Assessing and Addressing Pain in the Equine Patient

Advancements in pain management techniques have improved exponentially across all species; however, the dissemination of updates in multimodal analgesic use for equine patients is still incomplete. Despite the archaic perception that livestock are less receptive to analgesic medicine, analgesia is ethical no matter the species. While the NSAIDs phenylbutazone and flunixin meglumine are effective and widely used in equine patients, other drugs, when used adjunctly, can provide more robust pain management.

The veterinary nurse remains a frontline advocate for equine patients by assessing pain and participating in the implementation of pharmacologic and nursing management. This article focuses on the acutely painful horse; readers are encouraged to explore updates in anesthesia and analgesia specific to patients undergoing a surgical procedure.

Importance of Pain Management in Horses

When a horse is injured, inflammatory mediators are released from the damaged cells and transmit signals from the site of injury to the brain. In response, the brain initiates a cascade of physiologic responses. The sympathetic response triggers vasoconstriction, an increase in myocardial work and oxygen consumption, and a catabolic state. Entering a catabolic state causes hypomotility of the gut, sodium and potassium imbalances, and muscle wasting.¹ Hyperglycemia and insulin resistance may also result. These responses increase the risk of infection, slow healing, and increase morbidity. Psychologically, the fight-or-flight response to injury can cause hyperalgesia (increased perception of pain) and stress, which can cause delayed healing and negative behaviors. An effective analgesic plan can reduce the negative physiologic impact of pain and stress for patients, the length of time a patient is hospitalized, and client expenses and stress.

Pain Assessment

Use of validated pain assessment scales, critical observation of patient behavior and attitude, and a thorough physical examination all provide essential information about patient comfort. Veterinary nurses should communicate their findings clearly and accurately to the attending veterinarian.

Equine Pain Assessment Tools

Use of emotional or casual language in describing signs of pain can be confusing. While multiple subjective and objective pain grading scales have been developed and validated to enable the veterinary team to consistently identify and describe subtle changes in equine facial expressions and body posture due to common causes of pain, there is no single comprehensive grading scale at this time.² Therefore, the veterinary team should choose a scale—such as the American Association of Equine Practitioners Lameness Scale (TABLE 1) or the Obel Laminitis Pain Scale (TABLE 2)—that meets their practice’s needs or create a scale that incorporates aspects from other scales. Whichever scale is implemented, it must keep patient assessment unbiased and be used consistently, and staff must be trained on its use to reduce confusion.

The Horse Grimace Scale (HGS) is a powerful tool for the veterinary nurse when used alongside commonsense observations of equine behavior, physiologic changes, or other practical pain scales (RESOURCE 1). It is based on 6 facial expressions: ear position, orbital tightening, tension above the eyes, tension of the chewing muscles, strain on the mouth, and nostril flare (FIGURE 1).^4,5 These expressions are graded by their intensity (presence and prominence) on a scale of 0 to 2.² A score of 0 indicates that the qualifying feature is not present, a score of 1 that it is moderately present, and a score of 2 that the feature is obviously present. A proposal published in 2024 assigned more detailed anatomic structures to increase accuracy and decrease interpretation variability.⁵ The HGS is validated for evaluation of pain across multiple origins, making it a versatile option for communicating both within the team and with clients.

Figure 1. Patient grimace. Note the obvious stiffly backwards ears, orbital tightening, tension above the orbit, strained nostrils, and flattening of the profile.

Critical Observation

The instinct of the horse as a prey species is to hide pain, even as the survival pressures of the domesticated horse have lessened. This trait is particularly notable in draft breeds, older horses, donkeys, and mules (FIGURE 2). Young horses and “hot blood” breeds, such as thoroughbreds and Arabians, tend to be more sensitive to pain and more readily display signs of it (FIGURE 3).

Figure 2. A Belgian mare displaying subtle behaviors from a painful abdomen caused by idiopathic peritonitis. These subtle behaviors include a dull demeanor, ears positioned at 3- and 9-o’clock, squinted eyes, and some grimacing noted around the nostrils and mouth.

Figure 3. Neonatal foal displays obvious colic signs from a meconium impaction, which was successfully treated with a soapy enema.

Stress can induce either analgesia, as when a horse experiencing a fear response masks pain,¹ or hyperalgesia when the patient is anxious, overstimulated, or exposed to repetitive noxious stimuli (e.g., needle-shy horse).¹ Therefore, when critically observing an equine patient, the veterinary nurse should consider that a quieter patient is not necessarily comfortable. Conversely, pain management and careful handling should not be withheld from a reactive patient based on the perception that the patient is being unreasonable.

