Commercial pigs are frequently exposed to tail mutilations in the form of preventive husbandry procedures (tail docking) or as a result of abnormal behaviour (tail biting). Although tissue and nerve injuries are well-described causes of pain hypersensitivity in humans and in rodent animal models, there is no information on the changes in local pain sensitivity induced by tail injuries in pigs. To determine the temporal profile of sensitisation, pigs were exposed to surgical tail resections and mechanical nociceptive thresholds (MNT) were measured in the acute (one week post-operatively) and in the long-term (either eight or sixteen weeks post-surgery) phase of recovery. The influence of the degree of amputation on MNTs was also evaluated by comparing three different tail-resection treatments (intact, ‘short tail’, ‘long tail’). A significant reduction in MNTs one week following surgery suggests the occurrence of acute sensitisation. Long-term hypersensitivity was also observed in tail-resected pigs at either two or four months following surgery. Tail amputation in pigs appears to evoke acute and sustained changes in peripheral mechanical sensitivity, which resemble features of neuropathic pain reported in humans and other species and provides new information on implications for the welfare of animals subjected to this type of injury.
Tail docking is an undesirable mutilation of pigs. Currently virtually all young piglets are docked in conventional farming so as to prevent tail biting later in life. However, throughout Europe efforts are made to reduce tail docking. Often farmers provide additional enrichment to try and prevent tail biting. Nevertheless, stopping the practice of tail docking may, and frequently does, lead to elevated levels of tail biting, resulting in tail wounds. In relation to this farmers and policy makers would like to know what levels of tail biting would be equivalent to tail docking in terms of pig welfare, i.e. how much tail biting can be allowed before deciding it would be better to continue tail docking. But this poses the problem how to weigh the (lack of) welfare involved in tail biting of a grower or finishing pig against the pain of tail docking of young piglets. Is this possible? And if so, how?
We recently had a brainstorm session on this subject. This is an outline of what we came up with, including a very tentative personal estimate (by MB).
In my personal view when (in the end up to) about 12% of undocked pigs were tail bitten that would be roughly equivalent in welfare to the docking of all piglets. The uncertainty margin, however, is high, at least ranging from 5-25%. The reasoning underlying my estimate is as follows.
Firstly, piglets are normally docked using hot iron cautery. This is quite painful as it involves applying both heat and rather blunt trauma. The heat kills bacteria and thus may reduce the chance of subsequent infection of the tail wound. Tail biting at a later age, by contrast, is caused by even more (and multiple) blunt trauma (due to biting). It also has a substantially higher likelihood of infection. In addition, there is e.g. fear in the tail bitten pig due to being chased by a biter. Based on this I would say that pain (and stress directly related to tail biting) may roughly be about ten times as high in intensity and about ten times as long in duration, compared to tail docking. This would imply that 1 tail-bitten pig is off-set by about 100 docked piglets as regards the intensity and duration of the pain involved.
However, animal welfare encompasses more than just pain. An important additional factor is the level of stress which is not directly related to tail biting activity.
Firstly, there may be stress related to the treatment of tail biting, e.g. when biters and/or victims are taken out of the pen (resulting in social isolation and/or fighting). This stressor, however, is partly offset by the enhanced enrichment normally provided to pigs experiencing an outbreak of tail biting (though not all pigs are equally affected by the ‘costs’ and ‘benefits’). Note that there is another, more macabre, offset involving ‘happiness’ too, and that is the excitement experienced by the (sometimes fanatic) biter pigs when a tail-biting outbreak has started. Note also, that this biter ‘welfare’ is at the same time an indicator of the level of (background) stress experienced by pigs leading to this abnormal behaviour in the first place.
