By Jeannine Miesle MA, AAV

Part 1: History / Introduction

Part 2: Pain Perception and Signal Reception

Part 3: Pain Signal Transmission and Pain Pathways

Part 4: Types of Pain, Long-term effects, Referred Pain, and Pain Memory

Part 5: Pain, Stress, and the Body’s Physiological Response to Them  (Please scroll down)

Part 6: Pain in the Avian Species

Part 7: Anesthesia and Analgesia, Chronic Pain

Part 8: Quality-of-Life Issues

Part 9: Pain Assessment in Birds / Quality of Life

Part 10: Hospice and Palliative Care for Pets, Strategy for Comprehensive Care, & Conclusion

Part 5: Pain, Stress, and the Body’s Physiological Response to Them

An animal’s well-being should include

“The Five Freedoms”

Freedom from thirst, hunger, and malnutrition

  • Access to fresh water and a healthful diet

Freedom from discomfort

  • Suitable environment, shelter, and a resting place

Freedom from pain, injury, and disease

  • Prevention of cruelty and illness by care and rapid treatment

Freedom to express normal behavior

  • Provision of space, facilities, and company of the animal’s own kind

Freedom from fear and distress

  • Assurance of conditions that avoid stress and mental suffering

Wm. W Muir III

Stress is a biological response that an animal exhibits when it perceives a threat. Its internal state of well—being (homeostasis) is jeopardized. In the past, most vets only concerned themselves with the body’s response to injury or surgical trauma, but today they realize that there are many factors which can put homeostasis at risk.

The Central Nervous System (CNS) can modify the response of the body to the various stressors, such as pain, surgery, restraint, and confinement. Birds can experience trauma from auditory and visual stimuli identical to somatosensory input from damaged tissue. Stress modifies the animal’s memory, thus serving as a protective role in diverting the body’s resources to cope with the stressor. If the body cannot respond effectively to stress, it cannot maintain homeostasis, and the result is dysfunction, disease, and suffering. This “sickness syndrome” hastens death.

It is uncertain whether animals perceive pain and suffering in the same way humans do because they live in the present and cannot understand death or future suffering. Its interaction with its environment and external events determines its well-being. This interaction incorporates feeling, perception and awareness, so if he perceives an event as threatening, the response will be the same whether it is threatening or not; therefore, it is ethical and humane to minimize stress because of the adverse reaction to it all animals face.

Pain serves a protective function by warning the animal of real or impending tissue damage. Acute and chronic pain produce stress, and when severe, increase neuroendocrine activity and profound behavior changes. Environmental factors can produce anxiety and feather that sensitize and amplify the stress response to painful stimulus. The severity and duration of pain determine the consequences of the animal’s pain in terms of stress and suffering.

Stress alters the release of the hormones in the body. Some of the hormones that are affected are cortisol, epinephrine, glucagon (insulin for mammals), the growth and thyroid hormones, and vasopressin, which aids the body in retaining the necessary amount of water.

The metabolism is also affected by stress. Unbalanced carbohydrates can lead to hyperglycemia, poor wound-healing, infections, and morbidity. Fat and protein metabolism are disturbed, causing the body to be unable to metabolize these properly.

The Immune System

Although most people think the immune system’s primary purpose is to identify and destroy foreign substances, it also functions as a sense organ that communicates injury-related information to the brain. It can be activated or depressed by stress; thus, pain, whether accidental or intentional (e.g. surgery), modulates (changes) the immune response. The key elements that determine how the immune system responds to pain are intensity and duration. Chronic pain suppresses the immune system’s responses, and mild-to-moderate pain from tissue trauma activates the immune system’s messengers.

The Acute-Phase Response of the immune system is triggered by severe stress from any cause. This response releases proteins from the liver, which acts in repairing tissue. Excessive production of these proteins can contribute to Systemic Inflammatory Response Syndrome (SIRS).

Morphological Changes

Morphological changes brought about by chronic stress or pain are typical of long-term aversive stimuli and include failure to thrive, feather loss, poor feather condition, weight loss, and acceleration of aging.


Muir W III. Pain and Stress. In: Handbook of Veterinary Pain Management. Ed: James Gaynor, Wm Muir III. Mosby Inc, 2009