Physical Examination

In addition to use of a pain grading scale and critical observation, the veterinary nurse should assess pain with a thorough and systematic physical examination. A healthy horse is interested in its surroundings and has a good appetite to graze on small amounts of feed over a long period of time. In addition to normal vital signs (TABLE 3), it bears equal weight on all 4 limbs and is willing to walk and, if applicable, perform its discipline to the owner’s standards. A healthy foal is very curious and reactive to its surroundings and nurses every 20 to 30 minutes from its dam. In a painful horse, vital signs are often abnormally elevated, with tachycardia and tachypnea present, similar to a patient with a disease process.

Identifying Sources of Pain

The most common pain syndromes equine patients experience include surgical pain; lameness resulting from chronic, degenerative, or acute injury; laminitis; gastrointestinal distress (e.g., colic, ulcers); and pleuropneumonia.³ These syndromes fall into broad categories: somatic, visceral, and neuropathic pain. Somatic pain originates from skin, muscles, tendons, ligaments, joints, and bones, while visceral pain is caused by conditions associated with infection and inflammation of the major organs within the abdomen and thorax. Neuropathic pain originates within the peripheral or central nervous system.

Idiopathic pain is persistent pain without a specific source. It is often extreme and exacerbated by excitement, fear, and stress.

Somatic Pain

Horses experiencing somatic pain exhibit abnormal weight distribution or shifting, posture in a “camped under” or “parked out” stance, and are reluctant to move. In some instances, they become recumbent and are reluctant or unwilling to stand. They guard the affected limb (e.g., pointing, rotating, hanging) and are sensitive to palpation, flexion, or extension of the painful area (FIGURE 4). They sweat and exhibit anxious expressions around their eyes, ears, and lips.

Figure 4. Gelding with osteomyelitis of the right hind third phalanx presented with 5/5 grade lameness, demonstrating guarding of the limb by refusing to bear weight while standing or walking.

Visceral Pain

Visceral pain causes significant changes in physiologic findings. Affected horses sweat profusely, have muscle fasciculations or muscle tremors, and groan or grunt (FIGURE 5). Visceral pain from the abdomen causes horses to look at their flank; kick at their abdomen; paw frequently; lie down and stand up repeatedly; and sometimes roll, thrash, or violently fall to the ground. Visceral pain originating in the thorax might cause the patient to take a rigid or sawhorse stance or be reluctant to move.

Figure 5. American Mammoth Jackstock donkey 12 hours after resection anastomosis for strangulated small bowel exhibiting signs of severe visceral pain and systemic inflammatory response syndrome with profuse sweating, muscle fasciculations, splinted abdomen, and anxious appearance around eyes.

Neuropathic Pain

Neuropathic pain is difficult to address in equine patients, often due to their size. Damage or dysfunction of the nervous system can be caused by trauma, infection, ischemia, or cancer, but chronic laminitis is thought to be the most common cause of this pain.

Analgesic Approach

While the use of NSAIDs remains standard practice in pain management for equine patients, the use of adjunct analgesic drugs warrants exploration to provide thorough pain relief and decrease adverse effects of higher doses of NSAIDs. A multimodal analgesic plan targets different pain receptors to improve pain relief while minimizing adverse effects.

NSAIDs

NSAIDs (TABLE 4) work by inhibiting cyclooxygenase (COX) enzymes, thereby preventing the conversion of arachidonic acid into prostaglandins, which participate in the inflammatory cascade. There are 2 main types of COX enzymes: COX-1 and COX-2. COX-2 enzymes are specifically responsible for the prostaglandins associated with inflammation, pain, and fever. COX-1 enzymes primarily produce prostaglandins essential for maintaining healthy gastrointestinal mucosa. Maintaining the integrity of the gastrointestinal mucosa to prevent gastric ulcers is an important consideration in equine patients.

The most popular equine NSAIDs, phenylbutazone and flunixin meglumine, continue to be effective in addressing pain. They inhibit both COX-1 and COX-2 enzymes and are therefore referred to as nonselective COX inhibitors. Flunixin meglumine is best used for visceral pain and has antiendotoxic effects, while phenylbutazone remains the most reliable NSAID for orthopedic pain.³

Newer generations of NSAIDs target COX-2 specifically. Such COX-2–selective NSAIDs have the benefit of reducing the development of gastrointestinal ulcers. Meloxicam and firocoxib are more COX-2 selective, making them potentially safer for the gastrointestinal tract.⁷ Studies indicate that their analgesic effects are at least equal to those of phenylbutazone or flunixin meglumine.^8,9 However, regular use, overdose, or “stacking” of NSAIDs can cause an array of negative adverse effects, such as gastric ulcers, right dorsal colitis, nephrotoxicity, and liver disease.