A much more important source of stress that must be taken into account, therefore, is related to the general housing conditions to which the pigs are exposed prior to a tail biting outbreak. Tail biting is an unnatural behaviour that is triggered by (some kind of) stress. Pig farmers are aware of this and will try and prevent tail biting by generally improving the housing conditions when they (start to) raise pigs with intact (undocked) tails. Thus the expected level of stress to which the pigs are exposed is likely to be higher in the case of routine tail docking. When farmers stop tail docking they normally provide much better enrichment (rooting material & space). Farmers raising pigs with intact tails will also take other measures to reduce stress, e.g. provide better climatic conditions, better feed and better health care. These stress-reducing measures don’t just apply to the biters or the victims of tail biting. They apply to all pigs in the pen. Furthermore, they don’t just apply during an outbreak of tail biting, but they apply throughout the pigs’ lives. Hence, the reduced stress levels are a major factor reducing the off-set between docking and tail biting based exclusively on pain (and pain-related fear). I would estimate that the improved living conditions may reduce the off-set by at least a factor 10. This would mean that taking into account both pain and stress, 100(%) docked pigs (kept with minimal care and in a more barren environment) could be roughly equivalent to similarly-sized group of pigs with intact tails under enriched conditions and in which 10% of the pigs has been tail bitten.
Tail biting in docked pigs
However, we know that tail biting does not only occur in undocked pigs. It is also seen in docked pigs. Roughly 2% of docked pigs are tail bitten. It seems safe to assume that the level of pain from being tail bitten is roughly comparable in docked pigs and in undocked pigs (though docked tails may be more sensitive and thus less likely to get bitten). Taking this into account would imply that 100 docked pigs of which 2% also experiences tail biting later in life would be having a level of (poor) welfare comparable to 100 undocked pigs of which 12% gets tail bitten. This is about 6 times as much tail biting as the 2% base-line set under conventional docking conditions.
It must be emphasised again, however, that this level of 12% tail biting is a very rough estimate. So, a wide safety-margin applies, e.g. 5-25%. This may depend in particular on the quality of enrichment and the extra care provided under non-docking conditions.
Please note, that this post is the result of a brainstorm session only and presents a personal view. It illustrates how systematic reasoning (using principles of semantic modelling) can be used to start to answer this rather important welfare question. I have provided a very rough estimate. For a more accurate assessment more detailed studies would certainly be required, both in terms of more carefully including what is already known and in terms of accumulating more empirical knowledge about what is not known yet. At present the assessment is still very speculative, and meant to illustrate primarily how to in principle deal with the question of what level of tail biting is equivalent to a practice of routine tail docking.
Postscript: Excluded aspects and some feedback from readers
Note that, in my estimate I neglected several (minor) aspects.
Firstly, I neglected the fact that for tail docking piglets must be picked up. This results in stress, both in the mother sow and in the piglets. From an evolutionary perspective the procedure of catching piglets may be equivalent to experiencing capture by a predator. This would mean that the given estimate would be a moderate underestimation. However, tail docking may be performed in combination with other treatments such as iron injection and castration. If so, the additional stress from handling may be relatively minor. Note, however, that castration applies only to males and may be banned in the near future, and iron injection may be given orally as a kind of ingestible compost, or as has recently been shown, may not be necessary at all. Hence, combining such treatments with tail docking has a reducing likelihood.
Secondly, I assumed that teeth cutting will not be practiced to treat an outbreak of tail biting, neither in the docked pigs, nor in the undocked pigs. Or, more precisely, at least I assumed teeth cutting is not practiced in substantially different numbers of pig. Such teeth cutting is painful and illegal, so it could be considered appropriate to ignore the practice. However, if it were practiced more in undocked pigs (which are likely to experience higher levels of tail biting), then it would have a substantial impact on the level of equivalence, pushing the percentage back down again substantially.
A third point to note is that I did not include in the estimate other ethical considerations or our (anthropomorphic) emotional responses. An example of the latter may be related to the amount of blood seen in the pen, the farmer’s level of stress (unpredictability) associated to this, and the potentially adverse economic consequences associated with tail biting. An example of other ethical considerations is the fact that tail docking may be considered to be an infringement of the animals’ integrity or intrinsic value. In such a rights-based moral view tail docking may be considered ethically wrong, regardless of the level of tail biting when tail docking is stopped. Such aspects were excluded because these are aspects not directly related to animal welfare. They are more related to our human perception of ethics and/or human welfare, rather than animal welfare.
