According to the International Association for the Study of Pain, the definition of “pain” is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage.”1 In horses, pain may limit movement of an injured body part, which may assist healing. Treating and managing the source of pain can be important to healing as a horse recovers from an injury.

Horse owners and managers can often tell when a horse is in pain because they are familiar with the horse’s normal behavior. In many cases, it is easy to observe that a horse is experiencing pain, but in others, the signs that a horse is in pain may not be as obvious, since horses differ in their tolerance and their outward signs of pain. Some horses react differently from others to the same injury.

Individual pain tolerance may also vary based on factors like a horse’s age, previous experiences and environment. Some non-painful stimuli (e.g., anxiety, fear) may also give rise to similar signs as those of pain, making pain recognition in the horse challenging. 

Some indicators of pain in horses include:

• Elevated heart rate

• Elevated respiratory rate

• Sweating

• Dilated pupils

• Behavioral changes

• Ears partially or fully back

• Pacing, rolling, pawing, or kicking at their sides 

• Lameness2

Pain Classifications

Pain is often classified by the body system it is affecting (e.g., integumentary, musculoskeletal, visceral). Examples of musculoskeletal pain include osteoarthritis, laminitis or navicular disease. Colic is a common clinical sign of visceral (intestinal) pain.

Pain can also be classified according to duration, this being either acute or chronic. Pain is classified as chronic if it has been present for more than three months. Acute pain is more often of recent onset, although it can be associated with chronic illnesses.3

The intensity of pain is another way that pain is classified. In many horses, mild pain is often difficult to detect, while moderate to severe pain can be easier to identify because the horse will more readily display outward signs.

Pain, Inflammation and Lameness

One of the most common origins of pain is the body’s normal inflammatory response. Inflammation is also an important part of tissue healing mechanisms. Pain is the most common cause of lameness in horses. This may arise from short-term injury, mechanical lameness (e.g., the result of damaged connective tissues or an abnormality in anatomy) or neuromuscular disorders. 

Physically immature horses that are subjected to repetitive stress due to activity have been prone to lameness. Certain breeds of horses and those occupying specific areas of discipline are more commonly predisposed to developing lameness conditions. Orthopedic disease, poor conformation, improper hoof balance or shoeing and failure to adequately condition performance horses can also cause lameness.1, 3

Phenylbutazone for the Treatment of Pain in the Horse

Phenylbutazone (PBZ) is a Non-Steroidal Anti-inflammatory Drug (NSAID) that is used primarily for treating pain and inflammation in horses. Phenylbutazone has analgesic, anti-inflammatory, antipyretic, and mild uricosuric properties. The mechanism of action is via inhibition of cyclooxygenase, which results in reduced prostaglandin synthesis. Additional pharmacologic actions include reduced renal blood flow, decreased glomerular filtration rate, decreased platelet aggregation, and gastric mucosal damage.4

Following oral administration, phenylbutazone is absorbed from both the stomach and small intestine. It is distributed throughout the body with highest concentrations attained in the liver, heart, lungs, kidneys, and blood. Serum half-life in horses ranges from 3.5 to 6 hours and is dose dependent. In horses and other species, phenylbutazone is nearly completely metabolized, primarily to oxyphenbutazone (active) and gamma-hydroxyphenylbutazone.4

For the treatment of musculoskeletal conditions in horses, recommended doses of oral phenylbutazone are 2 – 4 g/454 kg (1000 lb) horse PO once daily, not to exceed 4 g/day. 

Adverse effects of phenylbutazone that have been reported in horses include oral and GI erosions and ulcers, hypoalbuminemia, diarrhea, right dorsal colitis, anorexia, azotemia, and renal papillary necrosis. Contraindications include known hypersensitivity, history of or preexisting hematologic or bone marrow abnormalities, preexisting GI ulcers; use in horses and dairy cattle intended for food is prohibited.4

Although gastric ulceration is frequently observed in adult horses and foals, evidence of an association between this disease and administration of NSAIDs such as phenylbutazone or flunixin at recommended dosages is lacking.4 Since phenylbutazone (PBZ) is known for its capacity to elicit equine gastric ulcer syndrome (EGUS), administering omeprazole concurrently can mitigate the risk for both equine glandular gastric disease (EGGD) and equine squamous gastric disease (ESGD).

Where to buy Phenylbutazone 

Phenylbutazone is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. PHENYLBUTAZONE 100 MG/ML ORAL SUSPENSION (500ML) by NexGen Pharmaceuticals is indicated for the treatment of pain arising from musculoskeletal conditions in the horse.

This product carries numerous potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Merck Veterinary Manual.

2aaep.org.

3equimanagement.com.

4Plumb’s Veterinary Drugs.