«October 2009 SCIENTIFIC COORDINATOR Pierre Le Neindre, Senior research scientist, INRA (French National Institute for Agricultural Research) ...»
In contrast, there is a more clear-cut position on other species such as fish. In general, fish appear to be more similar to amphibians, reptiles and cephalopods, which indeed have a neural network enabling efficient detection of noxious stimuli, the expression of protective responses, and the ability to remember stimuli which threatened their physical integrity. However, the characterization of emotional components of pain still remains to be established for these species.
Expertise scientifique collective "Douleurs animales" 41
3. How should pain in farm animals be assessed?
To avoid pain, it is essential to identify and if possible to quantify it. In most cases humans can describe and assess their own pain and communicate about it with others. In the absence of verbal or written communication (in babies or non-verbal disabled people, for example), this self-assessment of pain is not possible and it is necessary to resort to behavioural or physiological criteria (hetero-evaluation). In animals, self-assessment is obviously not possible and the problem of evaluating pain is very complex. Numerous scientific reviews and guidelines on the assessment of pain have been published. These studies draw mainly on examples taken from mammals and there are strong similarities between the criteria selected and those used in assessing human pain. What’s more rats and mice are very often used to test drugs to relieve pain in humans.
The discomfort of pain has a high biological value since it favours survival. It warns that tissue damage is taking place, is about to take place or has already occurred, enabling the individual to react in a manner to stop, prevent or reduce the damage that could endanger its health or survival. Most of the criteria for pain assessment correspond with physiological or behavioural changes, the function of which is precisely to stop the cause and / or reduce the effects of noxious stimuli that threaten the physical integrity of the individual (Table 2). These modifications can also be found in states of stress, anxiety or discomfort that do not necessarily involve a nociceptive component, so it is very difficult to identify criteria that indicate specifically the presence of pain. In addition, these alterations can be the cause of reduced animal performance. An approach based on tissue damage can be added to the physiological and behavioural assessment of pain. When tissue damage is identified it can be assumed that there are painful consequences.
Mellor D.J., Cook C.J., Stafford K.J. (2000). Quantifying some responses to pain as a stressor. /In/ The Biology of Animal Stress: Basic 1 Principles and Implications for Welfare, (Moberg G.P., Mench J.A., eds.), CAB International, Wallingford: 171-198. Prunier A., Hay M., Servière J. (2002). Evaluation et prévention de la douleur induite par les interventions de convenance chez le porcelet. Journées de la Recherche Porcine 34: 257-268.
42 Expertise scientifique collective "Douleurs animales" To identify and measure animal pain, one can rely on the monitoring of physiological parameters, observation of behavioural changes, clinical assessment of tissue damage, or reductions in animal performance. It must be borne in mind though that it is not possible to obtain a "pain score" from a simple chemical or electro-physiological test and it is necessary to combine several types of criteria. In this assessment, the criteria for evaluating pain are described for each target species, firstly in ruminants (mainly cattle and sheep) and in pigs, and then in birds and fish. Although it is generally accepted that mammals and birds can feel pain it is clear that there is not the same degree of complexity in the emotional component of pain in all of the species studied. This emotional component implies that the animal is conscious, therefore it is accepted that pain is not felt under general anaesthesia. In fish, we refer to nociception, as the existence of pain in this class of vertebrates remains controversial (see Chapter 2).
Very often, the criteria for assessing pain were defined in situations where pain was induced during common farm practices like, for example, castration in males. These farm interventions and the justification for them are not described in this chapter but are dealt with in Chapter 4.
In the particular context of slaughter, where pain is potentially acute and intense and where animals are usually stunned prior to bleeding/ exsanguination, the approach to pain assessment differs according to the stage in the whole slaughter process. Before slaughter, it is essential to identify the situations that may cause pain (e.g., fights between animals, use of electric goads by the abattoir staff). During slaughter the stunning phase is distinguished from the bleeding phase and the assessment of pain is focused on the animal’s level of consciousness, being the determining factor in its ability to feel pain, and behaviour, which allows the identification of possible signs of pain.
After slaughter the carcass can be checked for the presence of lesions and injuries that may have been source of pain prior to death.
3.1. Measures based on tissue damage Clinical examination of animals, necropsy or histopathological analysis can reveal tissue damage that may cause pain. Fractures, skin lesions, abscesses, inflammation and neuromas are likely to cause pain in mammals and birds, or nociception in fish.
In pigs and ruminants In mammals in general, tissue innervations and pain mechanisms are similar to those observed in humans. Injuries and lesions that cause pain in humans are hence considered to have the same effects on non-human mammals.
Histopathological analysis was used to assess the long-term effects of teeth clipping and tail docking in pigs.
Histological comparison of tooth sections at different ages shows many anomalies when the teeth are cut the day after birth: pulp cavity decay, dentine fracture, bleeding, pulpitis, abscesses and necrosis (see Chapter 4). This approach also was used to determine whether tail docking induces the development of scar neuromas (uncontrolled proliferation of glial cells or axons) known to cause painful phenomena in humans. In pigs, such cellular changes have been observed in histological sections of tail stumps, but their painful nature has not been demonstrated by other means (see Chapter 4).
A much more classic approach is to record injuries, bruises, abscesses and, in extreme cases, fractures. The number and severity of skin or hoof lesions/injuries are among the most frequently used criteria for all species, in addition to tail injuries specifically for pigs. Skin lesions are common when unfamiliar pigs are gathered in a pen/yard and tail biting reflects a form of cannibalism. Assessing the number and the seriousness of such lesions constitutes a measure for scoring the animal welfare status as determined by the Welfare Quality® programme.2 Welfare Quality® is a European research project, one of the aims of which is to develop a scoring system to determine farm animal welfare.