Finally, it is most important to emphasise that I have considered steady-state conditions, but realize that all practices are subject to optimisation. The practice of tail docking has already been optimised for over a period of at least 50 year. By contrast, the practice of raising pigs with intact tails still more or less has to enter the phase of optimisation in commercial practice. This implies that substantially higher levels of tail biting may be regarded as acceptable, provided this is only temporary and provided it leads to substantially lower levels of tail biting later on. In other words, it requires that farmers will persist in raising pigs with intact tails and have a chance to learn to deal with it over a certain transition period, both in terms of prevention and treatment of tail-biting outbreaks.
Feedback reader 1:
Regarding the painfulness of tail biting vs tail docking, I find it impossible to guess the relation – especially as tail biting comes in so many forms.
I absolutely agree that a weighing like this is necessary, but I also think it is a bit dangerous to throw out estimates that are not really based on any evidence (or at least you do not present any?), such as the 100 times worse pain experienced by bitten pigs than docked pigs. Also, tail biting is very heterogeneous, from just a small, one-time bite, to a chronic situation, where the entire tail is lost, so the way you estimate the pain simplifies the matter greatly.
As to the expected level of actual tail biting when docking is stopped: I estimate a two-fold increase in tail biting if no docking is performed. Perhaps somewhere between 2- and 4-fold, based on e.g. slaughterhouse data. There may be a 4-fold increase when the housing situation is not improved otherwise – which you also take into account in your text – when applying a non-docking policy the farmer would normally also improve housing conditions, thus reducing the risk further. I certainly agree that when a farm stops docking, they will probably have a higher incidence of tail biting initially, but on the long-term (as is shown e.g. in Finland where tail docking is totally forbidden, and the tail-biting incidence, based on abattoir data is around 2%), a 10 or 12% incidence is certainly higher than I would expect.
Feedback reader 2:
Having read your blog I think you need to factor in adaptive, compensatory pain modulation into your model.
It is sometimes too easy to fail to take into account post-injury peripheral and central modulation of pain signalling that occur as part of the normal healing process and only focus on the ‘pro-pain’ component.
I also don’t see how you can substantiate this claim?
‘Based on this I would say that the pain of tail biting may be about ten times as high and about ten times as long, compared to tail docking. This would imply that 1 tail-bitten pig is off-set by about 100 docked piglets as regards the intensity and duration of the pain involved’.
While I think it might be possible to attribute weighting to some risk factors within systems, I don’t think it can be applied to pain experienced by an individual (or even at group level as you are suggesting) because there are so many factors that contribute to an individual’s experience of pain? I don’t think you can quantify the painfulness of tail biting and tail docking.
Also when thinking about stress you might want to define what you mean by that in relation to chronicity?
Short-term compensatory responses to stress are in my view positive for the animal; however beyond that when there is a failure of compensation and ultimately homeostatic decompensation then they are undoubtedly negative.
I guess I’m suggesting that any weighting approach might need to accommodate (or factor in) changes over time (i.e. dynamic weighting?)
I hope you find my comments helpful?
As to substantiation, again, it’s my suggestion for a start of an argument to answer this in my view fairly important question. My answer is based on my personal experience as a vet and scientist, and on reasons indicated in the blog. It is certainly in need of further study, examination and assessment. I fully acknowledge the considerable level of uncertainty as well as the risk associated with trying to answer the question. At the same time, however, I would also argue that there is a considerable risk in refusing to try to answer the question, as this leaves the issue to stakeholders.
Feedback reader 3:
Joining the discussion rather late, but basically I agree with the points others have made. I think it quite reasonable to conceptually set out the trade-offs which would determine the level of tail biting above which tail docking could be ethically justified, but putting numbers on some of these things is rather difficult.
For risk of tail biting in docked and undocked pigs we have a growing number of published sources and comparative national data.
For experimental comparisons we have old data suggesting increases of 30-60% in pigs in unbedded systems.
More recently we have studies suggesting somewhat lower results if straw is given.
So this part is perhaps simple, but depends on your assumptions about which husbandry systems will pertain across Europe.
For the welfare detriment of tail docking and tail biting, data indicate that both have long lasting effects on pain processing pathways, but the implications of this for pain perception for the individual are uncertain.
