to the absence of a concise definition. From a behavioral perspective, this inability to arrive at a “Webster's Dictionary” type of definition is due in part to the fact that: 1) pain and distress are not discrete states, but are a continuum of experience; 2) signs differ between species, and most animals hide signs of pain because such a sign of weakness may provoke an attack from predators or subordinate members of the group; 3) there is a lack of specific behavioral indicators of pain; 4) interobserver variability can be large; and 5) there is a tendency to anthropomorphize, which is encouraged by US Government Principle IV. That principle states that “Unless the contrary is established, investigators should consider that procedures that cause pain or distress in human beings may cause pain or distress in other animals. ”
It is a well-established and accepted practice to use human experience to judge an animal's experience of pain and distress. Although I completely support this principle, I think we need to use this kind of assessment as a starting, not an end, point. We need to encourage investigators to include studies of pain and distress as they perform their research and to publish their findings.
Currently, there are several variables associated with pain assessment:
Assessments vary with the scale used, and they can be very subjective. What one person may view as a procedure that evokes moderate pain or distress, another may view as one that elicits minor pain or distress.
As Flecknell (1994) has noted, the absence of preprocedural scoring results in a lack of validation scores. There are no control data, so frequently confounding variables (such as those produced by analgesics) cannot be identified. For example, some of the consequences of surgery in rats, such as loss of body weight and suppression of food and water intake (signs frequently interpreted to be indicative of pain and/or distress), can also be produced in normal, unoperated rats by administration of opioid analgesics.
Chronic signs can be subtle and hard to detect. Changes in behavior due to pain and/or distress can be slow, incremental, and, individually, virtually undetectable. Whereas,
The dramatic, sudden onset of signs of pain is readily recognizable.
Distress is a generic term that can encompass anxiety, fear, boredom, frustration, and so forth. Thus, there are potentially multiple expressions of distress. Causes of distress that have been proposed include heat, light, sound, thirst, hunger, pain, novelty, exercise, pursuit, disease, and so there are also potentially multiple causes. But remember, the difficulties we continue to have in defining