Clinical Scoring of Visual and Behavioral Indicators of Pain

Pain assessment in veterinary practice has typically relied on subjective global pain scales, with clinicians or animal caretakers providing proxy assessments for the animals in their care. Intangible measures such as demeanor, attitude, or lethargy are often cited for clinical scoring of pain but are poorly defined. Hence challenges are likely to occur with interobserver and intraobserver reliability and interpretation. Subjective scoring systems may be improved when descriptions, photographic images (Fig. 3-1), or video clips of behaviors are provided for training clinicians and animal caretakers. A website developed by Dr. Joyce Kent and Dr. Vince Molony, Royal (Dick) School of Veterinary Studies, provides an excellent resource with photographs and video clips of pain responses in several livestock species to common clinical situations (“Guidelines for the Recognition & Assessment of Animal Pain,” http://www.link.vet.ed.ac.uk/animalpain/Default.htm).

Validation of subjective scoring systems for animal subjects is emerging and is a positive step in the evolution of pain research. A standardized facial grimace scale has been validated for assessing pain in mice, including five distinct components, three of which are also observed in the human facial pain grimace.⁸ Livestock may display similar facial grimace components such as orbital tightening and ear position. Because changes in posture are subtle and often characteristic of particular types of pain, it is wise to standardize the visual assessment of the animal, scoring sequentially from head to tail (Table 3-1). Furthermore, changes in motivation and motor patterns are also characteristic of the type and location of pain and should be scored systematically (Table 3-2). Effective pain scoring protocols are critical for evaluating the effectiveness of interventions and determining humane end points when alleviation of pain cannot be achieved. Scoring of affective states such as depression can be refined with transparency regarding the responses recorded and use of specific tests to evoke behavioral responses (Table 3-3). Subjective pain scoring is most refined in terms of lameness and locomotion scoring, with excellent training resources available for bovine and equine practitioners and caretakers. Locomotion scoring systems that provide detailed descriptions of the observable changes in gait and allow observers to score components of the gait separately have been validated for identifying cows with severe hoof lesions⁹ and cows that have received a local anesthetic.¹⁰

Pain behavior responses can also be measured objectively, particularly in experimental applications where detailed observations are collected during live observations or more frequently from video recordings. In ethology, the science of animal behavior, a key component of research methodology is development of an appropriate ethogram, which consists of a list of definitions for mutually exclusive behaviors.¹¹ For transparency and to avoid inherent bias associated with interpretation, definitions that focus on motor patterns or movements that an animal performs are preferred over those described in terms of an underlying motivation. Pain behaviors can then be quantified in terms of frequency and duration of occurrence. Experiments can also be designed to provoke behavior, and quantified in terms of latency to respond and thresholds of stimulus required for response. Although pain-specific behaviors are relatively straightforward to observe and quantify when detailed definitions are provided, manual collection of behavior data is labor intensive and difficult to record because they occur at low frequencies even when the animal is undisturbed by activity in the barn.

New technologies such as accelerometers for frequency and duration of activity and resting behavior are emerging for automated data collection (Fig. 3-2), and they show promise for detecting behavioral changes associated with lameness.¹² Accelerometers are increasingly being incorporated into dairy management software programs, with changes in activity flagged for further investigation by the stockperson. In addition to specific pain responses, general pain responses are associated with changes in motivation during painful conditions. During acutely painful procedures, livestock typically struggle or attempt to escape. Escape attempts can be quantified using exit velocity, determined by the latency for the animal to break an infrared beam, by strain gauges measuring the force the animal exerts on the restraint chute, or by software analysis of head movements captured with video images.¹³

Similarly, in some livestock species distress calls are performed during painful procedures and can be discriminated from other vocalizations such as contact calls by the increase in rate, pitch, and volume.¹⁴ Distress calls given during painful procedures are greater in intensity than other vocalizations such as short calls at lower fundamental frequencies that are emitted from cattle when isolated.¹⁵ However, vocalizations may not be a robust measure of pain for cattle because they rarely vocalize, and propensity to vocalize is associated with breed, perhaps due to variations in excitability.¹⁶ Conversely, goats are expressive vocally, despite also being a prey species.