For tail docking, the data I have seen are still contradictory on whether cautery is more or less painful than simple section (some suggest the cautery destroys the nerves whilst others suggest greater pain). There is also the possibility of tail docking with anaesthesia/analgesia as a route of adoption.
For tail biting, the short term pain will certainly depend on the severity and, even more, on the prevalence of infection. The data on this are currently lacking to my knowledge.
The welfare impairment of keeping in conditions which give rise to tail biting is clearly the greatest of all in magnitude (severity x duration x no of animals) but I don’t think we have any way of comparing the welfare severity of ‘behavioural frustration’ against that of injury/pain. I would be concerned about taking arbitrary figures in the absence of any logical basis.
So, I guess my suggestion would be to explore the framework for this decision, but be very wary about pretending we can quantify it.
I also think the issue not addressed in your blog is the time course of any transition to cessation of tail docking and how to manage this. What proportion of farmers would have the awareness, capital and staff training to implement the changes necessary to their existing housing if obliged to cease tail docking (some older, fully slatted and large group housing systems will pose much bigger challenges and possibly require replacement of buildings), and how long would it take across Europe to reach the ‘acceptable’ situation of relatively low differential in tail-biting prevalence between docked and intact tails, rather than the ‘unacceptable’ differential shown for “one off” change in tail-docking experiments (stopping docking without further improvement of the environmental conditions). I think it important to highlight that your analysis relates to a ‘steady state’ situation and the importance of how any transition is managed and the welfare implications which this will have.
Note that I have not been comparing docking versus non-docking in a mono-factorial way. I compared docking in a more barren environment versus not docking in a more enriched environment supplemented with special attention by the farmer, as that is what will normally happen in practice. I have now emphasised this more clearly in the text.
I largely agree that we currently largely lack the data needed to quantify more precisely. However, I also believe that in principle it is possible to do so, and that the estimate/assessment can be more or less verified empirically (as the body of knowledge accumulates and modelling principles are improved). Personally, I am inclined to try and quantify despite considerable uncertainty, because it provides a better starting point for further discussion. In addition, such preliminary but more science-based estimates are much needed to complement the inevitably politically-loaded figures and personal assessments presented by farmer-representatives and NGO’s arguing either (rather exclusively) against or in favour of ending tail docking as a routine practice to prevent tail biting.
An important point I’ve been trying to make is that pain is not the only relevant aspect of welfare involved in tail docking and tail biting, and that the levels of enrichment and care should also be taken into account. I don’t think it is even possible to honestly say it is not possible to ‘add’ these aspects, since proper political decision making (in all kinds of areas, not just tail biting) simply does and has to, whether it is considered scientifically possible or not. And if so, I would argue it is most reasonable to try and provide the best possible scientific support, while being as honest as possible e.g. about uncertainty margins and the relevance of incorporating more information. I also think the estimate provides broad support to ‘farewell-dock’ initiatives such as those in Finland, Sweden, Denmark, the Netherlands, the UK and Germany.
The FareWellDock factsheets are out. Below you find the cover factsheet as well as the factsheets on tail docking, enrichment, health and the prediction of tail biting. This post shows images of the English versions, and links to the pdf version of the English factsheets, as well as all factsheets in Danish, Dutch, Finnish, French, Italian, Norwegian and Swedish. Separate pages are available directly showing the factsheets in the other languages (Danish, Dutch, Finnish, French, Italian, Norwegian and Swedish).
By M.S. Herskin, P. Di Giminiani, K. Thodberg, 2016. Research in Veterinary Science 108: 60–67.
Lidocain reduced signs of procedural pain during tail docking in piglets but did not affect behaviour during 5 h after the procedure.
Meloxicam had only very marginal effects on behaviour of the piglets during and up to 5 h after tail docking.
Tail docking led to behavioural changes throughout the 5 h observation period.
Tail docking length affected procedural and post-procedural behaviour of the piglets.