2 This system includes measures for 12 welfare criteria which are pooled to obtain a final score. Among the measures retained figure injuries revealed by lameness and deterioration of the integumentary system (e.g. hair loss or tissue damage), animal health based on an evaluation of troubles in the respiratory (coughing), gastrointestinal (diarrhoea) and reproductive tracts (vulvar discharges), mortality and the replacement rates of the animals. Assessment protocols and a system for integrating the scores into a single overall assessment score have been defined.
Expertise scientifique collective "Douleurs animales" 43 In birds Pathological studies have been conducted in different contexts to cast light on the painful aspect of tissue damage in farmed species. This has been undertaken in studies on beak trimming, which consists of cutting off or otherwise removing the tip of the beak usually before 7 days of age. Examination of birds that have undergone late beak trimming reveals the formation of neuromas which are potentially painful (see Chapter 4).
Evaluation of the plumage condition and the wounds caused by pecking, common in a range of species, provides an indirect assessment of the seriousness of this abnormal behaviour. In a similar way, the scores used to measure the pododermatitis in chickens (skin lesions on the underneath of their feet) enable the discrimination between mere inflammation and secondarily infected ulcers. High scores are associated with withdrawal reactions triggered by touch, suggesting painful phenomena. As shown by this example, additional criteria, in particular behavioural responses, can be used to diagnose the painful nature of a lesion or injury.
In fish Several types of tissue damage are found in farmed fish. The most frequently described are fin or skin erosion, and injuries to the eyes. Such tissue damage has multiple causes (infections, environmental factors, food...) and affects the health of the animals concerned.
At slaughter It is difficult to conduct routine reports on animal pain in slaughterhouses given the work constraints. In most cases, the measurements are performed post-mortem on carcasses. The measures used to assess tissue damage are generally associated with factors that presumably involve intense pain (bruises, broken bones...) arising before or during slaughter. Bruising is mainly due to agonistic interactions between animals (pigs, cattle), sexual behaviour (mounting between bulls) or knocks against the walls of restraining areas and transport trucks. In the case of cattle, sheep and pigs, leg fractures and dislocations occur when animals slip or fall, often due to slippery floors, or lose balance during transport. In poultry, leg and wing fractures or dislocations, as well as intramuscular bleeding, often occur when the animals are loaded onto trucks or shackled by their legs at the slaughterhouse.
Fractured vertebrae may be observed in pigs. This generally occurs during electrical stunning and is therefore only indicative of pain if the pigs were inadequately stunned. In ruminants, evaluation of the impact of the stunning bolt can indicate the degree of stunning achieved and, when there is failure to reach loss of consciousness, give insight into the reasons why (see Chapter 4).
Conclusion Tissue damage is a key element in the identification of sources of pain in all farm animals. However, complementary observations are required to confirm the assessment of the wounds and lesions as having a painful or nociceptive nature, which often proves to be a difficult task.
3.2. Physiological responses
A painful stimulus activates structures of the nervous system directly involved in the perception of pain and emotion. This activation also triggers the pituitary-adrenal axis and sympathetic nervous system, which has numerous effects on the body (e.g. acceleration of heart rate and respiration rate, increase in body temperature, etc…), and on behaviours (Table 2). However, handling the animals or environmental disturbances (noise, general activity...) can induce the same physiological phenomena in the absence of pain. Measurements of the activation of these systems must be conducted under perfectly controlled conditions so that the effects due to pain are not confounded with those caused by stress, environmental factors or by the procedure itself (e.g. stress due to restraining techniques or to the insertion of the needle during blood sampling). Behavioural or clinical observations must complement these physiological measurements to ensure correct interpretation.
44 Expertise scientifique collective "Douleurs animales" In ruminants and pigs Many studies have reported an increase in blood cortisol levels after a painful procedure in pigs (castration), calves (castration, dehorning) and lambs (castration, tail docking). The use of local or epidural anaesthesia reduces the amplitude and the duration of the peak of cortisol after surgery demonstrating the role of pain in the activation of the HPA axis. Several other experiments have also shown that the use of non-steroidal anti-inflammatory drugs prior to surgery (analgesics) can limit the increase in plasma cortisol after castration (calves) or after tail docking (lambs).
Measuring plasma ACTH concentration (AdrenoCorticoTropic Hormone) can also help evaluating the activation of the HPA axis even if it is used much less frequently than cortisol. ACTH increases very quickly and sharply after castration in pigs. ACTH levels give a more sensitive measure than cortisol levels after painful procedures but the concentrations are also more easily affected by the stress associated with handling or by environmental factors (i.e. lack of specificity).
To evaluate the response of the sympathetic system within minutes or hours after an intervention on an animal the blood concentrations of the catecholamines adrenaline and noradrenaline can be measured directly. It is also possible to measure the concentrations of the catecholamines or their metabolites in the urine to study changes over several hours or days. Several other measures can also be used to assess indirectly the activation of the sympathetic system since this system has many effects on the body. For example, respiratory rate, heart rate variability, pupil diameter, surface resistivity of the skin, blood pressure, body or eye temperatures, and plasma concentrations of several metabolites (glucose, lactate and free fatty acids) can be measured. It should be noted that some changes, such as heart rate variability, actually result from a change in the balance between the tonus of the sympathetic system and that of the parasympathetic system. In general, the sympathetic system is very sensitive to the effects of noxious stimuli and the response times are very brief, but it is also very sensitive to animal manipulation and environmental disturbances. One should therefore be even more cautious in interpreting these results than for the HPA axis. Nonetheless, some results clearly show increases in plasma catecholamines and/or lactate as well as heart rate, or a transient decrease in eye temperature after castration or dehorning and there is evidence that local anaesthesia may reduce or even eliminate the changes observed.