Many people report their horses are in pain, and often think their discomfort is far greater than is evident on veterinary examination. Before we look at pain from the equine perspective, we should explore what pain really is, and what it is not. From a neurologic standpoint, all stimuli are received and interpreted by the brain. For instance, we feel a warm breeze as the small hairs on our skin move; the brain combines the warmth, hair signals, and the location on the skin and report the sensation as being caused by a breeze and not something else, such as a small bug.

THE PERCEPTION OF PAIN

All pain is perceived in the brain. What is experienced as painful is usually a sensation that is simply too much beyond normal. For example, too much touch, stretch, heat, cold, or any number of other things. Humans identify pain as stimuli causing a level of discomfort we are not willing to tolerate. The horse’s perception of pain can be similar, but humans may interpret a horse’s behavior as a response to pain, when it is not painful. From a cortical perspective, pain is not only felt but also enhanced by the chemical situation of the brain at the time. For instance, some stimuli may only be painful when the horse is under stress.

As with humans, the horse’s brain generates the sensation of pain felt in the body. However, unlike us, the depth of that feeling changes based on the horse’s hierarchy of needs. We will look at a few of these, along with some of the areas of the brain that decode painful stimulation from the body.

HORSES DO FEEL PAIN

When horses are wounded, they feel pain. The first thing felt when skin is cut is an intense pain at the moment of injury. The signal for this pain is conducted rapidly by the nervous system. The pain is followed by a slower, prolonged, dull ache, which is conducted by the slower nerve fibers. These stimuli help recruit a normal immune response to the wound. Using chemical anesthetics, scientists can block one type of neuron and separate the two types of pain.

Several factors contribute to the recognition of pain:

  • Mechanical stimulation from the sharp object
  • Potassium released from the insides of damaged cells
  • Prostaglandins, histamines and bradykinin from immune cells that invade the area during inflammation
  • Increases in temperatures that cause action potentials at sensory neurons
  • Substance P from nearby nerve fibers.

In human brains, frontal lobe decision skills can influence pain recognition. The pain from a cut on your hand eventually subsides or moves to a lower intensity. If you consciously distract yourself, it bothers you less. People given placebos for pain control often report that the pain ceases or diminishes.

Horses have smaller frontal lobes in their brains, although they are excellent at creating distractions from pain. We often notice these behaviors and label them as vices. Examples are horses that weave in their stalls, chew their limbs, or crib. Horses are good at distracting themselves from pain because, as prey animals, other functions take precedence over pain perception. Survival takes priority over pain if the horse is running from a predator that is likely to eat him.

MOTIVATION CAN ELICIT BEHAVIORS ASSOCIATED WITH PAIN

When humans are anxious, their pain levels increase because they are already primed by the hippocampus, which helps with emotions and decision-making skills. With horses, their surroundings, peers, or both can increase anxiety. The food horses eat and the water they
drink can contribute to their level of inflammation, which heightens their level of anxiety. But they don’t generally worry, the way we do, which means their threshold to pain is not as close to action potential on a day-to-day basis, as would be a human in the same state. Their brains don’t tend to make their pain worse, the way ours do.

Motivation is thought to be controlled in an important way by basic regulatory processes essential for survival — feeding, respiration, sex, self-protection, and temperature regulation. It varies as a function of deprivation — for example, deprivation of food (hunger), water (thirst), or sex. There is a great natural urge to repeat these behaviors.

Motivational states have general effects: they increase the horse’s level of attention, make him more sensitive and enhance his ability to act. Motivational states, like thought  processes, are internal ideas thought to explain the intensity and direction of a variety of behaviors, such as temperature regulation, feeding, thirst and sex. This is why we are able to use heart rate, respiratory rate and temperature as a measure of pain versus a measure of motivation. A horse’s respiratory rate might increase with pain, but it can also increase with excitement; it only indicates pain when compared to the other two parameters, heart rate and temperature.

FROM THE BRAIN DOWN

Anything the horse does that makes him feel good actually reduces pain. Opioid receptors reduce pain and make horses feel friendly. When these are stimulated, either because of an activity or by drugs, the horse responds by behaving more normally.

Pain relief can be achieved when opioid receptors are stimulated. They in turn stimulate other parts of the brain that turn on the thalamus. Eventually, the pain is relieved. This comes in the way of neurotransmitters, such as endorphins.

Normal movement is a means by which horses can dampen or reduce painful sensations because it turns on opioid receptors. When horses are placed on stall rest, it becomes difficult for them to inhibit pain signals to their brains and can even cause them to become more painful. This is also why horses fidget. Movement can relieve pain coming from sources not easily identified by a horse’s owner. By moving, the horse inhibits the pain fibers from firing and makes himself more comfortable.

OTHER FACTORS IN PAIN PERCEPTION

Damage to the brain or spinal cord can cause the sensation of pain from non-painful stimulation. The extent of the damage may limit the reaction of the brain’s pathways. The influences of the descending pathways might also be responsible for pain perception with no obvious physical cause. Thoughts, emotions and neurocircuitry can affect both ascending and descending pain pathways. Scientists are uncertain to what extent horses can experience neuropathic pain.

Physiological and psychological factors can influence pain perception:

Age — Brain circuitry accumulates waste products over time and with age, so older horses have lower pain thresholds and more problems dealing with pain, in part due to inflammation affecting the hypothalamus.

Gender — Because females tend to be more prone to hypothyroidism than males, they are more prone to pain sensitivity. In males, the protective effects of testosterone on the metabolic system help reduce their response to environmental toxins.

Fatigue — Lack of appropriate serotonin levels cause the sensation of more pain when the body is stressed from lack of sleep.

Memory — How a horse has experienced pain in the past can influence his neural responses (memory is influenced by the amygdala and learning processes come from the limbic system).

As you can see, a lot goes into your horse’s perception of pain. Horses already know that movement helps them address their pain. When your horse is in pain, look for an AVCA certified animal chiropractor to help with better movement. While your veterinarian will be able to help stitch the wounds, your animal chiropractor will align the nervous system that is responding to the painful stimulation. You horse will be able to heal once its nervous system can do its job.

REFERENCES
A Primate Memoirs, R. Sapolsky
Zebras Don’t Get Ulcers, R. Sapolsky
Essentials of Neural Science and Behavior, Kandel, Schwartz & Jessell
Neurobehavioral Disorders of Childhood, Mellillo


Dr. Hayek is a veterinarian who has been using Animal Chiropractic in her practice for over 13 years. She uses Functional Neurology to find windows into the brain to change the way horses respond to their pain. She practices in five states and lives in Meridian, Texas where she and her husband teach Animal Chiropractic to veterinarians and chiropractors.