In many countries, piglets are tail docked to prevent tail biting. The aim of this study was 1) to evaluate the efficacy of a local anaesthetic and/or NSAID to reduce pain caused by tail docking; and 2) to examine interactions with docking length. This was examined in 295 piglets docked by hot iron cautery 2–4 days after birth and based on behaviour during docking as well as the following 5 h. The study involved three main factors: local anaesthetic (Lidocain), NSAID (Meloxicam) and docking length. Either 100%, 75%, 50% or 25% of the tails were left on the body of the piglets. Irrespective of the tail length, tail docking led to signs of procedural pain, which could be reduced by administration of Lidocain. Preemptive use of Meloxicam did not affect the signs of procedural pain. The results show that tail docking led to behavioural changes throughout the 5 h observation period indicating that effects of this management routine are more persistent than earlier suggested, and suggesting that docking length may influence the post-surgical behaviour of piglets. By use of the present sites of injection and dosages, neither local anaesthetic nor NSAID had marked effects on post-surgical behavioural changes induced by tail docking. Hence, if tail docking is to be performed, more research is needed in order to develop practical methods for on-farm piglet pain relief.
The practice of tail docking is applied to young piglets to avoid a potential later problem of tail biting. But are there effects of docking on piglets? Very few researchers have asked that question, found Dr Pierpaolo Di Giminiani, researcher at Newcastle University, UK.
Is a tail actually a sensitive part of the pig’s body? Dr Pierpaolo Di Giminiani thinks for a while and says, “That is a very good question! I would say it is not more or less sensitive than other parts of the body. The tail is full of neuro-anatomical structures responsible for the pain response. It is comparable to our human skin.” Unlike in many other animals, the pig’s tail may not have a wide range of functions. At best it serves to chase insects away and when in a curl, it gives an indication of the animal’s health. Still, when something happens to that tail, whether this be being docked or bitten, this body part responds like any other.
Dr Pierpaolo Di Giminiani is a research associate with a focus on ethology (animal behaviour) at Newcastle University, UK. During his graduate studies at Linköping University in Sweden, he studied the cognitive impairment caused by anaesthetic protocols in rodents. At his doctorate at Aarhus University, Denmark, he focused on the assessment of behavioural measures of pain in pigs as a result of cutaneous inflammation. Currently, he investigates pain in pigs in relation to the practice of tail docking in piglets and tail injuries in older pigs.
Tail docking – and especially the effects of tail docking on piglets – has been the focus of Di Giminiani’s studies since the beginning of 2014. They form part of the FareWellDock research programme, an international conglomerate funded by the European Union, zooming in on the common problem of tail biting, the preventive solution of tail docking, virtually common everywhere in Europe, and what can be done to overcome both. Especially tail docking of piglets is a practice which is increasingly frowned upon in some European countries. For more information on FareWellDock, see box below.
Di Giminiani’s studies have mainly centred on the question of whether piglets in the short and long term suffer from any pain from tail docking. In an interview with Pig Progress, Di Giminiani points to pain being a very complex experience and it being difficult to measure properly. He says, “I find it fascinating because there is a lot that we can do, especially in a species like the pig. A lot has been done in humans and laboratory animals and we now have the opportunity to apply novel techniques in other animal species. In addition, pain mitigation is often not provided or done so arbitrarily due to the lack of valid measures of pain in non-verbal animals. Therefore, it still remains one of the big open questions in research.”
Measuring pain in animals
In many other animal studies, Di Giminiani explains, research on pain sensitivity is fairly common. Before he set himself onto the theme of pain in piglets, an academic journey took him from his native Italy to San Diego, United States and later Linköping, Sweden, to learn and discover more on pain perception in laboratory rodents.
In pigs, however, everything was different, he says, as similar research appeared to be virtually absent in pigs when starting his PhD in Denmark at Aarhus University. Indeed, a bit strange, considering that pigs have an important role to play virtually all over the world – and considering the fact that from a medical perspective, pigs and humans are very similar.
The current research on pain sensitivity around tail docking in piglets at Newcastle University roughly revolves around three questions, Di Giminiani explains:
Is pain actually occurring or not and how long does it last?
What is the level of pain experience for piglets?
Based on the outcome, what can be done – for instance the use of painkillers?
