Monitoring animals for pain and distress is effective when which of the following is true?

Introduction

We have long recognized the impact that pain, stress, and distress can have on the welfare of laboratory animals, or for that matter, any animal. Pain and stress can cause distress, and the biologic effects can compromise experimental results ( Moberg 1999 ). For these reasons, we are continually striving to control and ameliorate the effects of pain and stress on laboratory animals. Neither pain nor stress—a part of an animal’s life, as they are ours—can ever be totally eliminated. Pain normally serves as a protective function to warn of impending danger and is therefore adaptive. Animals have evolved appropriate biologic strategies to assist them in coping with stress. Thus, pain and stress are not inherently bad for an animal, unless these biologic strategies fail to protect the animal from stress or the biologic cost of coping takes too great a toll on the animal. then the animal experiences distress and its welfare is threatened. The question is, when does an animal cross over from nonthreatening stress to distress?

The recognition of pain, stress, and distress is critical in maintaining the well-being of laboratory animals. However, this important task is difficult because of a lack of agreed-upon definitions of these terms, as well as the absence of absolute, objective measures. Because animals cannot verbalize what they are experiencing, investigators and caretakers must deduce the animal’s condition based on appearance and behavior. Excellent sources of information on these issues have appeared previously ( Bateson 1991 ; CCAC 1998 ; IRAC 1985 ; Morton and Griffiths 1985 ; NRC 1992 ; Short and Van Poznak 1992 ; Smith and Boyd 1991 ; Soma 1987 ; UFAW 1989 ; Wallace et al. 1990 ). In this article, we discuss the biologic meaning of pain, stress, and distress, with the aid of a model, and review and update guidelines for the recognition and assessment of pain and distress in laboratory animals.

Pain

The International Association for the Study of Pain has defined pain in humans as “an unpleasant sensory and emotional experience associated with potential or actual tissue damage, or described in such terms.” In the absence of language, it is impossible to know an animal’s experience, which can be deduced only by observing the animal’s general appearance and pain-like (“nocifensive”) behavior. Thus, most mammalian species react in a manner to avoid and reduce the impact of acute noxious (potentially or overtly tissue-damaging) stimuli, suggesting that they perceive pain. Considerable evidence indicates that common laboratory animals possess nervous systems that receive and process noxious stimuli in a manner similar to humans, at least at subcortical levels. Although it may be safe to assume that higher mammals consciously perceive pain, it has been argued that because their cerebral cortices are smaller and less developed than humans’, they may not have the capacity to comprehend the meaning of pain or an awareness of impending doom ( Bateson 1991 ; Carli 1980 ; Melzack and Dennis 1980 ; Smith and Boyd 1991 ). Thus, it is arguable whether laboratory and companion animals suffer in the same sense as humans, but we believe that animals should be treated as if they do, recognizing that there are no objective or measurable indices of suffering. In the following sections, we attempt to identify indicators of pain, stress, and distress in mammalian laboratory species.

Acute pain normally warns of the presence of a potentially injurious stimulus and has great adaptive value. Delivery of a noxious stimulus to an extremity usually elicits a protective, stereotyped reflexive withdrawal of the limb away from the source of injury. This may or may not be associated with other nocifensive reactions such as vocalization, orientation toward the stimulus, escape or attack, shaking, rubbing, scratching, biting or licking of the stimulated area, and autonomic reactions such as increased blood pressure and heartrate, piloerection, and pupillary dilation.

Considerable evidence indicates that simple nocifensive behavioral reactions, such as limb withdrawal reflexes, are generally elicited at similar thresholds in animals and humans and that the magnitude of the response increases with stimulus intensity. The presence of other more complex, integrated behavioral reactions varies across and within species and even within individuals. The maximal stimulus intensity voluntarily accepted by a human is the tolerance limit. The pain tolerance limit in animals, assessed subjectively, varies widely across species with some (e.g., cattle) apparently being more “stoic.” The relatively few studies measuring the magnitude of nocifensive behavioral responses (e.g., operantly conditioned escape responses in monkeys) suggest that at least some species perceive increasing pain over the same range of stimulus intensities (from threshold to tolerance limit) as humans ( Cooper and Vierck 1986a ).