Nociceptive Threshold Tests

Several standard tests have been developed for assessing nociception in animal models for biomedical research, including von Frey filaments for mechanoreceptors and the Hargreaves’ or plantar test for thermoreceptors. Increasingly, nociception tests are being modified or applied to livestock to ask about the sensory discriminatory perception of pain. To tease apart pain from other aversive experiences such as fear or distress, attenuation of the response should occur when relevant analgesia is applied. Where possible, local anesthesia is preferable to opioids or sedatives due to confounding that results from direct impacts of the latter on animal behavior.

Pressure algometry is an example of a mechanical nociception test that has proven useful for assessing pain in livestock (Fig. 3-3). This tool measures the amount of force applied to a surface, and painful thresholds are determined using a withdrawal response. When applied around the horn buds, calves display significantly lower nociception thresholds following disbudding when compared with pressure tolerated on the day before surgery. Furthermore, calves display less pressure sensitivity when treated with a nonsteroidal antiinflammatory drug versus a placebo solution.¹⁷ Pressure algometry data suggest that the duration of postsurgical pain is greater than estimates using neuroendocrine markers because significantly lower nociception thresholds are observed for 3 days following surgery relative to baseline values.¹⁸ Pressure algometry appears to be a robust tool, with potential for clinical application because it is cheap and easy to use and differences in response are often large.

Thermal nociception threshold is quantified by the amount of time (latency) for an avoidance response to a radiant or laser thermal stimulus. A CO₂ laser thermal stimulator was found to provoke a foot lift or kicking withdrawal response when skin temperature reached 45° to 55° C, and because CO₂ laser stimulators use a monochromatic, long wavelength infrared source of radiation, absorption is not affected by pigmentation of the skin.¹⁹ Standard operating procedures for all thermal nociception tests require the skin surface to be free of hair, manure, and moisture, and safety precautions such as maximum duration of stimulus application must be followed to avoid tissue damage on different skin surfaces.²⁰ A ceiling value of 20 seconds was successfully used as a safety precaution when a radiant thermal nociception stimulus (constant 80% beam intensity, 200C) was used during a lameness study in sows (Fig. 3-4). However, these same criteria were insufficient when the thermal test was used in a disbudding study, resulting in blisters on two calves.¹⁸

Classical pain tests that quantify reflexive nociceptive withdrawal responses to mechanical or thermal stimuli provide information about the sensory-discriminatory aspects of pain, but not the affective (emotional) components. Evidence indicates that some species such as chickens and sheep choose to self-medicate to avoid painful conditions. Conditioned avoidance tests are based on an animal’s ability to recall an association of a stimulus such as a location or a person with a previous aversive experience. Conditioned place avoidance is commonly observed where livestock have previously experienced painful interventions such as branding, dehorning, or castration. Similarly, cattle may associate the appearance or odor of the veterinary practitioner with previous aversive experience, resulting in difficulties with handling. However, it is often difficult to tease apart aversion arising due to experiences associated with pain versus fear. This is especially problematic with extensively managed livestock that are unfamiliar with handlers and equipment. In several species, electrical stimulation results in startle and withdrawal reflexive behaviors, as well as higher pain processes associated with aggression and avoidance behaviors. However, it is used in livestock to facilitate movement (electric prods), for restraint (electroimmobilization), and for semen collection (electroejaculation). Aversiveness of electroimmobilization is evident by conditioned aversion responses by livestock, resulting in reluctance to move through handling chutes or avoidance of particular handlers.²¹ These results have been used to advise against the practice of electroimmobilization, such as animal welfare policies of the American Veterinary Medical Association and Canadian Veterinary Medical Association.