The case of catching poultry

Catching and transport of reproductive animals to another site or of all birds to the abattoir requires capture and carries a strong risk of wounds and fractures as a result of human intervention. As well, laying hens are very susceptible to problems of fracture at the end of their breeding life, particularly when they are removed from their cages. Effectively, they have brittle bones (osteoporosis) which results directly from the heavy metabolic toll associated with laying eggs as well as the cage rearing. The fracture rate can be as high as 25% but this is very variable and depends on whether they are reared in cages or on the ground, the competence of the collection teams and the type of obstructions in the building. Since the 1980s mechanical harvesting of broilers and turkeys has developed wherever the shape of buildings permits. This technique generally means that the harvesting of the animals has a lower level of mortality than when catching is done manually.

The case of force feeding geese and ducks

The consequences of force feeding and the conditions of housing of certain genotypes of fat waterfowl (geese (3%) and ducks (97%)) to produce fois gras and magrets can be a source of pain. Two situations involving stress are envisaged here: (1) the force feeding itself which is practised for 11 or 12 days in the mule duck and 15 to 18 days in the goose. This practice is claimed not to be painful due to anatomical characteristics of the tissue in the wall of the digestive tract of the birds. However it can lead to accidental wounds or encourage pathological dermatological conditions: (2) Hypertrophy of the liver and steatosis (steatosis is lipid storage in the cells of an organ, e.g. in the hepatocytes in the liver where it is called fatty liver. In birds and fish, lipid synthesis takes place overwhelmingly in the liver, whereas it occurs mainly in adipose tissue in mammals.) which is the equivalent of nutritional cirrhosis of the liver with neither tissue damage nor cellular degeneration and is totally reversible. Anatomical studies show that the liver cannot be the source of painful sensations in birds. The average cumulative level of mortality and culling of the ‘weakest’ birds is about 2.5% in force-fed ducks but varies with the rearing conditions and management of the animals. It can be much higher in extreme situations such as heat waves. Further analysis of the causes of mortality and need for culling is necessary to understand better the effects of force-feeding on the physiology of the animal. The existence of painfull stimuli of visceral origin associated with the amount and speed of introduction of the food for force-feeding has not yet been studied.

The case of fish

The protocols for rearing that are currently in use imply that fish are manipulated frequently. They are manipulated in the phases when they are sorted, when their tank or breeding site is changed, when they have veterinary treatment, when they are weighed or when they are taken for slaughter. All of these operations imply that the fish are removed from their breeding environment for a variable amount of time. Techniques presently in use have usually been modernised and use systems that manipulate the fish rapidly while maintaining an aqueous environment at all times. For specific tasks on a small number of individuals, such as applying treatments, removing tissue or marking individual fish, the use of anaesthetic is more or less obligatory for the success of the operation. Where fish production is done under well-run and managed conditions it is unlikely that these sorts of manipulations produce substantial nociception.

Examples from poultry

The degree of prevalence of often-observed foot lesions, such as dermatitis of the foot (pododermatitis), keratosis or swelling, depends on the rearing system and also on the genotype of the birds and the type of equipment. Pododermatitis is common in poultry species either when raised in fully closed conditions or have free range access. This is also true in poultry that have access to a run. Pododermatitis probably causes little pain when it remains superficial but in an advanced stage, such as when ulcerated, these lesions are often infected and the development of pain is very likely. Certain types of litter favour the development of inflammatory pododermatitis. Thus, moist litter is much more likely than dry litter to induce inflammatory responses in the tissue of feet. The prevalence of pododermatitis has been proposed as an indicator to evaluate the level of welfare within a farm and to establish acceptable densities for individual breeding establishments (European directive on broilers). The relevance of its use as an indicator is still under evaluation.