Consequent to surgery, accidental injury, illness, or disease, a persistent state of pain may develop. After an injury such as a surgical incision, tissue at and surrounding the site of trauma may exhibit increased pain sensitivity. This sensitivity is usually characterized by an increase in the intensity of pain (i.e., nocifensive response) elicited by a noxious stimulus, termed hyperalgesia. This condition is frequently accompanied by allodynia, in which pain is elicited by a nonnoxious stimulus (e.g., touch) that normally is incapable of eliciting pain. Hyperalgesia and allodynia are adaptive in that the animal is self-motivated to reduce movement to minimize reinjury and aid recuperation, which normally requires a few days. Although hyperalgesia and allodynia can be objectively measured experimentally (e.g., Bennett and Xie 1988 ; Tabo et al. 1999 ), these and other indicators of pain are difficult to quantify objectively in the clinic. The use of scaling procedures by veterinarians to assess an animal’s degree of postsurgical pain may prove to be helpful (e.g., Holton et al. 1998 ).

After injury to peripheral or central nervous tissue, or progressive disease states such as cancer, pain may develop over time and persist indefinitely. This chronic pain no longer has adaptive value but instead causes distress. Although more difficult to recognize than acute pain, it is crucial that signs of chronic pain are identified to afford appropriate treatment and minimize or prevent distress.

Neural Basis of Pain

Acute pain sensation is normally elicited by noxious stimulation of high-threshold receptors, called nociceptors, which exist in most tissues. Varieties of nociceptors exist that respond selectively to intense mechanical stimuli (mechanical nociceptor), or nonselectively, to noxious mechanical, thermal, and chemical stimuli ("polymodal” nociceptor). Some nociceptors do not respond to noxious stimuli initially but begin to respond during development of inflammation (“sleeping” nociceptors). The afferent fibers of nociceptors travel in peripheral nerves into the spinal cord, where they synaptically contact second-order neurons in the dorsal horn. Most nociceptors are thought to release the neurotransmitter glutamate, as well as the neuroactive peptides substance P and neurokinin A, from their presynaptic terminals. These transmitters excite the second-order dorsal horn neurons, which either send axons into ascending sensory pathways to transmit pain information to higher centers or serve as interneurons in segmental reflex pathways. Ascending pathways involved in the transmission of pain include the spinothalamic tract, the spinocervicothalamic tract, the spinohypothalamic tract, spinoreticular pathways, the spinoparabrachial amygdalar pathway, the postsynaptic dorsal column pathway, and a recently identified pathway for visceral pain in the dorsal columns. In addition, pain may be transmitted rostrally by multisynaptic connections (for recent reviews, see Millan 1999 ; Willis 1985 ; Willis and Coggeshall 1991 ; Willis and Westlund 1997 ).

After a peripheral injury such as surgical incision, the resultant hyperalgesia has two possible explanations. First, it is known that noxious stimulation can cause nociceptors to “sensitize,” that is, to give a greater response to subsequent stimuli. This action is due to the release of algesic chemicals from damaged cutaneous cells into the extracellular environment, where they can depolarize the fiber endings of nociceptors. In addition, injury can lead to sensitization of second-order spinal neurons, involving a multistep process starting with the release of an excessive amount of glutamate from the nociceptor presynaptic terminals. As a result of the dual action of glutamate to depolarize the dorsal horn neuron, and to open cation channels by its action at a subtype of postsynaptic glutamate receptor called the NMDA-receptor, Ca++ gains entry into the postsynaptic neuron to trigger second messenger systems ultimately leading to increased excitability of the neuron ( Dickenson et al. 1997 ). This process, called central sensitization, can lead to hyperalgesia because the hyperexcitable neuron now sends a larger pain signal to higher centers. The neuron can even be excited by a nonnoxious tactile stimulus; the heightened signal sent to the brain is interpreted as painful even though it was caused by a normally nonpainful stimulus. Both peripheral and central sensitization very likely contribute to increased pain sensitivity after peripheral injuries (for recent reviews, see Carstens 1995 ; Dubner and Ruda 1992 ; Millan 1999 ).

