Internal pain experienced by a horse can manifest itself in many different ways

Written by ELT. Ann-Helena Hokkanen

Pain originating in the internal organs is called visceral pain. It is a pain of its own, which differs in many different ways from, for example, somatic pain arising in the skin, muscles and joints. Identifying visceral pain is facilitated by knowing how animals sense and experience it.


Internal pain is called visceral pain. This "state of the body" pain originating in the internal organs differs from superficial or deep somatic pain originating elsewhere in the body, especially in the skin and mucous membranes and in the musculoskeletal system, in many different ways. While somatic pain is usually sharp or burning in nature and fairly easy to localize, visceral pain is very vague and widespread and dull or cramping in nature.


Internal pain can also be accompanied by symptoms caused by increased activity of the horse's autonomic, i.e. involuntary, nervous system, such as increased heart rate, sweating, diarrhea and changes in the movements of the digestive tract. Factors related to the functioning of the autonomic nervous system and the processing of pain originating from internal organs in the central nervous system also usually increase the fear and anxiety caused by pain and make visceral pain more unpleasant than somatic pain. For the same reason, the stress and negative emotional states experienced by the horse probably greatly worsen the experience of visceral pain. In addition, pain originating from the internal organs can be reflected on the animal's skin.


In addition to visceral pain related to the internal organs and the membranes surrounding them, injuries and diseases of the internal organs can also cause somatic, sharp pain. There are a lot of pain receptors in the wall of the chest and abdominal cavity, the outer layer of the peritoneum and the outer part of the pleura, i.e. the parietal pleura, and these areas are innervated so that the pain sensed by these nociceptors is somatic in nature. This pain is often called parietal pain to distinguish it from somatic pain arising elsewhere in the body. Therefore, both somatic and visceral pain are often associated with diseases of the cardiovascular system, respiratory tract, digestive tract and genitals of horses. It is also possible that an illness or injury causes nerve damage pain if it involves damage to the nervous system.

Internal pain is difficult to localize

When an animal's tissue is exposed to a potentially dangerous stimulus, a chain of events leading to the sensation of pain and the experience of pain begins. First, the pain receptor that senses pain, or nociceptor, is activated and the stimulus is coded into electrical impulses of the nerve cell, the frequency of which indicates the strength and duration of the stimulus. Then this coded information about the properties of the stimulus and impending tissue damage is transmitted along neurons from the tissue through the spinal cord to the animal's brain. This pathway is called the pain pathway.


The visceral pain signal from the internal organs travels to the central nervous system via the visceral nerve fibers that bring sensory information to the central nervous system, which are part of the sympathetic nervous system. Nerves transmitting pain in the animal's thorax and abdomen pass either through the vagus nerve or spinal nerves. The nerves that carry pain sensations in the pelvic area, on the other hand, pass through the spinal nerves. The vagueness of visceral pain is partly explained by the fact that there are only a few pain receptors in the viscera that react to things that are potentially dangerous to the body, such as chemical or mechanical stimuli, that are specialized for sensing tissue damage. These visceral pain receptors are therefore few and far between, especially when their small number is compared to, for example, the huge surface area of ​​the intestine. Therefore, one neuron is responsible for sensing pain from a very large area, and it is thus difficult for the brain to localize where the pain is coming from.

You can't stretch and you become sensitive to pain

The internal organs have so-called high-threshold pain receptors, which are only activated when there is a strong, potentially harmful irritation to the body. In addition, there are neurons in the internal organs that convey information about the intensity of the stimulus, which have a low activation threshold, but on the other hand, the ability to amplify their message according to the intensity of the stimulus. In this way, the horse's brain receives information from the tissue not only about the damage, but also about its severity.


Visceral pain receptors are specialized to react especially to lack of oxygen, stretch and neurotransmitters of inflammation. That is, for things that are typically dangerous for the internal organs, especially the intestines. For example, the stretching of the intestine causes a strong pain sensation. In addition, the lack of oxygen and the inflammatory reaction resulting from tissue damage, as well as the related inflammatory mediators, quickly intensify visceral pain. This is because a significant part of the visceral nociceptors are unresponsive pain receptors under normal conditions, so-called sleeping nociceptors, which are only activated due to an inflammatory reaction or lack of oxygen.


The pain receptors of the internal organs are therefore specialized to react especially to substances related to stretching and lack of oxygen and inflammation. Therefore, the stretching of the intestinal wall, for example as a result of an intestinal hernia or an intestinal spiral, causes severe pain for the horse. This pain is first transmitted to the central nervous system via high-threshold mechanoreceptors. Then, the lack of oxygen in the tissues and the resulting inflammatory reaction caused by damage to the mucous membrane activate dormant pain receptors and additionally sensitize high-threshold pain receptors. After this, even stimuli that normally do not cause pain to the animal cause the pain receptors to be activated. This makes the pain experienced by the animal worse. This is called local sensitization.