Joint injuries and locomotive problems are a potential source of pain in poultry. Thus, in broiler, there is little movement by the birds. They spend a lot of their time sitting which is in accordance with the rapid rate of growth that characterises the lines of birds used in this type of production. The incidence of postural problems and lameness is also high. Since the beginning of the 1970s, a regular census of lameness in industrial breeding establishments has confirmed this high prevalence. It shows especially the relationship between conditions of chronic physiological nociception in rearing and the behavioural adaptations that are observed such as lameness and postural problems. This confirmation of distressful conditions or true pain is based on several types of observation: (1) The existence of nociceptors in the feet of poultry, (2) anatomical and anatomo-pathological data on the feet and pelvic joints that show several potential sources of pain related to bone deformation together with stress on the joints and dislocation of the tendons or simple tendonitis and (3) studies of behavioural pharmacology that show positive effects of analgesics or anti-inflammatory substances on posture and locomotion. Nonetheless, it has been shown that cortical decerebration of chickens does not inhibit the protective postural behaviour of leg bending to avoid standing on a foot when it is made sore by intra-plantar injection of urate crystals. This suggests that some of the protective behaviour is controlled at the level of the brainstem, thus, in the absence of any conscious emotional component.

Examples from pigs

A significant source of pain in pig rearing arises from musculoskeletal problems, especially those caused by poor stance and walking difficulties and the resulting lameness. This lameness comes from many sources, including genetic factors in particular but also factors associated with the flooring, especially when it is too hard, with a lack of exercise when there is insufficient space and with the feeding regime when it encourages a rate of growth that is too rapid and hence problems with leg conformation.

Examples from cattle

Among the sources of potential pain many can be associated with:

• • restriction of available space for the animal either in stalls or on slats,

• • the quality of the ventilation and the density of the animals, factors that predispose animals to infections such as digestive or respiratory disorders in calves,

• • the quality of the hygiene of the bedding for the animals – a primary source of pathogens that can cause, for example, intra-mammary infections and diarrhoea in calves,

• • the type of flooring which, in cattle, can induce varying frequencies of lameness and foot disorders,

• • the quality of the feed and the modalities of its distribution which, if it is rich in fermentable carbohydrates, can lead to subacute ruminal acidosis along with many possible subsequent pathological problems,

• • the mode of gathering the animals which can lead to agonistic interactions, more frequent combat and thus wounds and bruising, particularly if the group is large.

Examples from fish

Fish being bred domestically are subject to attack from predators all through their breeding cycle, which causes significant losses. For salmonids bred in fresh water the principal predators are birds such as heron and cormorants, and otters and martens. In a marine environment, seals and birds are the main predators. The consequences of these attacks are multiple. The fish may be killed but also they may only be wounded and thereby made vulnerable to infection. Different methods are used to reduce the activity of predators, for example, the use of nets and cages, installation of acoustic or visual systems to scare the birds or seals.

Fin erosion is a major problem that has been described for several species of bred fish, particularly for salmonids in European and American farms. The term fin erosion covers a variety of tissue problems ranging from a simple crack in the skin to necrosis of the tissue and even, sometimes, haemorrhaging. Affected tissues show signs of inflammation with hyperplasia of the epithelium, thickening and formation of nodules. In addition, as nociceptors have been found in fins it is probable that these conditions cause nociceptive responses. However, there are no exiting studies and targeted research will be necessary to confirm this hypothesis. The main cause of fin erosion would be aggressive biting associated with fighting between individuals and the establishment of a dominance hierarchy. However, other, secondary causes are described in the literature such as handling the fish, abrasive substances in the interior of the rearing tanks, sun burn, exposure to stressful situations or to corticosteroids which can modify the structure of the skin and its capacity to regenerate.

In general though, there is little data available on the prevalence of these afflictions (all species considered) in the different systems of rearing.

The case of teeth clipping of piglets

Teeth clipping/grinding is seen as a means of reducing injuries to the teats or vulva of the sow as well as to other piglets in the litter. It involves clipping or grinding the canine and incisor teeth (needle teeth) of both jaws, making eight teeth in all. These milk teeth, which are very sharp at birth, are clipped with a pair of pliers or ground down with an electric grinder by the breeder on the day of birth or soon after. The proportion of the tooth removed varies between 1% and 31% depending on the operator and the tooth being treated. At the behavioural level, the piglets are seen to be defensive during grinding and display strong chewing movements just after teeth clipping/grinding. By contrast, the interval between the intervention and first sucking is unaffected compared with control animals, similarly to what one sees in other behavioural activities such as lying, standing or activity at the udder. Neither is there any evidence of activation of stress-response systems in the minutes and hours following teeth clipping. Histological analysis of longitudinal sections of clipped teeth at different ages shows anomalies that may be potential sources of pain. In particular, one sees breakages in the pulp cavity, dentine fracture, bleeding, pulpitis, abscesses, osteodentines (deposits of calcium that are similar to scar tissue) and necrosis. In all, the frequency of abnormalities is higher when the teeth are clipped with cutting pliers than when they are ground. In view of the fact that the dental pulp is innervated and that the structure of teeth in pigs is similar to that in man, it is highly likely that these sorts of anomalies induce severe pain.