Pain-transmitting spinal neurons are known to be under powerful descending modulation from the brainstem. It should be noted that descending pathways can either inhibit or excite spinal neurons ( Fields and Basbaum 1994 ; Maier et al. 1992 ; Millan 1999 ; Sandkuhler 1996 ; Willis 1985 ). These two forms of descending modulation probably play important roles in the complex effects that environmental stressors can have in increasing or reducing pain sensitivity.

The neural mechanisms underlying the development of chronic pain are not as well understood and are currently under active investigation. Chronic pain likely involves central sensitization (see above) and other long-term alterations in the function of pain-signaling pathways, such as remodeling of synapses within the spinal cord ( Lekan et al. 1996 ; Millan 1999 ).

Recognition of Pain in Laboratory Animals

It may not always be obvious that a laboratory animal is in pain, and careful observation of changes in appearance and behavior over time will aid in recognizing signs of pain. Knowledge of the animal’s treatment or procedure will obviously assist in this assessment. It is advisable to assume 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” ( IRAC 1985 ). A very general set of observations for assessing pain and distress has been proposed ( Morton and Griffiths 1985 ): change in body weight, external physical appearance, clinical signs, changes in unprovoked behavior, and behavioral responses to external stimuli. These criteria provide a good framework for additional assessment.

For animals that have been subjected to a localized surgical procedure, one would look for signs of pain in the affected body area. For example, in animal models of pain due to surgical incision or partial nerve injury, one should observe the affected limb for signs of spontaneous pain and/or hyperalgesia and allodynia. These signs include altered posture and gait with less weight placed on the injured limb, limb guarding, change in muscle tone and limb temperature, vocalization and/or exaggerated withdrawal of the limb to innocuous or noxious stimulation, and possibly also shaking, licking, scratching or biting of the affected area ( Bennett and Xie 1988 ; Kim and Chung 1992 ; Selzer et al. 1990 ; Tabo et al. 1999 ; Takaishi et al. 1996 ; reviewed in Millan 1999 ). However, scratching of the injured area might also reflect itch, which is very difficult to differentiate from pain. These changes may occur in the absence of more general alterations in appearance, feeding, and other behaviors. After a more severe injury such as transection of a peripheral nerve or dorsal roots, the animal may self-mutilate the denervated limb; however, it is not yet known whether this behavior represents a reaction to pain, a response to a nonpainful abnormal sensation such as tingling, or failure of the animal to recognize the denervated limb as part of its body schema ( Coderre et al. 1986 ).

Signs of chronic pain due to pathology may be more difficult to recognize initially because they are likely to develop slowly and vary in magnitude over time. Indicators of pain include a general deviation from the animal’s normal healthy appearance due to reduced grooming, loss in body weight, and a reduced locomotor activity ( Table 1 ). There may be postural changes, such as a “hunched” posture with abdominal pain. Rats may exhibit porphyrin secretions (“redtears”) around the eyes. The animal may vocalize spontaneously and/or when handled; however, vocalization is not specific to pain nor do animals in pain necessarily vocalize ( Cooper and Vierck 1986b ). Rodents emit vocalizations in the audible range as well as at frequencies >20 KHz; and although such ultrasonic vocalizations can be associated with pain ( Jourdan et al. 1995 , 1998 ), they are also emitted under other nonpainful circumstances. Finally, an animal in pain may fail to exhibit normal curiosity or social interactions, for example, withdrawing from the handler or, alternatively, showing increased aggressiveness. Here, knowledge of species-specific behavior is important in assessing pain. Table I provides a summary of many indicators of pain in several species of laboratory and companion animal. Detailed descriptions of signs of pain in other animal species are available in the literature ( Morton and Griffiths 1985 ; NRC 1992 ; Soma 1987 ; Wallace et al. 1990 ).

