Pain is something that all horses deal with at some point in their lives. Horses can experience pain for many different reasons, including injury, illness, or a result of surgery.
For example, castration is the most common surgical procedure performed on horses and is associated with significant post-operative pain.
Acute colic is another common painful experience for horses.  Pain is also commonly associated with degenerative joint disease, laminitis, gastric ulcers, and hoof issues.
Unlike humans, horses don’t always show it when they are experiencing pain, or they may only display subtle signs of discomfort. This is because, as prey animals, they have evolved to hide signs of pain and weakness in the presence of predators. 
As horse owners, we want to know how to alleviate pain for our horses. Pain management strategies will be different for every horse, depending on what’s causing the pain, the severity and duration of the pain, as well as the horse’s intended work.
Understanding Pain in Horses
While we may think of pain as bad, pain is an important feedback mechanism to help horses survive in their environment.
A basic bodily sensation brought about by noxious (harmful) stimulus, pain is associated with actual or potential tissue damage or nerve injury. Pain is a signal received by nerve endings and can be viewed as an internal warning system. 
Pain needs to be proactively managed. Anxiety and stress can enhance pain perception in horses and vice versa. 
Adaptive vs. Maladaptive Pain
When pain serves a purpose – such as to protect an injured area or to promote healing and recovery – it is referred to as adaptive pain. 
Horses can also experience pain due to necessary veterinary interventions such as surgery. Navicular syndrome, laminitis, and osteoarthritis can cause chronic (long-term) pain.
Both surgery and chronic, painful conditions lead to maladaptive pain. This form of pain is dysfunctional: it neither protects nor supports the healing and repair of bodily tissues. 
The goal of pain management is to eliminate maladaptive pain for our horses while keeping the adaptive component. It is important for veterinarians and horse owners to recognize the differences between adaptive and maladaptive aspects of pain. 
Traditionally, pain in horses is classified according to the following factors:
- Duration: acute (short-term) or chronic (long-term)
- Anatomic location: superficial, deep, visceral (related to the internal organs), or musculoskeletal
- Quality: dull, sharp, burning, stabbing, throbbing, persistent or recurrent
- Intensity: mild, moderate, severe, excruciating, crippling 
The above classifications are descriptive, but they do not always provide enough information to inform a treatment approach. A new classification has been added in recent years to identify three main types of pain:
1. Nociceptive Pain
This type of pain is caused by damage to bodily tissue, often from an external injury. Nociceptive pain includes a behavioural response characterized by a motor withdrawal reflex or more complex behaviours.
2. Inflammatory Pain
Inflammatory pain is produced by the activation of nerve-ending cells called nociceptors. Nerve endings then secrete inflammatory molecules in a process called neurogenic inflammation.
This type of pain is an adaptive response that encourages a decrease in mobility until the tissue heals. 
Inflammatory pain sometimes leads to changes that result in spontaneous and excessive pain. In this case, it becomes maladaptive and needs to be managed. 
3. Neuropathic Pain
This type of pain results from dysfunction or damage to the peripheral nerves and spinal cord / central nervous system. It is often secondary to trauma or chronic inflammatory diseases and has no apparent protective or reparative function.
Neuropathic pain is maladaptive and needs to be controlled. Equine conditions such as head shaking, chronic laminitis, and nerve damage during surgery can be associated with neuropathic changes and pain. 
Acute vs. Chronic Pain in Horses
Acute pain is short-term pain that generally follows an injury. Heat, swelling, redness, and loss of function may occur as the body tries to remove injured or damaged cells, eliminate foreign material, minimize further damage, and allow tissue regeneration.
The goal of managing an acute injury is to limit the inflammatory response and reduce immediate discomfort. 
Chronic pain, however, lasts for a longer period of time. It is considered maladaptive when it persists beyond the expected period of tissue healing.
Equine Pain Measurement Scales
In the past, veterinarians evaluated pain in horses based on objective measurements such as heart and respiratory rate. They also sometimes use endocrine measures such as circulating levels of cortisol.
Researchers have recently developed several pain measurement scales that give a clearer indication of pain in horses.
Effective pain measurement scales are easy to use, apply to most horses and situations, and provide consistent results. Pain scales must also be sensitive enough to differentiate between mild, moderate, and severe pain. 
Horse Grimace Scale
The Horse Grimace Scale is an evaluation tool that determines a horse’s pain by examining facial expressions.
This scale includes detailed descriptions and photographs to illustrate various expressions associated with different levels of discomfort in horses. 
Facial expressions most frequently associated with pain include: 
- Low and/or asymmetrical ear positioning
- Angled appearance of the eyes
- A withdrawn or tense stare
- Dilated nostrils
- Tension in jaw, chin, lips or other facial muscles
Obel Laminitis Pain Scale
The Obel laminitis pain scale is widely used in horses with laminitis and classifies the severity of lameness by grade from I to IV.
This scale also includes behaviours such as weight shifting, resistance to having limbs lifted, and disability. 
A modified Obel scale has been developed for use in the research setting for horses with endocrinopathic laminitis. 
AAEP Lameness Grading Scale
The American Association of Equine Veterinarians (AAEP) lameness grading scale is also commonly used by veterinarians because it is simple and more repeatable than many other lameness scales. 
The scale ranges from 0 – 5, with 0 indicating no lameness and 5 indicating extreme lameness. 
- Grade 0: Lameness is not perceptible under any circumstances; manipulating the limbs does not produce gait abnormalities.
- Grade 1: Lameness is difficult to observe and is not consistently apparent regardless of circumstances.
- Grade 2: Lameness is difficult to observe at a walk or trot in a straight line but consistently apparent in certain circumstances.
- Grade 3: Lameness is observable at a trot under all circumstances.
- Grade 4: Lameness is obvious at a walk.
- Grade 5: Lameness produces a complete inability to bear weight or a complete inability to move.