In summary, the information from the literature suggests that there is mild and moderate pain during, and for some hours after teeth clipping to which we can probably add the later discomfort associated with inflammatory reactions and abscesses. Given that several studies suggest that the benefits of teeth clipping are limited, it has been suggested that this technique, which according to the law is not to be used routinely anyway, be abandoned.

The case of beak trimming poultry

Beak trimming and other mutilations of the beak are the practices that have been most studied and are the most controversial. The reason for these practices is to reduce the prevalence and consequences of pecking which can produce wounding and cannibalism. Pecking behaviour is a potential source of pain in animals that are victims and can also lead in certain cases to high levels of mortality in breeding establishments. Such deaths oblige the farmer to carry out beak trimming later in life and so provoke further pain in the beak trimmed animals. In practice, three techniques are used: (1) cutting the end of the beak, or just the upper jaw for ducks, with small secateurs after immobilising the head, (2) cauterisation using a heated blade, a technique called hot blade beak trimming, (3) infrared beak treatment using a machine that focuses a high intensity IR beam at the tip of the beak. The different terminologies associated with operations on the beak are thus a function of the extent of the amputation, the method used and the age of the animals (early if done before 10 days and late if done after 10 days); debeaking (more than a third of the beak sectioned early by cutting), beak trimming (less than a third of the beak sectioned early by cutting) or IR treatment of the end point of the beak realized at time of hatching (less than a third of the end point of beak will fell down latter).

Behavioural and electrophysiological indicators as well as neurobiological knowledge lead us to believe that there is severe pain particularly in the hours after beak trimming. Debeaking is more traumatic than trimming and IR treatment of the beak particularly if the operation is done at a late stage. In the chicken, discharges are seen in the intramandibular branch of the trigeminal nerve during the first 4 h after trimming. In addition, the long-term risk associated with debeaking is the formation of benign tumours of the nerves which are tissue masses formed by uncontrolled and extensive regrowth of Schwann cells and nerve fibres. These tumours, called neuromas, have been described in humans and can provoke severe pain for the life of the animal. This risk is also higher when the amount of beak removed is greater and when the operation is carried out when the bird is 4 weeks or older. Functionally beak trimming affects the anatomical integrity of the bird and, as a consequence much of its behaviour such as ingestion of food and drink, preening of feathers, gathering material for nest building, attack and defence. A hypothesis of chronic pain similar to ‘phantom pains’ experienced by humans after amputations (see pages 297–309) has been proposed whereby the birds have a ‘phantom beak’ but this has been put to question by recent studies on the anatomical organisation of nerve centres in the brain.

The case of tail docking of piglets

The aim in tail docking piglets is to avoid cannibalism post-weaning or during fattening, a problem encountered especially in piglets reared on slatted floors. Tail docking is a routine practice in a lot of piggeries regardless of the sex of the piglets. In 2007 more than 90% of piggeries in the European Union were involved. The tail is cut on the day of birth or soon after, either with a scalpel, cutting pliers or a cauterising knife. The amount of tail removed varies from a few millimetres (the tip only) to more than three quarters of the length. Physiological and behavioural studies indicate that the cutting itself is very probably a source of pain. The tail is innervated along its whole length in newborn piglets and they react defensively and cry out as the tail is cut. However no activity in the stress-response system has been seen in the minutes and hours after tail docking. We could presume that there would be chronic pain following the removal of the tail similar to that described in humans after amputation (see pages 297–309). Indeed, several studies refer to the presence of tumour-like neuromas, known to cause hyperalgesic phenomena, or phantom pains like those in human amputees but this question is still unresolved since it has not been confirmed by behavioural studies in the pig.

In summary, we can suspect mild pain in the hours that follow tail docking and chronic pain in the days and weeks that follow. However the frequency and intensity of chronic pain from this source suffered by piglets are not known.