Table 1

Indicators of pain in several common laboratory animals’

a

Adapted from Morton DB, Griffiths PHM. 1985 . Guidelines on the recognition of pain and discomfort in experimental animals and an hypothesis for assessment. Vet Rec 116:431–436; NRC [National Research Council]. 1992 . Recognition and Alleviation of Pain and Distress in Laboratory Animals. Washington DC: National Academy Press; Soma LR. 1987 . Assessment of animal pain in experimental animals. Lab Anim Sci 37:71–74; and Wallace J, Sanford J, Smith W, Spencer V. 1990. The assessment and control of the severity of scientific procedures on laboratory animals. Report of the Laboratory Animal Science Association Working Party. Lab Anim 24:97–130.

Table 1

Indicators of pain in several common laboratory animals’

a

Adapted from Morton DB, Griffiths PHM. 1985 . Guidelines on the recognition of pain and discomfort in experimental animals and an hypothesis for assessment. Vet Rec 116:431–436; NRC [National Research Council]. 1992 . Recognition and Alleviation of Pain and Distress in Laboratory Animals. Washington DC: National Academy Press; Soma LR. 1987 . Assessment of animal pain in experimental animals. Lab Anim Sci 37:71–74; and Wallace J, Sanford J, Smith W, Spencer V. 1990. The assessment and control of the severity of scientific procedures on laboratory animals. Report of the Laboratory Animal Science Association Working Party. Lab Anim 24:97–130.

Influence of Stress on Pain

Although pain is an important stressor, many forms of environmental or psychological stress can also increase or decrease pain perception. Examples of stressors that can reduce nocifensive behavioral responses in animals, resulting in so-called “stress-induced analgesia,” include inescapable footshock, centrifugal rotation, noxious stimuli, and presumably more psychological stressors such as fear or aggression ( Amit and Galina 1988 ; Tricklebank and Curzon 1984 ; Watkins and Mayer 1982 ). Stress-induced analgesia is thought to be mediated in part by descending pathways from the brain that can modulate the spinal transmission of pain signals (see above). Reduction of pain would be advantageous under highly stressful circumstances, such as fight-or-flight, in which it would be life threatening for an animal to attend to a painful injury rather than delaying until after it has reached safety. The capacity for endogenous systems to modulate pain in humans is well documented, a good example being the soldier who is injured on the battlefield but perceives no pain until long after he has been removed to the safe environment of the hospital. Indeed, a substantial fraction of patients who were admitted to the emergency room of a major hospital for severe injuries, such as broken bones, reported that they did not feel any pain for up to 7 hr after incurring the injury ( Melzack et al. 1982 ). Thus, in assessing pain in animals, it must be recognized that the animal’s level of pain might well be influenced by the history of other stressors that have impinged on it, as in humans.

In this context, the poorly understood relationship between pain and illness deserves brief mention. Sickness and fever can be induced in rats by a bacterial lipopolysaccharide (endotoxin) ( Romanovsky et al. 1996 ; Watkins et al. 1994 ). During the initial phase of lipopolysaccharide-induced fever, rats exhibit hyperalgesia and show increased locomotor activity, whereas they exhibit hypoalgesia during the later stage that is characterized by lethargy and poor temperature regulation. The significance of this biphasic change in pain sensitivity is not clear but certainly suggests that the assessment of pain in animals might be further complicated by the presence of systemic infection.

Animal Stress and Distress: A Model

It has never been possible to accurately define stress because the term is applied so broadly. Some authors use stress to describe the biological response of an animal to a threat. Others use stress to describe the actual threat, such as restraint stress or temperature stress. For this discussion, we define stress as the biological responses an animal exhibits in an attempt to cope with a threat to its homeostasis. The threat to its homeostasis is the stressor. No stress occurs unless the animal perceives a threat, either consciously or unconsciously. Although animals are conscious of most stressors, there are potential threats to an animal’s homeostasis, such as tumor growth, that will elicit stress responses even though the animal may never be conscious of the threats. For this reason, we define perception very broadly to include both conscious and unconscious recognition of a stressor. Regardless of how an animal perceives a stressor, the resulting stress may or may not prove to be distressful.

