Equine Pain Pathways

By understanding how pain works and addressing its various pathways, we can improve healing and better ensure equine welfare.
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Equine Pain Pathways
While our horse's pain is never pleasant, it's typically an important warning signal to help protect the body and promote healing, our sources say. | Photo: Thinkstock

Understanding different types of pain in horses and when and how to control them

Nagging, aching, throbbing. Dull, sharp, piercing, burning. Extreme, excruciating, unbearable—so many faces of pain, words to describe it, and ways to experience it. Pain comes in many forms and manifests in a variety of ways. While our horses’ pain is never pleasant, it’s typically an important warning signal to help protect the body and promote healing, our sources say. Sometimes, however, that biological mechanism goes too far. And for equine health and welfare, we must intervene to stop, or at least reduce, that pain.

The Science of Pain and Nociception

The body is full of sensors called nociceptors that recognize when the body encounters something dangerous, including extreme temperatures, unsafe objects, or chemical compounds, says Alonso Guedes, DVM, MS, PhD, Dipl. ACVA, associate professor of anesthesia and pain medicine at the University of Minnesota’s College of Veterinary Medicine, in Saint Paul.

Nociceptors nestle in the tips of long strands of neurons called nerve fibers, which connect to the spinal cord and brain, Guedes says. The fibers join up with the spinal cord. Some fibers play a role in detecting sudden, immediate dangers, such as pain from stepping on a nail, while others are associated with longer-lasting nonemergency threats, such as joint inflammation from overuse.

When the nociceptors detect these dangers, they send signals along the neural fibers up to the spinal cord. While those signals might include messages about a serious burn, a major fracture, or a gushing gash, we can’t yet describe it as pain, says Guedes. Up until the dorsal ganglion, it’s only nociception, he says.

When the messages reach the brain, cerebral structures identify and validate the signals, locate the affected part of the body, and appreciate the intensity. The brain then rapidly reacts, prompting the animal to jerk a foot away from a sharp object, for example. Once the pain signals reach the brain’s somatosensory cortex and the horse is aware of the sensation he’s experienced, the feeling becomes pain, says Guedes.

Equine Pain Pathways
Photo: The Horse Staff

Nature’s Warning Signal

As unpleasant as it is, pain has a critical purpose, says Guedes. “Pain is really important for injury healing and survival,” he says. “It’s a good adaptive mechanism for protecting the organism from harm in the environment.”

Thijs van Loon, DVM, PhD, Dipl. ECVAA, assistant professor at Utrecht University, in the Netherlands, agrees: “Pain has important functions, whether it’s getting the animal away from something harmful or taking rest to recover or protecting it from further harm .”

Pain also teaches animals to avoid danger. “Pain creates a memory, and horses will remember to stay away from things that have hurt them before,” says Guedes.

In fact, pain blends into horses’ natural fight-or-flight response, he explains. “With pain, there’s a stress response activating the different central nervous system areas to produce hormones, affecting the sympathetic nervous system and increasing heart rate, respiratory rate, and blood flow to the muscles that need to activate, and general awareness,” he says. “Basically, pain goes back to survival and, in essence, it can be a good thing.” 

The Categories of Pain

You probably already know basic definitions of acute and chronic pain, related to how long they last. Acute pain comes on suddenly and is generally short-lived, while chronic pain lasts longer, explains van Loon. The actual definitions, however, are a bit more complex.

“The borderline between acute and chronic pain depends not only on the time that the animal is suffering from it but also on the changes that may occur in the nervous system due to this ongoing pain,” he says. Once these changes in the nervous system have taken place, the pain becomes much more difficult to alleviate and can become a disease in itself.

Chronic pain could also be pain that recurs intermittently or at low levels over weeks, months, or years. Acute pain can flare up over existing chronic pain, compounding it. This scenario is common in orthopedic cases, such as an arthritic joint that becomes acutely painful with certain movements or even as arthritic changes in the joint evolve throughout the disease process. Horses with chronic laminitis can experience acute flare-ups if their diet or activity changes, he says.

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You can also differentiate between chronic and acute pain by their signs, van Loon says. “With acute colic, for example, you might have a horse that’s lying on its back really suffering severe signs or a horse that is showing less dramatic signs if the disease is mild,” he says. “A horse with chronic colic might be losing weight over time or having coat problems, sometimes even without obvious colic signs.”

Pain can also be categorized as either somatic or visceral, van Loon continues. Somatic pain arises from the musculoskeletal system and the limbs. Humans often describe it as being “sharp” and can usually pinpoint its exact location. Visceral pain, on the other hand, originates in the internal organs and is usually duller and more vague in location.

“Obviously, we can’t ask horses what their pain feels like, but there are so many similarities in anatomy and the nervous system between horses and humans that they’re probably working in the same way, although we can’t know for sure,” van Loon says.

Another kind of pain—neuropathic—stems from overstimulation of the central nervous system, resulting in changes to the nervous system itself. It can develop with unrelenting chronic pain, causing horses to become extra sensitive to stimuli, he says. Things that usually don’t hurt—the wind, for example—become painful.