The case of dehorning cattle

The main aim in dehorning calves is to minimise the risks of animals causing injury to each other or to humans by horn butting. It also allows easier access to feeders and systems of confinement. Practically all dairy calves that are to be reared are currently dehorned. It is common in France to dehorn beef breed heifers between 16 and 24 months of age. Animals of beef and hardy breeds generally have horns that are bigger than those of dairy breeds because they were mostly bred for pulling yokes. Cultural or aesthetic factors enshrined in breed standards have maintained these characteristics. The methods of dehorning should be adapted to take into account the specific conformation of the horns. Late dehorning is more frequent in France than in many other countries particularly Anglo-Saxon countries which have favoured the rearing of genetically polled cattle such as the Aberdeen Angus or the selection of genetically polled cattle including lines of beef cattle such as the Charolaise or Limousine that originally come from France. Despite the fact that the practice of dehorning is covered by precise recommendations in the European agreement and the code of good farming practice, it seems that very few breeders use local anaesthetic and analgesics (around 20% for anaesthetic and 10% for analgesics, see also pages 333–342). Legislation in France prevents them from obtaining anaesthetics and practicing local anaesthesia. Dehorning without anaesthetic or analgesia is known to be painful for both young cattle and adults. It induces, in particular, a big change in cortisol levels and behaviour. Among the methods used, the technique of dehorning (disbudding) by heat cauterisation seems to be less painful than the use of chemical pastes or crayons which can lead to lesions of the eyes if these products run into them. In turn, these methods would be less painful than dehorning by metal shears.

Overall it appears that for dehorning calves:

Disbudding by cauterisation is the least painful method.

The ideal analgesic protocol is a combination of sedation with α2 agonists like xylazine, a preoperative administration of non-steroidal anti-inflammatory drugs (NSAIDs) and local anaesthesia of the cornual nerve before the operation (cf. pages 333–342).

Regulations for dehorning cattle and goats: recommendations about dehorning of cattle are laid out in a Council of Europe agreement on farm animals but there is, so far, no such regulation in France. The agreement makes clear that dehorning by methods other than surgical excision should be banned with two exceptions: (1) cattle <4 weeks old for which disbudding without anaesthetic by cauterisation or chemical means is possible; (2) for cattle more than 4 weeks old dehorning, destruction or ablation at an early stage of the part producing the horn by surgical methods or by cauterisation is allowed if it is done under anaesthetic by a veterinary surgeon or qualified person. Investigations in France and Canada report various effective field practices carried out by breeders, without systematically resorting to an analgesic protocol. The code of good practice proposed by French professionals recommends dehorning of cattle before 6 weeks of age. For the dehorning of adult animals, it is recommended that an analgesic or, preferably, anaesthetic be used.

The case of removing combs and/or wattles from birds

Several procedures are used in birds: (1) removing the comb, known as dubbing, is not often carried out in France. It is often associated with caponisation or with particular genotypes of laying and broiler breeds in which the comb is excessively large. The comb limits the field of vision and is associated with injuries. It reduces feed intake and interferes with copulation; (2) wattle removal is done for the same reasons in reproducing turkeys (Wattle removal is the act of cutting the protruding portion of the caruncle (or wattle), which is a bright red fleshy membrane that covers the head and extends over part of the beak and neck.); (3) operations on the feet, like declawing and despurring are practices that are designed to prevent wounding during interactions between pen-mates during the growth phase (Muscovy ducks) and during mating during the reproductive phase in certain species.

These practices probably cause pain because some of the tissue removed is richly innervated. However, as far as we know, there are no studies that have analysed the degree to which these practices cause pain.

The case of tail docking in cattle

In France tail docking in cattle was practised to avoid injuries to the tail and consequent gangrene after trampling underfoot particularly when animals were run on slatted floors. In other countries, especially in New Zealand it was practised to avoid contamination of the clusters of milking machines by pathogens. It is almost never practised in cattle any more in France because it has now been shown that it gives almost no advantage to the animal except in certain therapeutic applications. In addition, the removal of the tails is a source of acute and chronic pain particularly associated with the development of necrotic growths where the cut is made, particularly when done on adult animals.

This practice is authorised in France, including by the code of practice for organic agriculture, even though it is banned in many countries such as Switzerland, Germany, Denmark, Sweden, the United Kingdom and several states of Australia.