To understand how stress leads to distress, we will use the model of animal stress depicted in Figure 1 to place the diverse literature on animal stress into a working framework. Because the development of the model is discussed elsewhere ( Moberg 1985 , 1999 , 2000 ), we focus herein on the aspects of the model that are relevant to distress. All stress responses, good, bad, or inconsequential, result from the animal responding to a stressor by exhibiting one or more of four general biological defense responses: behavioral, autonomic nervous system, neuroendocrine system, or immune. The responses of these systems to the stressor alter biological functions, resulting in the stress responses. Because most stressors are brief, the changes in biological function required to cope with the stressor are minimal and of no significant consequence to the animal ( Figure 2 ), that is, the biological cost of the stress is of little consequence to the animal’s well-being. As schematically depicted in Figure 2 , the biological resources required by the stress response do not affect other biological functions ( Moberg 1999 , 2000 ). Once the stressor is alleviated, the animal returns to prestress conditions.

Figure 1

Model of the biologic response of animal to stress. Reprinted with permission from Moberg GP. 1999 . When does stress become distress? Lab Anim 28:422-426.

Figure 2

Hypothetical scheme of how stress diverts biologic resources during mild stress. In this scheme, biologic resources are arbitrarily assigned to various biologic functions (Fl-F”n”). During mild stress, only reserve resources are used to cope with the stressor. The total stress response extends from the time biologic resources are diverted until the reserves have been replenished. Reprinted with permission from Moberg GP. 1999 . When does stress become distress? Lab Anim 28:422—426.

When the stressor is severe, of long duration, or characterized by the cumulative effects of several stressors, then the total biological cost of the stress response may require the diversion of resources from other biological activities ( Figure 3 ), disrupting other biological functions that are critical to the animal’s well-being. When these functions are disrupted, the animal enters into a prepathological state ( Figure 1 ), rendering it vulnerable to the development of pathologies ( Moberg 1999 , 2000 ). During this time, when normal function is disrupted and the possibility for pathology exists, the animal experiences distress and its welfare is threatened.

Figure 3

Hypothetical scheme of how the diversion of biologic resources necessary to cope with severe stress significantly impacts other biologic functions leading to distress. As compared with mild stress ( Figure 2 ), the biologic cost of distress requires a much longer recovery period. Reprinted with permission from Moberg GP. 1999 . When does stress become distress? Lab Anim 28:422—426.

In this model of stress, we view biological resources and pathology in the broadest sense. Although shifts in energy utilization characterize many stresses ( Elsasser et al. 2000 ; Moberg 2000 ), other changes in function, such as altered behavior or loss of immune competence, are biological resources effected in an attempt to cope with the stressor. Likewise, disease is not the only type of pathology an animal experiences during stress ( Moberg 1985 ). Loss of reproduction, abnormal behavior, or failure to grow are examples of stress-induced pathologies. The presence of any of these pathologies or the biologic conditions leading to these pathologies (prepathological state) clearly indicates that the animal is in distress and its well-being is threatened ( Moberg 1985 , 1999 , 2000 ). Although it is difficult to objectively assess the degree to which an animal’s well-being is compromised, a list of some obvious indicators is provided in Table 2 ( NRC 1992 ).

Table 2

Some behavioral, physiologic, and biochemical indicators of well-being a , b

a

Departures from normal behaviors and characteristics are suggestive of changes in well-being. A knowledge of species-typical and individual-specific behaviors and clinical values is essential.

b

Reprinted from NRC [National Research Council]. 1992 . Recognition and Alleviation of Pain and Distress in Laboratory Animals. Washington DC: National Academy Press, with permission.

Table 2

Some behavioral, physiologic, and biochemical indicators of well-being a , b

a

Departures from normal behaviors and characteristics are suggestive of changes in well-being. A knowledge of species-typical and individual-specific behaviors and clinical values is essential.

b

Reprinted from NRC [National Research Council]. 1992 . Recognition and Alleviation of Pain and Distress in Laboratory Animals. Washington DC: National Academy Press, with permission.

