticated horses experience a certain degree of stress from modern-day practices such as stall confinement. Horse owners might not pick up on or appreciate many of these stressors, and equine researchers say this can amount to a welfare issue.
“I believe that responsible horse owners essentially ‘owe’ it to their horses to either reduce or manage stress wherever and whenever possible,” says Camie Heleski, PhD, an equitation science researcher and senior lecturer for the University of Kentucky College of Agriculture, Food and Environment Equine Programs. The onus is also on trainers and handlers.
Before delving into specific causes of stress, what a “stressed” horse looks like, and what you can do to minimize stressors, let’s take a look at stress itself and how scientists measure it.
What is It?
Stress, as described by the American 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 hormones such as adrenaline and cortisol to surge through the body.”
Heleski says, “Most of us in applied animal behavior or animal welfare science 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 stressful 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 reflexlike immediate activation of the sympathetic 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 adrenocorticotropic 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 Consulting, in De Knipe, The Netherlands, described the pros and cons of each of these stress measurements during the 2014 International Society for Equitation Science conference. 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 assessing 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., rolling, 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 behaviors 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 common stressors in horses and the most recent research on each.
Researchers have linked dietary restrictions (i.e., not permitting free-choice forage consumption, such as grazing or constant access to hay throughout the day) to the emergence of oral stereotypies or other abnormal behaviors, along with gastric ulceration in horses. In broodmares, these stressful dietary restrictions can even impact reproductive efficiency.
“It is well-known that horses are trickle feeders that would naturally consume a semicontinuous supply of forage for 40-70% of each 24-hour period,” explains Martine Hausberger, PhD, director of the Laboratory of Animal and Human Ethology, a branch of the French national research center (CNRS) and the University of Rennes. “It is also known that horses can experience gastrointestinal discomfort if deprived of food for a mere one to two hours.”
A good measure of stress in horses, which don’t always reveal signs of GI discomfort, can be its effects on reproduction. After feeding 100 Arabian broodmares in either a standard pattern (fed forage only at night) or a continuous feeding group (fed forage morning and night), Hausberger et al. found that despite having been fed the same total amount of roughage, mares in the continuous feeding group had significantly fewer estrous abnormalities and an almost 30% higher conception rate than those in the standard group. These results suggest that stress related to noncontinuous feeding hinders reproductive performance.
The study authors deduced that semicontinuous feeding of roughage might be a way to fulfill the basic physiological needs of the horses’ digestive system, reduce stress, and promote reproductive success.
Reproductive system stress
Tom Stout, VetMB, PhD, an equine reproduction researcher in Utrecht University’s Department of Equine Sciences, in The Netherlands, says physiological stressors such as pain, systemic disease, weaning, transport, changes in group structure, poor nutrition, or temperature extremes can predispose mares to pregnancy loss.
Even a mare’s transrectal pregnancy check—using an ultrasound probe to “see” the uterus through the rectal wall—can cause significant stress. “Transrectal ultrasound examinations are standard for detecting ovulation and identifying pregnancy in horses,” says Harald Sieme, DVM, DrMedVet, professor of Equine Reproductive Medicine at the University of Veterinary Medicine in Hanover, Germany. Sieme and colleagues determined that conducting the exam in nonlacting pregnant mares caused “significant disruption of homeostasis and stress,” evident from heart rate, heart rate variability, and saliva cortisol levels measurements in 25 mares.
This was in line with previous research findings that rectal examinations can induce stress, potentially contributing to welfare issues and even pregnancy loss.
Sieme and his colleagues determined that transabdominal ultrasound, which is performed through the abdominal wall (as in pregnant women) is a better approach for nonlacting pregnant mares after Day 90 of gestation.
Says Stout, “Although the exact role of stress in pregnancy loss is not clear, it seems prudent to minimize stress during pregnancy.”
Gastrointestinal illness and colic
Not surprisingly, colic is extremely stressful for both horse and owner. Researchers in the U.K. recently confirmed this by measuring heart rate and circulating cortisol levels in 179 referred colic cases and comparing them to 30 systemically healthy control horses. Horses with serum total cortisol concentrations (STCCs) above 200 nmol/L (nanomoles per liter) were more likely to colic than controls, and colicking horses with STCCs above 200 nmol/L were more likely to display moderate to severe colic signs (e.g., heart rate higher than 45 beats/min). The study authors concluded that STCCs might provide additional decision-making (e.g., whether to go to surgery) and prognostic information in horses with colic.
“This study confirmed that horses with colic demonstrate elevations in STCCs and suggests that an increase in STCCs relates to the severity of the underlying disease,” they wrote.
In a 2015 study, researchers found that fecal cortisol levels increased in 2-year-old Thoroughbreds after their transport and arrival at an auction in South Africa. Study authors suggested that this “reflected a cumulative series of stressful events associated with transport and sales arrival.” The levels decreased as the horses became accustomed to the environment and routine. The authors also noted that this physiological stress response and commingling might increase the risk of infectious upper respiratory disease developing in young horses. They suggested further investigation into transport and arrival phases and management practices to improve horses’ health and welfare during events such as sales.
In a 2015 study, researchers from the University of Sassari, in Italy, collected and analyzed blood samples from 21 horses participating in the Sartiglia, which is a costumed horseback joust. They found increased sugar, enzymes, cortisol, beta-endorphins, and reactive oxygen metabolites on the day of competition, but those values returned to baseline levels quickly by the day after the competition. In light of these results, the authors suggested that owners and riders try to minimize the stress horses in similar tournaments might experience.
In another 2015 study, researchers from Nottingham Trent University, in the U.K., evaluated 16 horses housed in four different scenarios for five days each. When housed individually and with no contact with other horses, study subjects had higher fecal cortisol levels and were more difficult to handle than when housed in groups or individually with horses nearby. Eye temperature, a noninvasive measure of stress, was significantly lower in horses in group housing. The study authors concluded that both physiological and behavioral measures indicated that social contact during housing of domestic horses could improve equine welfare.
It is our responsibility to provide the best possible environment to minimize stress and ultimately maximize our horses’ quality of life.
Try not to be too hard on yourself if you’ve only just identified certain situations you didn’t realize were stressful for your horse. Managing stress is not the same as minimizing or avoiding stress altogether, says Heleski.
“For example, for me, giving oral presentations is somewhat ‘stressful,’ but it is also invigorating and an important part of my job,” she says. “This is similar to taking my horses to a show. I am reasonably certain that this is somewhat ‘stressful,’ but I try to manage their stress by getting to the show early and allowing them time to settle, making sure they have a lot of hay while they are at the show so they can perform foraging behavior during much of their stalled time, etc. Meanwhile, in their home environment, I try to minimize stressors by allowing them a great deal of time at pasture with lots of foraging opportunities and the chance to socialize with other familiar equids. I work to keep most of their routines as consistent as possible.”
Importantly, Heleski adds, “Where possible, I try to reduce environmental stressors by providing the option of shelter during extreme weather conditions. By maintaining a veterinarian-approved health protocol, I work to reduce the stressors of disease and parasites.”