The case of tail docking in sheep

Tail docking of sheep is practised to (1) reduce soiling of the rear end by faeces and urine and consequently reduce fly-strike and (2) facilitate obstetrical practices and avoid complications at parturition. It is performed most often in lambs under 2 weeks of age and is carried out in three major ways: surgically, by cauterisation or by constriction, usually using rubber rings. It is mainly directed towards future breeding animals and especially maiden ewes for reasons of hygiene and the surveillance of lambing. The information in the literature shows that all the methods used are sources of acute and chronic pain even if they do not lead to death. Constriction by rubber rings appears to be the most painful procedure.

Piglets

The major motivation for castrating piglets is to improve the quality of the meat of male pigs by eliminating male odours in the carcass brought on by sexual hormones. In addition, it makes rearing easier by reducing the amount of aggressive and sexual behaviour of the animals and hence the amount of wounding. In Europe 80% of pigs are castrated by surgical means. In France, as in most other European countries, castration is performed by surgery without anaesthesia or analgesic treatment (see pages 333–342 for the cultural, regulatory and economic constraints that explain in part this situation). Behavioural observations and physiological studies show that this practice is a source of acute pain as the tissues involved are innervated. During castration most piglets scream loudly and at a frequency that is characteristic of painful situations. Just after the castration the HPA axis and the sympathetic nervous system are activated but this activation disappears by the next day. The behaviour of the animals also changes following castration. This happens in the first hours after the operation and is marked by a reduction in movement, isolation, prostration, less time at the udder of the sow, stretching of the hind legs, trembling and spasms. Some of these disturbances continue for several days after the operation, for example, huddling up, showing less synchrony of activities with litter mates, scratching the incision in the rump and tail wagging. Finally, there does not seem to be any clear effect of the age at castration on the outcome in terms of the pain caused by the procedure. Even though healing is better when the piglets are castrated very young, it seems that they have a slower growth rate due to being disadvantaged in the competition for teats which results in their use of the less productive ones.

In summary, castration is a practice that has positive effects in the long term on potential sources of pain in intensive piggeries by limiting aggressive behaviour and thus lesions, wounds, lameness and fractures. However the information in the literature clearly shows that pain is acute during castration, then strong for several hours afterwards, followed by more moderate pain for several days following castration.

Cattle and sheep

Castration of cattle and sheep results principally in a reduction in the level of testosterone. There are a range of reasons for castrating. The first is to allow joint rearing of young females and castrated males (heifers and steers for cattle) on the same pastures in grazing systems. Second, it reduces between animal aggressiveness and sexual activity and improves the docility of the animals. It also results in greater marbling of the meat (increased intramuscular fat), a quality appreciated by the consumer, but leads to a slower rate of growth than in entire males. There are three major methods of castration of calves; crushing of the testicular cord, usually by the Burdizzo clamp, constriction, usually using rubber rings and surgical removal.

Studies into castration of cattle show that it is a source of acute and chronic pain. This is the case regardless of the technique used and/or the age of the animal being castrated. However there are some differences in intensity according to the technique and age. Increases in cortisol levels and the incidence of abnormal behaviours and postures are manifestations of this effect. The existence of chronic pain is inferred from specific behaviours associated with the site of castration and abnormal posture. It seems that the intensity of the pain at the time of castration with the Burdizzo clamp is less strong than with surgical castration, although the Burdizzo is rarely used in France because of its cost, its danger to the operator and the possible failures of the technique. Surgery is the technique that causes the briefest pain. It is as dangerous for the operators as the Burdizzo but is much less onerous. There are no failures or setbacks except for possible post-operative problems. The rubber ring is the means of castration most used by breeders, except in the United Kingdom. It induces chronic pain that is more difficult to detect and to manage than that observed following castration by other methods. Finally, the intensity of the pain is proportional to the age of the animal. Only a minority of breeders use anaesthetics to castrate their calves except in Switzerland where its use is mandatory. In contrast most veterinary surgeons use anaesthetics for castration (see pages 333–342). In sheep, the situation is similar: The Burdizzo method is recognised as being the least painful while the rubber ring method is the most practical for its ease of use. However the practice is not common in France.

In summary, it is evident from the literature that castration of calves should be practised:

• • as early as possible; preferably by one week of age and not later than 45 days of age,

• • by using the Burdizzo clamp method,

• • using an analgesic protocol associating administration of NSAIDs 20 min before the operation with a local anaesthetic by infiltrating 2% lidocaine (about 5 ml) into the distal scrotum. Adding the use of an α2 agonist at or around the time of the operation is also worth considering.