Although providing a biologic definition of distress is not difficult, recognizing when distress occurs is far more difficult. Certainly the presence of pathologies such as disease, selfmutilation, or death are obvious indicators of distress; however, waiting for the occurrence of such endpoints as an indication of distress occurring in laboratory animals is inhumane and unacceptable. Our goal must be to recognize distress before the animal reaches the pathological state. Because the key to distress is the biologic cost of stress, we need to focus on the biologic processes leading to the change in biologic function or the alteration in function as indications of distress occurring. Because there is currently no litmus test for distress, we need to approach the recognition of distress on almost a case-by-case basis.

Nonpain Distress

In some aspects, recognizing nonpain distress is more difficult than recognizing the discomfort associated with pain; although the source of pain is usually obvious, our challenge is to gauge the level of discomfort. In contrast, many nonpain stressors confronting the laboratory animal are less obvious and their biologic effects on the animal are poorly understood. These stressors fall into two general categories: stressors associated with the experimental manipulation of the animal and stressors resulting from routine husbandry practices.

Both categories share the same biology of the stress response, but recognition of these stressors requires different approaches.

Distress does not occur without stress. As discussed, distress occurs only when the biological cost of stress negatively affects biological functions critical to the animal’s well-being. Stress, however, has proved difficult to measure ( Moberg 1985 ). Returning to the model of animal stress ( Figure 1 ), it would seem that the most reasonable strategy for measuring stress would be to monitor the responses of the four major defense systems (behavior, autonomic nervous system, neuroendocrine system, and immune system) inasmuch as they are responsible for the biological changes that occur during stress. Indeed, all four systems have been used to measure stress ( Moberg 2000 ); however, none have proved to be a reliable measure of stress, let alone distress. No one system responds to all stressors. Frequently, they respond comparably to both threatening and nonthreatening stimuli. In addition, there are intraanimal variations in responses to the same stressor that are very difficult to monitor outside the controlled conditions of the laboratory.

The stressor responses of the hypothalmic pituitary adrenal axis (HPA 1 ) provide an example of the difficulty encountered in measuring stress. Measuring the secretion of the HPA hormones, especially the glucocorticosteroids cortisol and corticosterone, has been the most popular tool for evaluating stress, and researchers frequently use increases in circulating glucocortico steroids as proof of stress. Certainly numerous stressors do elicit an increase in circulating steroids but contrary to Selye’s (1950) prediction, not all stressors elicit an HPA response ( Mason 1968 ). Further complicating the use of the HPA axis as a measure of stress is the comparable response of the axis to both potentially threatening and nonthreatening stimuli. This is perhaps best exemplified by studies of Colborn and colleagues (1991) who found that stallions secreted similar amounts of cortisol whether the horses were restrained, exercised, or allowed to mate. It is difficult to argue that mating has the same negative impact on the stallion’s welfare as being restrained.

Further complicating stress measurements are the intraanimal differences in how the four general defense systems respond in attempting to cope with the stressor ( Engle 1967 ; Henry 1992 ; Moberg 1985 , 2000 ; Weiss 1972 ). Early experience, genetics, age, and physiological state are examples of a multitude of modulators that influence the nature of the stress response ( Moberg 1985 , 2000 ). With traditional laboratory animals such as rodents, many of these variables can be controlled and accounted for in the experimental design; however, for some laboratory animals (e.g., nonhuman primates or random source animals), it is extremely difficult to account for these modulators of the stress response because simple measures of hormones, autonomic nervous system activity, or immune responses may be unreliable measures of stress outside the experimental paradigm ( Moberg 1987b ).

In monitoring the responses of the neuroendocrine, autonomic, and immune systems to potential stressors, it is very difficult to obtain the requisite samples for analysis without allowing the sampling procedures to stress the animals. To evaluate these systems, it is usually necessary to obtain blood samples or attach monitoring equipment. The act of restraining animals and drawing blood is a stressor for the animal, resulting in significant changes in circulating HPA hormones as well as changes in other neuroendocrine hormones or autonomic nervous system activity. Sometimes it is possible to incorporate sampling methods into the experimental design, permitting the investigator to obtain samples via cannulae or some other methods without further stressing the animals; however, this approach is feasible for only a limited number of animals and is certainly impractical for monitoring distress related to the husbandry of laboratory colonies, especially for such species as nonhuman primates.