“The pain receptors never come back to a normal state, and the pain becomes a syndrome itself,” says van Loon.

Headshaking syndrome is a manifestation of neuropathic pain, he explains. “Normal stimuli are extremely painful in these horses,” he says.

When Helpful Becomes Harmful

Neuropathic pain isn’t the only situation in which “good” pain becomes bad. “When pain no longer serves an active process of survival or healing, it’s considered maladaptive,” Guedes says.

Pain—or even nociception—that’s excruciating can push the horse into a state of negative metabolism known as a catabolic state, he explains. “This actually reduces the body’s ability to heal, and that in itself is going to contribute to the injury and interfere with healing.”

Essentially, it comes down to too much pain for too long. “When pain becomes too prolonged or too severe, all the body’s responses to that pain start draining the organism, diminishing its reserves to repair itself or function normally,” Guedes explains. “So when that response continues for a long period of time, it starts working against the organism.”

On a behavioral level, horses in extreme levels of pain can make things worse by further hurting themselves, he adds. “If there’s significant discomfort, they could self-traumatize,” he says. Horses experiencing severe colic pain, for example, could thrash violently against floors, walls, and objects.

Treat it or Leave it?

So what do you do when a horse is in pain? Let the pain do its “good” work, or stop it from wreaking havoc on the healing process? Our sources say it’s a balancing act.

“We’re not looking at pain as something that we need to eliminate at all times,” Guedes explains. “We also have to appreciate the times and situations where pain is not helpful, and we absolutely need to control it. The idea here is that we want to maintain the pain sensitivity at a minimum level that is protective and there is no outward suffering of the animal, as well as no detrimental biological effect from that.”

For example, a horse suffering from laminitis still needs to be able to feel some discomfort to keep some of his weight off the laminitic foot and prevent potentially irreversible damage during treatment, Guedes says.

Even during surgery it’s important to control pain or, more specifically, nociception, says van Loon. Horses under general anesthesia might not feel the pain, but their nociceptors are still transmitting signals from the surgery site to the spinal cord. That’s why local anesthetics or other analgesic (pain-relieving) drugs to stop or modulate nociceptive signaling are critical, he says.

“The nociceptive effects during surgery can be so widespread and serious, and there’s so much trauma and so many inflammatory markers, that any positive effects are completely lost,” says van Loon.

Study results also show that effective pain management correlates with better healing. Over the past few decades researchers have noted that when hospitalized horses’ pain wasn’t managed well, the animals took longer to get better, and their wounds healed more slowly, he says.

“So there’s not only the ethical point of pain control but also the practical point of providing the best conditions for ­healing,” he adds.

This balance of pain means never letting horses suffer, Guedes says. “A lot of people misunderstand what it means to let the horses feel a little bit of pain,” he says. “But we have an ethical duty to ensure that horses don’t suffer or that suffering is minimized as much as possible.”

Owners must understand these distinctions and “mitigate unnecessary or maladaptive pain while maintaining the pain that is protective or adaptive,” Guedes says. “And that is easier said than done. Oftentimes the line between adaptive and maladaptive is not very clear. It requires a lot of study, observation, and good judgment to be able to do the right thing.”

The Role Drugs Play

Did you know the body creates its own analgesics? Endogenous (self-made) peptides similar to opioids help control pain naturally, says Guedes. Their molecules have a shape that fits into receptors in the nervous system, interfering with the signaling process. In cases of extreme sudden pain, the body can trigger high levels of pain control for a brief period—what’s known as stress-induced analgesia, he says. But for other kinds of pain, the opposite can occur. “The problem is the body doesn’t produce enough to control the pain completely in many, if not most, situations,” he says.

That’s why scientists developed drugs that fit into those natural opioid receptors. Opioids such as morphine and butorphanol, as well as drugs such as xylazine or detomidine, can provide relief by keeping pain signals from reaching the brain. Local anesthetics such as lidocaine or mepivacaine block signals from the nociceptors completely, says Guedes.

Anti-inflammatory drugs, including phenylbutazone (Bute) or flunixin meglumine (Banamine), work to reduce the inflammation in a painful area, he says. Tissue swelling resulting from injury can be itself a source of pain, causing the nociceptors to transmit signals. Inflammatory chemicals produced at the injury site can activate nociceptors even in response to stimuli that are not normally painful. Reducing inflammation with these drugs diminishes nociceptor activity.

Scientists are currently working on new kinds of anti-inflammatories that could be more potent, particularly for laminitic pain, says van Loon. Other pain-relief research is focused on existing drugs, especially opioids, because “there’s not enough evidence of what their properties would be in certain situations,” he says.

Take-Home Message

Pain is a sophisticated mechanism designed to protect and facilitate healing. By understanding how pain works and addressing its various pathways to manage it, our sources say we can improve healing and better ensure our horses’ welfare.

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Written by:

Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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