Poultry

Castration of poultry, or caponisation, is mainly practised in chickens and more occasionally in guinea fowl. It has a double purpose: (1) to avoid pecking and the significant mortality that can result and (2) to obtain marbled meat of the desired gustatory quality. Caponising consists of removing the testes, which are internal, by way of one or two lateral incisions, most often on conscious animals without anaesthesia or analgesia. Typical behavioural indicators such as vocalisation and raised blood concentrations of corticosterone indicate a high level of stress from the time of capture and restraint onwards then strong reactions during the surgical act. Combined use of a tranquiliser and an anaesthetic has been proposed. General anaesthetics are difficult to use in poultry because there is a wide variation in the duration of effect of the anaesthetic and the first animals to wake up must be immediately separated from their mates to avoid cannibalism and post-surgical complications resulting from them soiling the wounds of their still-anaesthetised companions (see also pages 333–342).

The period before slaughter

This period consists of loading the animals at the farm, transport to the abattoir (with or without passing through a saleyard), unloading at the abattoir and holding them in lairage until it is time to lead them to the stunning area. This period is complicated with several factors that could potentially cause pain. Handling procedures and aggressive interactions between animals are the most frequent reasons for animals having pain. Many steps have been taken by the industry in this domain. The first is the training of the persons associated with transport and handling. The design of trucks has been overhauled and the transport fleet has been improved to a great extent. Handling yards have also been improved considerably to avoid mixing young cattle, to facilitate the movement of animals and to minimise risks for both them and the handlers. It should be pointed out that these improvements concern not only animals being sent to abattoirs but are equally applicable to animals that are transported long distances to change farms. In particular this concerns cattle from dairy herds intended for fattening (young bulls fattened in Italy for instance).

Poultry are gathered either manually or mechanically (see Section 4.2.2.) and transported to the abattoir. At the abattoir, when stunning by electric stunning is in use, the bird is hung upside down by the legs and moved, head down, to an electrified bath. This technique is probably painful, especially if the shackle size is badly adapted to the size of the legs.

Ruminants have to be moved from the lairage area at the abattoir to the stunning box. The most frequently used methods to encourage forward movement of the animals through the facilities comprise not only vocal means, but also include prodding with sticks and electric goads, both of which cause pain. A recent study showed that more than 97% of carcasses of large cattle had contusions, some due to blows with a stick, but also because the animals bash against barriers when being moved or due to fighting among themselves. The frequency and degree of contusions are a function of the density of the animals in trucks, whether or not they have been through a saleyard and the quality of roads and conditions during transport. In the abattoir, these contusions may be due to poorly designed or unsuitable equipment in the raceways and the stunning box (absence of devices for head restraint, swinging rails and anti back-up devices). In slaughter calves, sliding and falling, linked to their being frightened or having spent a long time in transport and waiting at the abattoir, also cause contusions and potential pain.

In pigs, a common practice is to mix several rearing pens of animals together for transport and this leads to fighting between animals, resulting in lesions and wounds.

In fish, the preparation for slaughter requires the animals to be grouped. They are either slaughtered next to the rearing pond, transported a short distance to a slaughter room at the breeding site, or transported over a distance of variable length to a specialised abattoir. All of this requires that the fish be removed from their normal environment and often exposes them to air. At the moment we do not know if exposure of fish to air causes nociception. Nevertheless, harvesting the fish in big nets constrains them and can damage scales or gills, and is a potential source of nociception. Different techniques for loading fish are used depending on the species (by pump, fishnet or conveyor belt). Harvesting and loading result in hormonal changes and behavioural responses that are more or less marked according to the species and the conditions. These responses vary according to the techniques used.

Stunning

Except under exemption, pre-slaughter stunning (either reversible or not) is mandatory. The objective of stunning is to induce rapidly an unconscious state that lasts long enough so as the animal does not regain consciousness during bleeding. The technique itself should not cause pain. The method of choice depends in general on the species. Depending on how the technique is implemented, it may induce psychological stress due to isolation or unfamiliarity with the environment. The main techniques used are: electrical stunning, the captive bolt and stunning by gas.