Of the four major defense systems, behavior is perhaps the most promising as an unobtrusive way to monitor for stress ( Mench 1998 ). Although promising, the use of behavior as a measure of stress is hampered by our lack of understanding of animal behavior as it relates to stress ( Rushen 2000 ). Accepting behavior as an indicator of distress requires the correlation of reliable behavioral changes with stress-induced biologic changes that force the animal into the prepathological state.

Even if an animal experiences stress, it does not mean that the animal suffers distress. As we have stated, stress is a part of life. If the biologic cost of the stress has no impact on other biologic functions ( Figure 2 ), then there is no distress. However, if the biologic cost of the stress affects other biologic functions, distress does occur ( Figure 3 ). From this argument, it would appear that the most appropriate measures of distress would be changes in biologic functions (see Figure 1 ) that lead to the prepathological and pathological states ( Moberg 1996 ). Although morbidity and mortality are not acceptable for monitoring distress, their presence must not be ignored as indicative of severe distress occurring in the colony. Likewise, failure to grow, reduction in reproductive success, and markedly abnormal behavior are indications of distress. Nevertheless, we need methods to determine when the prepathological state is developing. Reduction in immune competence ( Blecha 2000 ), disruption of the reproductive neuroendocrine axis ( Moberg 1987a ), altered metabolism ( Elsasser et al. 2000 ), and growth ( Moberg 1999 ) have been used to identify distress; however, use of these measures is as tedious and difficult as measuring hormone secretion or autonomic nervous system activity. The advantage of evaluating these biologic functions for detecting distress, as opposed to simply measuring hormone secretion, is that we know the stressor has disrupted biologic functions that truly affect the animal’s well-being health, reproduction, and growth.

Although difficult, taking such measures of stress and distress can be incorporated into the design of many experiments. However, using them to identify distress in animal colonies resulting from improper husbandry practices is far more difficult because of the sheer number and types of animals involved. Modem animal colonies, through controlled environments and standardized management practices, do not experience many potential stressors such as extreme temperatures, inadequate nutrition, or abnormal lighting schedules; however, many other subtle environmental factors may cause distress. In their superb report on the recognition and alleviation of pain and distress in laboratory animals, the National Research Council ( NRC 1992 ) identifies such potential factors as the animal’s relationships with conspecifics, the necessary available social space, the potential effects of feeding and foraging behavior, and the physical nature of the captive environment. Unfortunately, apart from studies of environmental enrichment in certain primate species, not enough research on these topics has emerged since this report was issued to guide us in deciding how to house our animals to reduce or even evaluate potential distress related to these concerns.

In the absence of simple, definitive measures of distress, our best approach is to use our intuition and sensitivity to reduce potential distress in laboratory colonies. Because the presence of distress is related to the biologic cost of stress, the simplest approach is to reduce the burden of this cost. Although a single stressor may induce a stress response with a sufficient biologic cost to induce distress, it is probably more common for distress related to husbandry to result from the biologic costs of several small stressors accumulating to result in a total cost of stress that is sufficient to induce distress ( Moberg 1999 , 2000 ). Recognizing this cumulative effect is especially important in developing management practices for laboratory animals. Any one manipulation, such as transfer to a new cage, might be inconsequential to die animal. However, if several such practices are combined within a time frame when the individual costs can accumulate, the animal may experience distress, its welfare may be jeopardized, and the distress stemming from routine husbandry of the animal might confound experimental results. By the same token, what might be viewed as innocuous manipulation of the animal may confound experimental results. for example, a growing mouse restrained for a single 4-hr period will require 48 hr to return to normal body weight and both protein and lipid energy (K. D. Laugero, University of California, San Francisco, and G. P. Moberg, 1998, personal communication). By being sensitive to the potential biologic costs of individual stressors, it is possible to develop husbandry and experimental protocols that eliminate or ameliorate distress, even in the absence of definitive measures of stress or distress.

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Abbreviation used in this article: HPA, hypothalmic pituitary adrenal axis.

© Institute for Laboratory Animal Research

© Institute for Laboratory Animal Research

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