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may withdraw from contact with other animals. They may become listless and refuse to eat or reduce their eating and drinking. They may avoid being handled. These are all possible signs of pain, but none alone is sufficient to determine the presence or level of pain. For example, many animals vocalize intensely when they are handled even if they are not in pain (e.g., Stafleu et al., 1992). Multiple criteria should therefore be assessed (Bayne, 2000; Wallace et al., 1990).
An important step in determining that an animal is in pain is recognition of a departure from normal behavior and appearance (Dubner, 1987; Kitchen et al., 1987; Morton and Griffiths, 1985; NRC, 1992). But as Bayne (2000) indicates, assessments vary with the scale used, and the scales can be very subjective. Flecknell and Silverman (2000) noted that preprocedural scoring is necessary to obtain an appropriate baseline so that confounding variables (such as behavioral effects produced by analgesics) can be identified. For example, some of the consequences of surgery in rats, such as loss of body weight and reduction in food and water intake (signs frequently interpreted to indicate pain or distress), can also be produced in normal, unoperated-on rats by administering opioid analgesics.
Recent evidence indicates that some signs of pain may not be perceived by personnel, such as the ultrasonic vocalizations of infant mice (Nastiti et al., 1991), but are detectable with appropriate equipment. Several excellent references discuss species-specific behaviors that are indicative of pain (Carstens and Moberg, 2000; Hawkins, 2002; Morton and Griffiths, 1985; NRC, 1992; Roughan and Flecknell, in press; Soma, 1987; Wallace et al., 1990).
Assessment of pain should not be influenced by the biases of the observer (Sanford et al., 1986), and the observer should be well trained in both normal and abnormal behaviors of the species in question. Variability among observers can have a substantial effect on the interpretation of assessment data (Holton et al., 1998).
Chronic or persistent pain differs from acute pain because it may not be associated with any obvious pathologic condition and does not serve any protective function. Signs of chronic pain can be subtle and difficult to detect in that an animal’s behavior may change slowly and incrementally. Chronic or persistent pain is also more likely to lead to distress and maladaptive behavior. Signs of chronic or persistent pain include decrease in appetite, weight loss, reduction in activity, sleep loss, irritability, and decrease in mating behavior and reproductive performance (Soma, 1987). Alterations in urinary and bowel activities and lack of grooming are often associated with persistent pain. Severe chronic pain can reduce body temperature, cause a weak and shallow pulse, and depress respiration. As noted above, animals cannot control chronic or persistent pain, and it is important to assess the intensity of the pain by using behavioral measures.