Equine Pain

INAG 120 – Equine Health Management

October 10, 2011

What is Pain?

• Human pain: An unpleasant sensory and emotional experience

associated with actual or potential tissue damage or described in terms of such damage

(IASP - International Association for the Study of Pain, 1979)

What is Pain in Animals?

• An aversive sensory experience that elicits protective motor actions, results in learned avoidance and may modify species specific traits of behavior, including social behavior

Or…

• Animal pain is an aversive, sensory experience representing  awareness by the animal of damage or threat to the integrity of its tissues (note that there might not be any damage).  It changes the animal’s physiology and behavior to reduce or avoid the damage, to reduce the likelihood of its recurrence and to promote recovery. 

• Non-functional (non-useful) pain occurs when the intensity or duration of the experience is not appropriate for damage sustained (especially if none exists) and when physiological and behavioral responses are unsuccessful in alleviating it

Pain Classification: Sites of Origin

• Somatic pain

• Visceral pain

• Neuropathic pain

• Referred pain

Pain Classification: Duration

• Acute

• Chronic

• Chronic inflammatory

• Chronic neuropathic

Type of Pain

• Acute Pain – Sudden onset– Short duration– Variable severity– Identifiable cause– Focal to injury site – Acts as symptom or warning– Equine Acute Pain?

• Chronic Pain– Pathologic/

diseased state– Unrelenting– May have no

identifiable cause– May spread

beyond original site of injury

– May serve no biological function

– Equine Chronic Pain?

Production of Pain

• Pain is in the brain…• Peripheral mechanisms

– Skin, muscle, bone and other tissues have multiple nerve endings

– These nerve endings, when stimulated send a signal to the brain through the nervous system• Takes from 0.001 – 1 second to be recognized

Tissue Stimulation

• Noxious– Stimulation that makes the horse aware of stimulation

that could or already has done damage

• Non-noxious– Used by the horse to make it aware of the state of its

body and of its immediate environment (i.e. touch) • Travel along thick, myelinated nerves and rarely produce

pain unless the fibers are sensitized such as by inflammation

Abnormal Pain Response

• Allodynia– Not known/reported in horses

• Hyperalgesia– Primary (peripheral tissue)

• Conversion of arachidonic acid to prostaglandins can be significantly reduced:

– COX-1 is inhibited by drugs such as Aspirin – COX-2 is inhibited by drugs such as Celecoxib and Rofecoxib

• COX-2 inhibitors generally have fewer unwanted effects.

– Secondary (occurs in surrounding uninjured tissue)• Contributes to development and maintenance of chronic pain

Substance P

• Substance P– Neruopeptide (short chain of amino acids)

produced by nociceptors and used to activate neurons in the spinal cord.

– Can also be used for local inflammatory responses (i.e., a scratch) without going through spinal cord.

Inflammatory Responses

• Increased blood flow REDNESS• Increased blood flow WARMTH• Swelling as fluid leaks out of blood vessels

Injury Pain Sensation

• Nociception – Transduction, transmission, and spinal cord

modulation• Perception

– Perceiving the unpleasant experience• Cognition

– Behavioral response• How can the sensation of pain be

lessened?

Pain Assessment

1. Assessment of pain in your horse is a value judgment relying on measurement of physiological and behavioral changes (indices)

2. Use a wide range of indices– Training improves usage– Know the species, breed and individual animal well – Know when to seek advice from those with more

expertise (including owners or typical caretakers)

Pain Assessment

3. Experienced judge says horse is not in pain? Acceptable

– HOWEVER: If judge not available take appropriate action to treat pain

– Better safe than sorry…

4. Leave your emotions at the stall door (pasture gate, etc.).

Objective Assessment of Pain

• Measurements of physiological, biochemical and behavioral responses:– Threshold testing – Behavioral response to a visual or auditory stimulus

• Physiological responses – Changes in heart and respiration rate, blood pressure and stress

hormone values (cortisol)• Biochemical changes

– blood glucose, acute phase proteins and muscle enzymes• Behavioral responses

– precise measurements of the occurrence of well-defined activities and postures

Physiological Responses to Pain

• Dilatation of the pupils and/or wide opening of the eyelids

• Changes in blood pressure and heart rate• Increased respiration rate and/or depth • Pilo-erection (goose bumps)• Changes in skin and body temperature • Increased muscle tone • Sweating • Increased defecation and urination

Biochemical Pain Responses

• Changes in blood or cerebrospinal fluid levels of certain chemicals– Neurotransmitters, enzymes, other hormones– Changes in things such as lactate levels in

blood are not specific indicators of pain– Damaged tissues release enzymes into blood– Values don’t necessarily reflect level of pain

Behavior

• Responses that allow the animal to reduce or avoid the pain recurrence– Involves emotional experience and learning

• Reflex responses– Licking, biting attacking or running away

• Minimization of pain and assist healing

• Response designed to stop another animal (or person) from causing pain

• Failure to act normally

Behavior - Posture

• Normal Postures:– Horses spend less

than 3% of the day lying down

– If not in pain or ill, normal horse will get up when approached

– When lying, horses lie in sternal (ventral) posture

– May also lie laterally (flat out)

Behavior - Posture

• Abnormal Postures:– Lying in lateral posture– Lying ventrally with hind

legs extended– Normal sternal position but

trembling– Standing or walking

abnormally– Statue standing

• sustained standing still• trembling, stretching, a

hunched back, a tucked in tail and tucked up abdomen

Behavior – Changes in Gait

• Later lecture on lameness…

Behavior - Activity

• What level/type of activity are they involved in?• Examples:

– Pain in the mouth “quidding”– Pain in the head headshaking or twitching ears– Pain in the tail tail rubbing

• Incidence of these activities increases with increasing pain severity

• Can alter animal’s placement in herd hierarchy

Behavior - Activity

• Chronic Pain– Same activities shown but usually at lower

incidence– More difficult to detect than acute pain– “Ain’t doin’ right…” “Something just isn’t

right…”

Behavior – Facial Expression

• Horses communicate primarily using SIGHT (i.e., posture)

• Some horses perform flehmen behavior in response to pain

Behavior - Vocalization

• Grunting• Groaning• Squealing

• Not all that common in horses unless severe pain

Behavior – Mental State

• Adjectives:– Dull, depressed, unresponsive, unaware,

sleepy, apprehensive, anxious, timid, bright, alert, aware, excitable, hypersensitive and aggressive

• Play may be decreased

• Decreased ability to learn new tasks• Inability to perform learned tasks

Subjective Assessment of Pain

• Verbal descriptions:– In humans: mild, moderate, severe,

excessive, excruciating, localized, sharp, dull, burning

– Horses?

Systematic Assessment

• Details of animal

• Clinical history of the problem

• Clinical exam (vet)

• Physiological responses to pain

• Biochemical responses to pain (vet)

• Behavioral responses to pain