«October 2009 SCIENTIFIC COORDINATOR Pierre Le Neindre, Senior research scientist, INRA (French National Institute for Agricultural Research) ...»
48 Expertise scientifique collective "Douleurs animales" Birds inhibit some spontaneous movements and postures to avoid stimulating specific painful areas. As an illustration, late beak trimming of chickens and turkeys reduces pecking. Removing the end of the beak may temporarily reduce food intake, drinking and pruning. As in mammals, lameness is sometimes observed, with birds adopting a posture that allows them to avoid stimulating a painful area of a leg. In comparison to healthy animals, lame chickens spend less time walking, feed less frequently and are more often seen lying when they eat. The time spent lying or standing on one leg decreases after an analgesic treatment and the effect on behaviour is dose dependent.
It seems that after a while chickens that cannot escape from painful stimuli cease to show defensive or avoidance reactions. Feather-pecked chickens, for example, eventually adopt a lying posture with the head drawn into the body.
In fish In very few studies has the relationship between behaviour and nociception in fish been analysed. In the studies that have been conducted, behavioural responses were not measured in a systematic and standardized manner.
However, the results indicate that some fish are able to learn to avoid noxious stimuli. In studies on trout that were submitted to a noxious stimulus (subcutaneous injection of acetic acid) the fish displayed avoidance behaviour, stopped ingesting food and expressed behavioural changes, such as rubbing the injected area and body swaying.
These behavioural changes could last for several hours but were diminished when an analgesic treatment (morphine) was given which shows that they were the direct consequence of the injection of acetic acid.
At slaughter Numerous studies describe aggressive interactions (pigs, cattle), mounting behaviour (bulls), slipping and falling (all species) as possible sources of pain. Behavioural indicators (drooping posture, avoidance reactions, for example) can also be used to assess the level of consciousness.
Conclusion Behavioural responses are very sensitive criteria for detecting pain and are suitable for observations on farm or in slaughterhouses. However, considerable methodological precautions are required and the conditions of observation of the species as well as the animal’s physiological state and experience must be taken into account in the interpretation. Observers should be sufficiently trained to identify and interpret the behavioural indicators of pain correctly.
3.4. Criteria related to livestock productivity
Pain may have detrimental effects on some behaviours, like feeding, and may stimulate the release of stress hormones that affect metabolism, immunity and reproductive function. A decrease in livestock productivity can be expected which is the case for growth rate (average daily weight gain), milk production, egg production and feed conversion ratio. However, as opposed to the behavioural and physiological impairments, these changes do not take place immediately. They appear at a later stage if the impairments persist for a length of time. They can still be useful criteria for alerting farmers, particularly when it is not possible to observe all animals individually. In addition, animal mortality can be used as an on-farm indicator of pain as it is likely that death is preceded by painful phenomena.
In pigs and ruminants Assessing the direct impact of pain on cattle production has been the purpose of only a few studies. The adverse effects of pain are more commonly extracted from work investigating the impact of stress on economic criteria and on agricultural productivity or on health problems in cattle. Some studies do show that the use of local anaesthetics to reduce pain limits the weight loss that usually results from castration.
Expertise scientifique collective "Douleurs animales" 49 In cattle, stress has detrimental effects on the reproductive performance of males and females, due to interference with the secretion of sex hormones. In general, musculoskeletal disorders affect the reproductive performance in both sexes as well as milk production in cows, but whether this is due to a lower feed intake, the activation of the HPA axis or inflammatory processes is unknown. The economic impact of other diseases has been quantified in dairy cattle (e.g. mastitis) however, as in the case of lameness, what is attributable to the disease itself cannot be distinguished from what is due to pain. Nevertheless, studies comparing sick animals that either received or did not receive analgesic treatment suggest that pain plays a significant part in the economic loss. There are hardly any studies on beef cattle available.
In pigs, several authors have tried to characterize the impact of teeth clipping, tail docking and castration on the growth of piglets in experimental situations. These interventions do not seem to affect the growth rate, excepting when the intervention penalizes the piglet’s access to the teats of the highest milk-producing glands of the udder (as was the case in studies on selective teeth clipping where only some piglets in the litter had their teeth cut) or in the case of castration performed on males before 3 days of age. Tail biting may cause reduced growth rate, increased morbidity and mortality, and any injured parts of the carcass may be seized at slaughter after inspection and removed from sale. Lameness may be the cause of reduced growth in young pigs, low reproductive performance in mature animals, and premature culling of sows and boars from the piggery.
In birds In poultry, feather pecking can be a major welfare problem in laying hens since victims of repeated pecking show apathetic behaviour and reduced food intake leading to reduced egg laying. Beak trimming is carried out to alleviate this problem (see Chapter 4). It is known that lameness may result in reduced growth rates or even mortality but overall there is little data on the consequences of pain on animal production. As for mammals, pain is often associated with diseases that may themselves have a direct effect on the performance of the animals, in which case it is difficult to identify the part played by pain itself. In addition, because of bird density it is impossible to evaluate individual performances on a poultry farm, which makes the use of criteria based on animal productivity to identify pain difficult when only a small percentage of birds are affected.
In fish In the only studies in which the negative consequences of exposing farmed fish to stress were investigated, the focus was on animal performance (growth, reproduction, immunity or adaptation). In none of these studies were the effects of noxious stimuli taken into account.
Conclusion The direct impact of pain on livestock performance has been assessed in very few studies. Criteria based on livestock productivity are of little use in assessing pain and are most often used in addition to other criteria or as warning signals, especially when the observation of individual animals is impossible.
