Assessment and Alleviation of Pain and Distress in Animals

Stress is a normal feature of life for all animals and serves important adaptive functions, such as flight-or-fight, predation, or "social-climbing." Animals used in research are subject to many stressors that are not normal and under circumstances that do not offer normal adaptive options.

Distress occurs when an animal is unable to adapt completely to a stressor (scientific definition).

Distress may be manifested by some behaviors and by biochemical and physiological changes; however, some animals hide fear and distress. It is critical, therefore, that protocols be designed to minimize the possibility of stress on the animals.

NOTE:* Behavioral, biochemical, and physiological changes indicative of distress can markedly affect research data; therefore, preventing distress in research animals is of prime importance, from both a humane and a research standpoint.

Possible stressors for animals being used in research:

  • inadequate space and overcrowding
  • social hierarchy
  • social deprivation
  • lack of environmental conditions needed for species-specific behaviors (e.g., gnawing for rats, exercise for dogs)
  • inappropriate handling and restraint
  • noise
  • odors
  • pheromones
  • fear
  • high-intensity lighting
  • irregularities in temperature, humidity, and light cycles
  • weaning
  • diet and feeding schedules
  • disease
  • injury
  • surgery
  • experimental procedures
  • and more

Response of an animal to a stressor can vary depending on age, sex, past experience (e.g., handling, previous experimental procedures), genetic profile, physiologic state, psychological state

Not all stressors can be treated with drugs, so planning experiments to minimize stress on the animals is of paramount importance.

Ways to reduce or eliminate distress

  • careful attention to animal husbandry (i.e., light, temperature, humidity, caging, bedding, etc.)
  • provision of species-appropriate environmental enrichment (in some species, this is mandated by law)
  • social housing for social species
  • training (of people AND animals)
  • gentle, quiet handling
  • limiting the numbers of stressors imposed on an animal
  • use of anesthetics, analgesics, and anti-inflammatory drugs for intra- and postoperative or experimentally induced pain

Although we cannot know exactly what an animal perceives as painful, the rule of thumb is to consider any stimulus we humans would consider painful as also painful to animals

Use of anesthetics and analgesics

  • required by law unless withholding is scientifically justified and IACUC-approved
  • should be planned in consultation with an RARC veterinarian; must be planned while writing protocols
  • use of paralytic drugs not permitted without anesthesia
  • "surgical plane of anesthesia" = unconsciousness, immobility, and analgesia
  • anesthetized animals must be monitored and the monitoring recorded
  • there is NO one "perfect" anesthetic for all animals

Choice of anesthetic depends on species, procedure, available equipment, expertise with anesthetic regiment, goals of the experiment.

Clinical and Physiological Signs of Pain in Laboratory Animals
Species Weight Heart Rate Respiration Other
cat Decreases due to dehydration or inappetence Increase in acute pain, Decrease in chronic pain Increase in acute pain, Decrease in chronic pain 3rd eyelid protrusion, circumanal gland discharge
cattle Decreases due to dehydration or inappetence Increase in severe pain Increase and shallow teeth grinding, lack of grooming, violent when handled
chicken dehydration Increase Increase allows handling
dog Decreases due to dehydration or inappetence Increase in acute pain, Decrease in chronic pain Increase in acute pain, Decrease in chronic pain Increase in urine specific gravity, Decrease in volume, pupils dilated
guinea pig dehydration Increase Increase upper respiratory congestion
horse dehydration Increase Increase, with flared nostrils interrupted feeding with food held in mouth uneaten, pupils dilated, limb-shifting, reluctance to move
nonhuman primate dehydration, no eating or drinking Increase Increase looks "miserable," lack of grooming, glassy eyes
other birds Decreases, dehydration Increase Increase
rabbit inappetence (prolonged); dehydration Increase Increase upper respiratory congestion
rodent Decreases due to dehydration or inappetence Increase Increase eats neonates; excessive licking and scratching, hunched posture, porphyrin around eyes in rats
sheep Decreases due to dehydration or inappetence Increase Increase and shallow grunting, grinding of teeth
swine Decreases, will still approach food, dehydration Increase Increase allow handling, hide in bedding

Reprinted from Rollins and Kessel. The Experimental Animal in Research, Vol. 1., CRC Press 1990.

Species Specific Behavioral Signs of Pain
Species Vocalizing Posture Locomotion Temperament
cat growl or hiss, but mostly silent stiff, hunched in sternal recumbency, limbs tucked under body reluctant to move, may carry limb reclusive
cattle grunting; teeth grinding rigid; head down; back humped limps; reluctant to move painful area dull, depressed; act violent when handled
chicken gasping stand on one foot; hunched; huddled none lethargic; allow handling
dog whimper, howl, growl Increase in acute pain, Decrease in chronic pain drag hind legs subdued, quiet, restless, or vicious; varies from acute to chronic pain
guinea pig urgent repetitive squeals cower, crouch, recumbent reluctant to move; walk in circles or pace docile, quiet; or terrified, agitated
horse grunting, nickering rigid; head lowered; kicks at abdomen favor area in pain restless; agitated; an become aggressive
nonhuman primate scream, moan, grunt head forward, arms across body; huddled and crouching excessive motion to tonic immobility, depending on pain severity docile to aggressive
other birds chirping huddled; hunched and "fluffed up" unwilling to move; unable to stand inactive, drooping; miserable appearance
pig increase in squealing to no sound at all all 4 feet close together under body inactive; drag hind legs passive to aggressive, depending on pain severity
rabbit piercing squeal on acute pain hunched; face back of cage Increase apprehensive; dull; sometimes aggressive depending on pain severity; eats neonates
rodent squeak, squeal rounded back; head tilted; back rigid ataxia; running in circles docile or aggressive, depending on pain severity; eats neonates
sheep teeth grinding; grunting rigid; head down limps, reluctant to move painful area disinterested in surroundings; dull, depressed

Reprinted from Rollins and Kessel. The Experimental Animal in Research, Vol. 1, CRC Press, 1990.