Exploring the perception and physiology of pain in horses
"It's not a matter any more of wondering whether these animals actually do experience pain or feel pain," Pettifer says. "We know they have the capability of doing that."
Horses certainly experience pain from mechanical, thermal and/or chemical insults, but whether they also feel the emotional aspects of pain is difficult to assess.Horses known to be in pain, especially chronic pain, may seem depressed or at least more sedentary, and exhibit reduced self-maintenance behaviors. They certainly are in discomfort and their quality of life is negatively affected.
Smith notes that horses with untreated pain show distress and may be difficult to work with. Other symptoms include decreased water and feed intake, a predominantly catabolic metabolism, abnormal endocrine profile, delayed recovery from surgery, rapid-shallow respiration, muscle spasms, disuse atrophy and decreased mobility.
Acute and chronic pain
Horses experience both acute and chronic pain.
Although felt, a horsefly bite or the smack of a racing whip would be considered mild acute noxious stimuli, with no lasting sensation.
Acute pain usually is of sudden onset and short duration (less than three months), of variable severity, has an identifiable cause, is focal to the injury site and usually is a symptom or warning, an endogenous protective mechanism that signals real or potential serious trauma and/or tissue damage.
For horses in acute pain, there is a reluctance to be handled; restlessness, which may evolve into depression; often a rigid stance and an anxious appearance. They may show dilated pupils, glassy eyes, flared nostrils, muscle tremors, profuse sweating and an increased respiratory and pulse rate.
Acute pain also usually occurs with colic, fracture or surgery and often can be treated with NSAIDs or opioids.
Chronic or debilitating pain is considered pathologic, or having the characteristics of a diseased state. It often is unrelenting, may have no identifiable cause, may spread beyond the original site of an injury or insult and serves no biological function.
Due to increased production of excitatory neurotransmitters, peripheral and spinal-chord sensitization, chronic pain often is associated with musculoskeletal injury or disease (e.g., laminitis, osteoarthritis or degenerative joint disease). It requires complex treatment strategies and typically has a poor prognosis.
Horses suffering chronic pain show reduced food and water intake, often accompanied with weight loss; changes in sleeping habits and increased periods of recumbency; and changes in social behavior, such as irritability or aggressiveness toward people and other horses.
Visceral and somatic pain
Visceral and somatic pain occur within both acute and chronic pain.
The form of pain a horse feels when it steps on a nail or develops laminitis is referred to as somatic. It is processed differently than the visceral pain felt with an illness such as colic.
Somatic pain is associated with bones, ligaments, and tendons. It tends to be more localized and tends to be sharper than visceral pain.