Both stress and distress are meaningful terms that describe a state of being. While the biological responses to stress are better understood, the scientific, regulatory, and animal welfare communities disagree with respect to a universally accepted definition of distress. Although most definitions of distress characterize it as an aversive, negative state in which coping and adaptation processes in response to stressors fail to return an organism to physiological and/or psychological homeostasis, philosophical differences center on the inclusion of emotions and feelings affected by this state of being. Similarly, while it is accepted that failure of the organism to return to homeostasis adversely impacts an animal’s well-being and leads to poor welfare, defining well-being without relying on some form of anthropomorphic measures is a challenge. Scientific research does not yet support objective criteria or principles with which to qualify distress, objective scientific assessment of subjective emotional states cannot be made, and while there is often a measure of agreement on the interpretation of physiologic and/or behavioral variables as indicators of stress, distress, or welfare status, there is not always a direct link. Further, the Committee postulates that even if a universally accepted definition existed, it could not be applied across all species and all conditions, because of the differential impact of the strain, age, gender, genetic background, and environment.

The transition to distress, which occurs when the body cannot cope against the assault of one or more stressors, depends on several factors. Of clear importance are stressor duration, stressor intensity, and the capacity of the individual animal to respond; changes in any of these increases the likelihood of behavioral or physical signs of distress. Thus, minor perturbations may be stressful and/or negatively affect an animal’s moment-to-moment emotional state but they would not impair its adaptive capacity and therefore not cause distress (this may be unrelated to the state of the animal’s welfare as illustrated in Figure 2-2). In contrast, a major homeostatic disruption (e.g., postsurgical infection), which causes measurable behavioral (e.g., withdrawal) and physiological (e.g., fever) changes that impair an animal’s adaptive capacity, would be considered distressful and indicative of poor welfare. However, distress may not manifest itself with recognizable “maladaptive behaviors, such as abnormal feeding or aggression” (NRC 2003a, page 16) but instead begin with subclinical pathological changes (e.g., hypertension or immunosuppression) that can lead to overt disease. These physiological concepts should be integrated within and evaluated in concert with animal welfare principles.


While there are some specific behavioral measures of stress, relatively little is known about behavioral correlates of stress (i.e., behavioral changes

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