In addition to the changes occurring at the site of the injury, pain sensation and inflammatory mediators also lead to sensitization to pain in the animal's central nervous system, i.e. the spinal cord, brain stem and brain. Then the function of the pain pathway changes so that it is even easier for the sensation of pain to pass from the damaged area to the cortical layer of the animal's brain, where the experience of pain always takes place. In addition, the body's mechanisms that normally restrain the flow of pain sensation in the pain pathway also begin to function worse than normal. All of this further intensifies the pain experienced by a horse suffering from visceral pain.

Even touch can hurt

Usually, pain related to a part of the digestive tract or an internal organ is felt as a vague pain in the abdominal or chest cavity and is aching and dull in nature. However, we don't always understand that pain symptoms on the horse's skin can also be a sign of internal pain, as this pain can be reflected on the animal's skin. The most familiar example in medicine is probably the pain associated with a heart attack, which in some patients is also reflected in the left arm in addition to the pain felt in the chest. In this case, the reflex pain is caused by the fact that the visceral pain-sensation-carrying nerves from the heart reach the spinal cord in the same area as the pain-carrying nerves from the skin of the left arm. Both afferent nerves form the first interneuronal junction of the pain pathway with the same neuron traveling in the spinal cord. In this way, this neuron, which carries the pain sensation further to the brain, can be activated both by the visceral pain sensation coming from the heart and by the somatic pain-causing stimulus directed at the skin of the left arm. Since the brain has no way of knowing whether the pain sensation originates from the heart or the arm, and furthermore, painful stimuli are directed at the skin much more often than at the heart, the brain connects the information that arrives via the nerve cell in the spinal cord to the stimulus from the skin of the arm. In this way, the pain information coming from the heart can also be interpreted as pain in the arm.

However, we don't always understand that pain symptoms on the horse's skin can also be a sign of internal pain, as this pain can be reflected on the animal's skin.

In humans, each structure of the abdominal and thoracic cavity has its typical reflection area on the skin, and in addition, certain muscles are also often sensitive. In addition, sensitization to pain caused by visceral pain also increases the experience of reflex pain. In human patients, irritation of the diaphragm can be felt as pain in the shoulders, uterine inflammation often causes lower back pain, and liver disease can cause pain in the right shoulder blade area in addition to the upper abdomen. The phenomenon is also familiar to those of us who work with horses; a mare in estrus can be sensitive

strongly the skin and muscles of the back when riding, and the intestine irritated by the sand causes the horse to avoid brushing its flanks.

Although very little is known about the reflection of visceral pain on the skin in horses and more research is needed, it is good to keep the phenomenon in mind. If, for no apparent reason, the horse starts to resist touching a part of its body, the possibility of pain should be kept in mind and investigated further.

For example, the pain caused by intestinal inflammation in horses can be accompanied by strong sensitivity of the skin of the hind legs.

Multiple triggers

For horses, visceral pain is caused by injuries and diseases of the organs of the thoracic and abdominal cavity and the pelvic area. Common painful diseases of the chest cavity, especially in foals, are respiratory tract infections. Internal pain in the pelvic region is caused by, for example, the opening phase of foaling, difficult foaling, uterine infections and urinary tract diseases.

The most common problems that cause visceral pain in the abdominal cavity are related to the horse's long and complex digestive tract. Ahky is a general term for pain in the horse's abdominal cavity. Its symptoms can vary greatly depending on the situation and the cause of the pain. And since visceral pain is rather vague and difficult to localize, it is difficult to determine from the symptoms what the problem is. Typical causes of colic include various malpositions, twists, blockages and infections of the intestines.

That's why sudden pain in the horse's abdominal cavity is always an emergency that should be taken seriously, the cause of which can be any number of options, from harmless, transient flatulence to a rapidly life-threatening intestinal twist requiring immediate surgical treatment.

Recognize the symptoms

Often the first sign of internal pain experienced by a horse is somehow abnormal behavior. Restlessness or apathy, decreased appetite or inability to eat, a pained face and an intense, introverted gaze are typical symptoms of visceral pain. Staggering, digging, kicking under the stomach, looking at the sides, repeatedly lying down, lifting the upper lip and grimacing are also alarming signs.

A sick horse can lie down more than normal and also in abnormal positions. The amount of stool may have decreased. Repeated attempts to urinate or defecate are also signs of problems. A horse experiencing internal pain often also grinds its teeth, yawns, belches and can make low neighing sounds.

A horse in pain, on the other hand, can stand at the back of the pen, look at the wall and not react to its surroundings or to the person entering the pen.

A restless horse shakes its head, digs, looks at its sides, kicks under its belly, swishes its tail, shifts its weight from one leg to another, walks on the ground and gets up again, moves restlessly around the stall, reacts strongly to sounds and can be aggressive. An apathetic horse, on the other hand, stands or lies still, hangs its head and does not pay attention to its surroundings.

In addition to the expression, the position of the horse's body and head can indicate pain. A horse normally carries its head high, while a painful horse's head is at or below the level of the withers. The reaction to the environment and the position in the pen also change. Normally, horses stand at the front of their stall and follow their surroundings attentively. They also usually react immediately to an approaching person. A horse in pain, on the other hand, can stand at the back of the pen, look at the wall and not react to its surroundings or to the person entering the pen.