Constant-Rate Infusions

A constant-rate infusion (CRI) is intravenous administration of a drug at a specified rate over a certain amount of time. In a hospital setting, a fluid pump intended to infuse small amounts over a long period of time is often used; some patients may require multiple infusions at the same time (FIGURE 6). While ambulatory settings present logistical hurdles in administering a CRI, several sources recommend the use of a disposable flow meter that can be set to control the fluid administration rate (FIGURE 7).¹⁰

Figure 6. Multiple constant-rate infusions to provide hydration, nutrition, blood pressure support, pain relief, and plasma for immunoglobulin G therapy in a neonatal foal 3 hours after abdominal exploratory surgery.

Figure 7. Example of a disposable flow meter (Dial-A-Flo; Hospira, icumed.com) setup to administer a ketamine constant-rate infusion during a general anesthesia case. Courtesy Lauren Kasnet, LVT, Rood & Riddle Equine Hospital

Opioids

Intramuscular injections of an opioid, such as butorphanol or morphine, are convenient but insufficient by themselves for relief of severe pain. Increasing the dose of morphine and butorphanol significantly affects the motility of the gastrointestinal tract.³ A CRI of an opioid ensures stable plasma drug concentrations, thus preventing a “yo-yo” effect between comfort and pain.^11,12 The effects of hypomotility may also be decreased, but not eliminated. For a butorphanol CRI, a loading dose of 18 µg/kg is delivered intravenously, followed by a CRI of 23 to 25 µg/kg/h.¹³ Morphine given as a CRI is started with an intravenous loading dose of 0.16 mg/kg, following by 0.1 mg/kg/h intravenously.¹⁰ If the patient becomes excitable, administration of a low-dose α₂ adrenergic agonist might be appropriate.

Lidocaine

Lidocaine is a sodium channel blocker that is used in CRIs in many practices to treat visceral and neuropathic pain. Because the pharmacokinetics are not completely understood, this use of lidocaine is controversial for some practitioners and warrants more research. Multiple studies have found it inhibits inflammatory mediators in the gastrointestinal tract as well as the GABA (γ-aminobutyric acid) uptake that is responsible for neuropathic pain.^10,14 This has made lidocaine a popular therapeutic drug for colic patients at risk for postoperative ileus. It also targets the pain pathway of acute and chronic laminitis.¹⁵ Other studies report that patients have a desired decrease in heart rate and improvement of lameness within hours of administration.

An initial loading dose of 1.3 mg/kg 2% lidocaine is administered intravenously over 15 minutes, with a subsequent rate of 0.04 to 0.05 mg/kg/min.¹³ It is important to note that lidocaine is protein bound in equine plasma, and patients with hypoproteinemia might reach a toxic threshold at a normal dose. Lidocaine toxicity appears with muscle fasciculations, agitation, ataxia, and collapse.

Ketamine

Ketamine is an NMDA (N-methyl-D-aspartate) receptor antagonist that targets receptors responsible for hyperalgesia, which can cause “wind-up” pain.¹⁶ Its analgesic effects when given at a low dose as a CRI are notable, although some research indicates better analgesia when combined with another drug.^10,12 When administered alone, a ketamine CRI of 0.4 to 0.8 mg/kg/h improves appetite and comfort in patients with synovitis, osteomyelitis, burn wounds, and colic.^12,13,16 Some clinicians might order an increase up to 1.5 mg/kg/h, if needed. Gastrointestinal motility can be negatively impacted by ketamine administration, warranting the consideration of a combination of 2 or more analgesic drugs.

Combination Therapy

A synergistic combination of drugs uses a lower dose of each, which reduces adverse effects while still offering the benefit of analgesia. Ketamine can be paired with an α₂ adrenergic agonist, lidocaine, or morphine to create a potent analgesic plan.¹⁰

Abrahamsen is credited with the development of pentafusion, an extremely potent combination of lidocaine, ketamine, morphine, detomidine, and acepromazine.^12,13 It is recommended to set up the CRI with two 1-L bags of normal saline and 2 delivery systems so the veterinary team can adjust the rate based on patient response (TABLE 5). The dosages are based on an average 450-kg (992-lb) adult horse, and it is recommended that the delivery rate be calculated to the patient’s weight using a ratio. An intravenous loading dose of lidocaine (1.3 mg/kg) and morphine (0.1 mg/kg) is administered slowly before the pentafusion CRI is started.¹² When the patient is more comfortable, the fluid rate for the bag containing acepromazine, morphine, and detomidine can be slowed by half. Abrahamsen found that this method reduced the incidence of hypomotility of the gastrointestinal system. While pentafusion is supported by empirical evidence, it has not been validated through research at the time of publishing.

Local and Regional Anesthesia

The use of local and regional anesthesia aids in interrupting the pain impulses along peripheral nerves.