Bearing in mind the signs covered in pages 310–318 under the heading of slaughter, the source or sources of pain at each of the stages of stunning and/or bleeding were evaluated on the basis of the time taken for the animal to lose consciousness. We will discuss unconsciousness rather than pain, because the state of unconsciousness guarantees the absence of pain.

By passing a current through the brain, electrical stunning consists of inducing a synchronised discharge of the neurones (epileptiform EEG). These neurones no longer function properly until they are repolarised. In addition, the electric stunning causes a release of glutamate and aspartate in the brain which are also partly responsible for the loss of consciousness. Depending on the species (pig, cattle or sheep), and according to the age of the animal, ‘head only’ electric stunning causes epileptiform waves for 25 ± 2 s in calves (250 V for 3 s), or for 65 ±3 s in sheep (200 V for 3 s). We often see a tonic stage then a stage of more or less regular contractions (clonic), of variable length according to species. If a third electrode is put on the breast (‘head-body’ electric stunning), it leads to cardiac fibrillation. Preliminary studies show that ‘head-body’ electric stunning causes cardiac fibrillation that extends unconsciousness and often results in the death of the animal by cardiac arrest. ‘Head-body’ electric stunning therefore seems to augment, in most species, the effectiveness of electric stunning (unconsiousness in 24 ±12 s in sheep), but these results are yet to be confirmed.

In France, electric stunning is used principally for pigs, sheep and poultry. Electric stunning has the advantage of having an instantaneous effect and being employable for all species. One of the major disadvantages of electric stunning, especially when it is automated, is associated with poor handling and difficulties in positioning the electrodes and adjusting the settings. If wrongly used electric stunning can stimulate pain receptors and cause pain without inducing unconsciousness. Depending on the abattoir, the quality of the equipment and the size and shape of the animals, the amount of current passed can be insufficient and painful. Thus, a New Zealand study in 2001 reported extreme values of percentages of unsatisfactory electric stunning varying from 2% to 54% in sheep depending on the circumstances. In these animals, the presence of wool might have contributed to the ineffectual electric stunning.

The second technology for stunning is the captive bolt. The bolt can be penetrating (the most commonly used which produces mechanical tissue damage) or not penetrating. In both cases, the technique produces a percussion, or, in other words, jolts the brain within the skull causing axonal damage. The captive bolt also produces considerable lesions on the skull and in the brain. It can be a reliable and very efficient way of inducing an instantaneous and lasting loss of consciousness. However, in practice, and depending on the type of animal, failure rates may range from 6% to 16% in cattle at commercial abattoirs, and therefore there is a risk of pain.

The third technology is stunning by gas. This comprises immersing the animal in a mixture of gasses, often containing a high concentration of CO₂; in general 40% for poultry and at least 70% for pigs. The behavioural responses of the animals, notably agitation and the attempts to escape, strongly suggest that the perception of this gas is unpleasant, or even painful. In poultry, the addition of oxygen reduces the adverse reactions. The time taken to induce loss of consciousness varies with gas concentration, but is around 17 s for the pig (collapse of the animal) and from 32 to 34 s for poultry (closing of the eyes). The technique is widely used in some countries of Northern Europe. In France, several pig abattoirs have recently been equipped with this system (cf. pages 333–342).

Bleeding

The objectives of bleeding are to cause death of the animal and drain the blood from the carcass. There are two choices of incision site from which cattle and sheep can be bled: the throat (severing both carotid arteries and both jugular veins) or the thorax. For the latter in cattle, the jugular groove is first cut at the base of the neck. In sheep and pigs, thoracic bleeding is done with a single stroke. The lowered arterial pressure and blood loss result in a lack of nutriments and oxygen of the tissues. In the brain, these phenomena prolong and deepen the state of unconsciousness finally leading to death.

In calves and adult cattle, the effectiveness of bleeding from the throat is very variable. This variability is explained by the formation of blood clots, which appears to be related to the retraction of the artery within its connective tissue sheath. Impaired blood flow is reported in some studies to occur comparatively often (in up to 16% of adult cattle and 25% of calves). It is especially a problem if the stunning is reversible (electric stunning) or in slaughter without pre-stunning (special case of ritual slaughter, see below). It postpones the death of the animal and can result in its regaining consciousness in the case of reversible unconsciousness. In general, thoracic bleeding is more efficient than bleeding from the throat as it allows better blood flow, but it takes longer and can only be used if the animal is stunned.