3.5. Multi-parametric scales for assessing pain
The physiological and behavioural responses to pain provide a clinical overview from which pain can be assessed in a reasonably objective manner. The simultaneous observation of several indices evocative of pain is indeed correlated with a higher probability that the animal in fact feels pain.
Furthermore, a multi-parametric approach is recommended in the assessment of pain in animals as it appears that pain intensity is usually proportional to the number and the severity of the indices observed.
Although each individual parameter cannot reflect the level of pain taken on its own, there is an overall relationship between all the clinical signs observed and the level of pain. This global view should enable the assessment of the intensity (low to high), the frequency (occasional to continuous), the duration (acute to chronic) and the type of pain
In pigs and ruminants There are no real multi-parametric pain scales for these species, but relatively simple scales for assessment are available to help detect locomotor problems, particularly in cattle (Table 3). These are only based on observation of animal postures and gait.
More complex evaluation scales ought to be developed for assessing pain on farm. Criteria based on tissue damage and behaviour could be used on farm for pigs and ruminants. The observations could be made by farmers, veterinary surgeons and technicians even though a training course might be necessary. One could add criteria based on livestock productivity knowing that they are generally less sensitive than behavioural criteria, as well as some basic physiological parameters like changes in respiratory rate. These evaluation scales could be designed using some of the measures that were selected for use in the Welfare Quality® project on the assessment of the welfare of farmed animals, such as those specific to tissue damage and health (lameness, coughing, diarrhoea).
Any information gathered in slaughterhouses on carcasses being removed by an inspector because of health problems or cannibalism, should also be included.
In birds No multi-parametric scales are available in poultry for the identification of pain or the measurement of pain intensity. The only scoring system available is based on observations of walking activity to measure the severity of locomotor abnormalities. Behavioural criteria are very rarely used given the difficulty posed by the huge numbers of animals raised together on poultry farms. In consequence, the assessment criteria could be based on tissue damage and livestock productivity instead.
Existing multi-parametric scales Multi-parametric scoring systems are only available for rodents, dogs and cats (post-surgery care) and horses.
They cannot be extrapolated directly to pigs, ruminants and poultry in on-farm situations but they will be mentioned as examples. These scoring systems have in common a strong emphasis on behavioural parameters. In most cases, the aim is to detect abnormal behaviours induced by pain. However, behaviours are specific to the species, the breed, the individual temperament and the context more than clinical and physiological variables are. In addition, the location and the type of pain, and its source, influence behavioural expression. It is therefore necessary to validate the behaviour displayed in a specific situation for each species and each physiological state.
In France, the most frequently used scoring system in veterinary medicine was developed by the Veterinary Association for Animal Anesthesia and Analgesia for the care of dogs after surgery. It takes into account a global subjective assessment of pain, the general demeanour of the dog, its interactions with the observer, its heart rate and its reaction to manipulation of the operated area.
This type of scoring system reduces the inter-observer variability. A score is assigned for each parameter. The total score provides information on pain intensity and is compared to the classification of the World Health Organization (WHO) so that the most appropriate treatment can be given if treatment is deemed necessary. Three levels of pain
have been defined in this manner, corresponding to three types of treatments with increasing antalgic properties:
NSAIDs (e.g. salicylic acid), weak opioids (e.g. codeine) in combination with NSAIDs, and strong opioids (e.g.
butorphanol) given in combination with NSAIDs.
The decision to change from one level of pain treatment to the next is taken after the assessment of the pain experienced by the patient and the pain relief afforded by the existing treatment.
A multi-parametric scale has been developed recently for horses that were subjected to experimental orthopaedic pain (Table 4). This scale was evaluated by comparing animals receiving different types of painkillers. The results suggest that the behavioural responses, including posture and pawing, and the reactions to palpation, are the best criteria because they are reproducible, sensitive and specific.
Bussières G., Jacques C., Lainay O., Beauchamp G., Leblond A., Cadoré J.L., Desmaizières L.M., Cuvelliez S.G., Troncy E. (2008).
4 Development of a composite orthopaedic pain scale in horses. Research in Veterinary Science 85(2): 294-306.
52 Expertise scientifique collective "Douleurs animales" Conclusion Whatever the level of sophistication of the scoring systems, pain can only be assessed correctly if the evaluator has been properly trained and if the scoring system is well adapted to the species and the situation encountered.
In other words, the method of evaluation will be different for pain-related lameness in sows, mastitis in dairy cows and feather pecking in broilers. The considerable task of developing and validating scoring systems for assessing pain in farm livestock has yet to be undertaken.
The various types of measures available for assessing pain can be combined to evaluate pain in farm animals as objectively as possible. The criteria for assessment are more or less detailed depending on the species considered (Table 5) with a much wider range of criteria for mammals than for birds, and even less for nociception in fish.
The existing measures, whether they are based on tissue damage, physiological responses, behavioural responses, or livestock productivity, are not sufficient to make a reliable diagnosis of pain in farm livestock when used individually. The solution therefore lies in the development and validation of multi-parametric scoring systems that are based on the combination of these criteria. Such methods have been developed in other species for postoperative care (dogs, horses) and should be adapted for use with farm animals. This will be challenging as these new scoring systems must be adapted for routine on-farm use within the constraints posed by the rearing conditions.
Given the progress in knowledge of farm animals, the requirements for future research on pain assessment differ according to the species. There is a need for developing and validating multi-parametric scoring systems for ruminants and pigs, identifying and validating criteria to characterize nociception in fish, and for research in all areas for farmed birds. Concerning slaughter, research is needed to clarify the relationship between physiological and behavioural responses of unconsciousness, and between these criteria and the absence of pain.