Perineural anesthesia targets specific nerves to block sensation to a particular area. These “blocks” include the palmar/plantar digital, abaxial sesamoid, low 4-point, and high 4-point nerve blocks. Abaxial sesamoid nerve blocks are effective against the pain of laminitis, subsolar abscesses, and other diseases within the distal structures.¹⁰ Lidocaine, mepivacaine, and bupivacaine are all used for performing a local anesthetic block in the distal limb, although the duration of bupivacaine (up to 8 hours) makes it the ideal choice for long-term relief.¹⁷

Injection of local anesthetic into the caudal epidural space significantly reduces perception of pain in the hindlimbs and the perineum and is used to address the pain of osteoarthritis, laminitis, or traumatic injuries (e.g., fracture) (TABLE 6).¹⁷ An 18- or 20-gauge, 1.5-in hypodermic needle or 18-gauge, 3-in spinal needle is used to inject the anesthetic within the first intercoccygeal space above the tailhead following aseptic preparation of the site.¹⁸ An indwelling epidural catheter can be placed when the patient requires frequent dosing. Mepivacaine (0.22 to 0.31 mg/kg) lasts up to 120 minutes; however, combinations of a local anesthetic with an opioid, α₂ adrenergic antagonist, or ketamine allow for longer analgesia, less risk of ataxia, fewer cardiopulmonary effects, and mild sedation.^10,18

Gabapentin

Gabapentin is an antiepileptic drug designed to inhibit the α₂δ subunit of voltage-gated calcium channels, which disrupts the pain process most associated with chronic pain.⁷ This mechanism of action has been widely used in human medicine to address chronic and neuropathic pain and has gained traction in veterinary medicine for the treatment of the same.

The implementation of gabapentin in horses is relatively new, with varied conclusions in studies regarding its efficacy against chronic osteoarthritis; laminitis; and headshaking from trigeminal neuropathy, temporohyoid osteoarthropathy, or an idiopathic cause.  Gabapentin can be administered at 20 mg/kg PO q12h; however, because oral gabapentin has limited bioavailability in horses, higher doses and more frequent administration are often needed.¹⁹ One study indicated that horses tolerate doses up to 120 mg/kg PO q12h without adverse effects.²⁰

Oral gabapentin is ineffective when used as a monotherapy.¹⁹ While more research is warranted to conclusively understand its pharmacokinetics and better implementation, its adjunctive role alongside other analgesic drugs is promising.

Other Pain Management Practices

The practice of compassionate care cannot be overstated in keeping painful horses comfortable. This includes basic care, corrective shoeing, and manual therapies. While it may seem obvious to the veterinary nurse, intentionally ensuring that patients are groomed at least daily decreases stress and alleviates discomfort by triggering the release of serotonin, dopamine, and oxytocin. Grooming also provides social interaction that hospitalization discourages. Deeply bedding stalls with shavings and straw encourages horses to lie down, rest, and take weight off overburdened contralateral limbs. If deep bedding obstructs ambulation, bedding can be banked to the back of the stall to allow a “sofa” for the horse to rest upon. Deep sand bedding can assist in the comfort of horses with acute laminitis.

Horses that are restrained, recumbent, or unwilling to stand regularly should have water and feed offered to them frequently and kept within reach (FIGURE 8). If the patient is reluctant to lower its head or is cross-tied to prevent further injury, feed and water should be elevated. If the patient is experiencing disease, healing from a surgery that causes dysphagia or quidding, or is inappetent, offering more digestible or palatable feed might prevent a negative energy balance.

Therapeutic farrier work includes corrective trimming, therapeutic horseshoes, and orthotics. A temporary sole-support orthotic that eases solar pain in laminitis cases can be applied using a dense foam pad. Commercial orthotic boots (e.g., Soft Ride Boots [softrideboots.com]) can be purchased by the client or stocked within the practice to rent for immediate use.

Cryotherapy is an age-old practice that reduces the inflammation and edema of injured tissues and is essential in the treatment of acute laminitis and flare-up pain in chronic laminitis patients.²¹ Thermotherapy, especially when paired with manual therapies, supports the healing process. Manual therapies serve to strengthen soft tissue structures as much as relieve pain and include massage, passive range of motion stretching, and soft tissue and joint mobilization.²² There are many indications to implement these therapies in patient recovery plans. They should be performed by an educated and experienced member of the veterinary team, and a veterinary nurse could receive an added credential as a Certified Equine Rehabilitation Practitioner from a reputable program.

Summary

Pharmaceutical pain management has developed significantly in the past decade as more studies have validated novel analgesic drugs, doses, and adjunct therapies. Multimodal pain management should be considered in every equine practice, with competent veterinary nurses making recommendations within the team based on observation and assessment of their patients.