Unilateral bleeding gives poorer results in pigs and poultry than bilateral bleeding.

In the special case of ritual slaughter, the animal is not in a stunned state during bleeding. The throat is cut in a single gesture with a very sharp, long knife. The operator cuts the skin, the muscles, the trachea, the carotid arteries, the jugular veins and the vagus nerve, but leaves the spinal cord and the spinal arteries and veins intact. Cutting the arteries causes a loss of arterial pressure which reduces the exchange of nutrients and oxygen between blood and organs, including the brain, resulting in a variable delay in the animal losing consciousness and dying. The effectiveness of ritual slaughter in inducing unconsciousness varies according to the species, but also according to the technical competence of the operator and the equipment used and also according to the individual animal. In sheep, unconsciousness develops rapidly, after 14 s on average. In calves and adult cattle, there is wide variability in the time taken to loss of consciousness, with extremes from 8 s to 11 min, the longest interval being due to the formation of blood clots. In the absence of such blood cots, isoelectric EEG is acquired between 36 and 54 s. A limited number of studies in the field show that blood cots occur in 17% to 18% of the animals during Muslim (Halal) and Jewish (Shechita) slaughter. In addition, in poultry, as in cattle, depending on the abattoir and the effectiveness of the operator, manual bleeding gives very variable results in terms of the time delay until loss of consciousness. For example, in chickens depending on whether bleeding is uni- or bi-lateral, the time taken to loss of consciousness varies from 60 to 120 s.

There are other potential sources of pain during exsanguination in conscious or poorly stunned animals, such as inhalation of blood into the trachea in cattle or technical problems in poultry (e.g. section of the spinal cord during bleeding), but, so far, there are no studies that confirm the existence of pain linked to these practices.

The specific case of slaughter of farmed fish

Farmed fish are slaughtered by different methods, according to species and their habitat.

Bleeding and asphyxiation in air are methods that bring about slow death (several minutes to several hours depending on the temperature and the species).

Percussive stunning by a blow to the head is a technique which, if correctly performed, causes virtually instantaneous death. It can be done manually or, less commonly, with an automatic mechanical stunning system. This automated system has been shown to be useful for several species but its use is still delicate, and can also make bleeding difficult.

Anaesthesia by CO₂, achieved by saturating the water with CO₂, causes, intense muscular activity in salmon for the first minutes followed by death after 5 min. It is not known if the perception of CO₂ is nociceptive for fish, but, in several species, it leads to changes in plasma levels of hematocrit, glucose and cortisol.

For electric stunning an electric current is passed through the water. With the right settings, it can render eel unconscious (lack of response to a nociceptive stimulus) almost instantaneously. In turbot, this electrical stunning induces a short period of unconsciousness, associated with the secretion of a lot of mucus which could be a sign of discomfort. Unconsciousness can be induced in salmonids by using the appropriate settings of voltage and time.

The partial destruction of the brain by inserting a spike (spiking) requires skilled technical ability, but it causes very quick death to sea bream. However if the procedure is badly carried out, signs of nociception and intense muscular activity have been noted.

Immersing fish in a chilling tank of very cold water (around 0°C) can be considered equivalent lethal anaesthesia for some species. This method is widely used for sea bass and sea bream which are temperate water fish species. It is not suitable for turbot or trout and strong stress reactions have been seen in these two species.

The most adapted method of stunning depends mainly on the species because depending on their habitat, certain fish are very resistant to being immersed in a chilling tank or to an increasing shortage of oxygen. For certain species such as the sea bream, the eel and the African mackerel, the morphology of the skull prevents sufficient transmission of a concussive blow to the brain necessary to induce unconsciousness. Because of the level of precision required, ‘spiking’ is only used for species like salmon and tuna that are of sufficient size, >4 kg, and can be handled individually. Finally, in an attempt to optimise the conditions of slaughter, different methods can be used together. Thus, the method of ‘live chilling’, which associates cold anaesthesia in oxygenated sea water at 1°C, with asphyxiation in CO₂ at 1°C, followed by bleeding has been developed for salmon with good results. Likewise, it is possible to render eels immediately unconscious and maintain them in an unconscious state until they are killed by combining electric stunning with exposure to a strong concentration of nitrogen in the water.