Di Giminiani says, “Basically, there are 2 common methods to assess pain in animals:
To observe spontaneous behaviour. You just observe the activity of the animals, e.g. how much they walk – their locomotion – how much they lie, how much they stand, how do they drink, how do they eat, etcetera.
“Another method revolves around stimulus and reception – how do animals react to certain controlled challenges? We apply a controlled challenge to evoke a reaction.” This last method had not been applied in pigs a lot, but was used at Newcastle University to figure out short and long-term effects of tail docking on piglets.
Di Giminiani’s research team applied a gas-heated iron for tail docking, so that any wound would immediately be closed to avoid infections. He says, “In addition to measuring responses to controlled stimuli, we developed a grimace scale to measure the facial expressions of piglets. They do seem to grimace, particularly that they squint with their eyes in the minutes immediately following tail docking.” A full scientific paper related to the findings will be sent for publication in the summer of 2016.
For the last three sections of this interview (Pain in animals in the longer term; Does it matter?; FareWellDock) see the original article, as well as several related articles, on the Pig Progress site.
“Freedom from pain, injury and disease” is one of the fundamental aspects of good animal welfare. However, in commercial pig production there are a number of situations where animals may experience pain. This may result from procedures carried out deliberately for management purposes, or from spontaneous health disorders. In order to make decisions on the ethical justification of procedures and the provision of pain alleviation by appropriate anaesthesia and analgesia, it is necessary to assess the intensity and duration of pain experienced by the animals. A number of behavioural, physiological and molecular methods now exist for such assessment but, since pain is a subjective experience which the individual may express in different ways, interpreting these measures can be a challenge. Better methods are required for the practical on-farm assessment of pain and the provision of analgesia when this occurs.
Introduction: Concerns exist over the long term consequences for tail stump pain experienced by piglets after docking, especially in relation to traumatic neuroma development in caudal nerves after docking injury. Neuroma formation may cause detrimental sensory changes in the tail due to altered axonal excitability leading to abnormal sensation or pain.
Aims: To characterize pig tail histopathology at time intervals up to 16 weeks after tail docking and to measure expression of key neuropeptides in caudal dorsal root ganglia and spinal cord neurons associated with (i) peripheral nerve regeneration; activating transcription factor-3 (ATF3), (ii) inflammatory pain; Calcitonin gene-related peptide (CGRP) and (iii) the maintenance of chronic pain; N-methyl D-aspartate (NMDA) ionotropic glutamate receptor subtype 2B (GRIN2B) at the same time points after tail docking injury.
Materials and Methods: Thirty-two female piglets (Landrace/Large White x synthetic sireline) were used (16 docked/16 sham-docked). Piglets were tail docked (amputation of approx. 2/3 of the tail) on post-natal day 3 using a gas hot docking iron. Equivalent sham-docked piglets served as intact controls. Pigs were euthanized by barbiturate overdose 1, 4, 8 and 16 weeks after sham/tail docking. Tail stumps (2 cm) were collected post-mortem for histopathological assessment. Caudal dorsal root ganglia (Ca1-Ca4+) and associated spinal cord were collected for gene expression analysis by real-time quantitative PCR of mRNA.
Results: Non-specific epidermal and dermal changes associated with healing were observed after tail docking. Mild inflammation, ulceration and oedema were present at 1 week. Traumatic neuroma development was a consistent feature from 4 weeks after tail docking. Neuroma axonal dispersion in the tail stump was on-going 16 weeks after tail docking. ATF-3 mRNA was significantly upregulated in caudal DRGs up to 8 weeks after tail docking, but did not differ at 16 weeks compared with sham controls. Both CGRP and GRIN2B mRNA expression was significantly upregulated 1 week after tail docking in caudal spinal cord neurons but were not significantly different from sham-docked pigs thereafter.
Conclusion: Histopathological lesions that occur shortly after tail docking (beyond 1 week) are not likely to induce or maintain pain. The effects of tail docking on peripheral nerve axonal proliferation and dispersion are relatively short-lived and, although still present, are attenuated by 16 weeks after tail docking injury. Changes in peripheral and spinal nociceptive processing associated with possible inflammatory and chronic pain appear to resolve by 4 weeks after tail docking injury.
Introduction: Tail biting is a global welfare problem in the pig industry leading to significant tail injury and potential carcass rejection. The temporal effects of such injuries and subsequent healing are presently unknown, although limb amputation in humans can lead to abnormal neural activity and decreased nociceptive thresholds. In order to evaluate potential sensitisation following tail damage, we created a model by surgical amputation of tails, and assessed mechanical nociceptive thresholds.
Materials and Methods: Surgical tail resection was performed to assess the influence of age, extent of tail amputated and time since amputation on thresholds of mechanical nociception. To evaluate the effect of age at the time of injury, female pigs underwent surgery at 9 weeks (±3 days ‘weaner’) (n=19) or 17 weeks (±3 days ‘finisher’) (n=43). The effect of time after amputation was evaluated on 24 pigs at 8 weeks, and 38 pigs at 16 weeks after surgery. The effect of the extent of tail amputated was assessed by assigning the pigs to 3 treatments (‘Intact’: sham-amputation; ‘short tail’: 2/3 of tail removed; ‘long tail’: 1/3 of tail removed). A Pressure Application Measurement device was used to record mechanical nociceptive thresholds (tail flick or tail clamp withdrawal responses). Within a single session, three stimuli were applied to a skin area proximal to the site of amputation, 3 days pre-surgery, 1 week and either 8 or 16 weeks post-amputation.
Results: Across the two amputation ages, results indicated that tail amputation induced a significant reduction (P<0.05) in mechanical nociceptive thresholds in short and long tails one week after surgery. The same treatment effect was observed at 16 weeks after amputation performed at 9 weeks of age (P<0.05). For surgeries performed at 17 weeks of age, thresholds tended to be lower in short compared to intact tails (P=0.081) and significantly lower (P<0.05) in long tail pigs 8 weeks after amputation. No significant difference was observed at 16 weeks following surgeries performed at 17 weeks of age.
Conclusion: These results show that surgical amputation of pig tails leads to decreased cutaneous mechanical nociceptive thresholds in the skin area proximal to the site of injury. Results indicated that severe tail injury occurring in the weaner period may be associated with sensitisation up to 16 weeks following the injury. In contrast, injuries occurring in the finishing period appeared to be associated with shorter lasting mechanical sensitisation, resolving within 16 weeks.
• Mechanical nociceptive thresholds were quantified for the first time in pig tails.
• The PAM device allowed determining anatomical and age-specific thresholds in pigs.
• A platform for the assessment of painful conditions in pigs is proposed.
The assessment of nociceptive thresholds is employed in animals and humans to evaluate changes in sensitivity potentially arising from tissue damage. Its application on the intact pig tail might represent a suitable method to assess changes in nociceptive thresholds arising from tail injury, such as tail docking or tail biting. The Pressure Application Measurement (PAM) device is used here for the first time on the tail of pigs to determine the reliability of the methods and to provide novel data on mechanical nociceptive thresholds (MNT) associated with four different age groups (9, 17, 24 and 32 weeks) and with proximity of the target region to the body of the animal. We recorded an overall acceptable level of intra-individual reliability, with mean values of CV ranging between 30.1 and 32.6%. Across all age groups, the first single measurement of MNT recorded at region 1 (proximal) was significantly higher (P b 0.05) than the following two. This was not observed at tail regions 2 and 3 (more distal). Age had a significant effect (P b 0.05) on the mean thresholds of nociception with increasing age corresponding to higher thresholds. Furthermore, a significant effect of proximity of tail region to the body was observed (P b 0.05), with MNT being higher in the proximal tail region in pigs of 9, 17 and 24 weeks of age.
There was also a significant positive correlation (P b 0.05) between mechanical nociceptive thresholds and age/body size of the animals.
To the best of our knowledge, no other investigation of tail nociceptive thresholds has been performed with the PAM device or alternative methods to obtain mechanical nociceptive thresholds in intact tails of pigs of different age/body size. The reliability of the data obtained with the PAM device support its use in the measurement of mechanical nociceptive threshold in pig tails. This methodological approach is possibly suitable for assessing changes in tail stump MNTs after tail injury caused by tail docking and biting.