Optimal Methods of Documenting Analgesic Efficacy in Neonatal Piglets Undergoing Castration

Two restraining devices in connection to surgical castration with or without local anesthesia: effects on piglet stress

BACKGROUND

Surgical castration causes severe pain to young piglets. In addition, piglets experience intense stress from handling and restraining during the procedure. Furthermore, piglets must be restrained twice when receiving local anesthesia before castration, and the injections are painful. Unfortunately, strategies to reduce piglets' stress during handling, local anesthetic injections, and painful procedures are limited and poorly understood. Thus, we randomized 179 male piglets aged 3 to 4 days to be restrained with either a commonly used commercial tubular bench with a dorsal restraining posture or a custom-made castration rack with a vertical restraining posture. Piglets received local anesthetic or sham injections (mimicking local anesthesia but without skin penetration) 10 min before castration. We then compared the effect of these devices on the piglets' behavioral reactions, vocalizations, and body temperatures at the following four times: when restrained for the first time, when given local anesthetic injections or receiving sham injections, during the second time restrained, and during castration.

RESULTS

Piglets were given higher mean reaction scores when restrained in the tubular bench than the castration rack. Piglets showed differing vocalization patterns in the two devices, with more grunts and screams in the castration rack and more squeals in the tubular bench. Moreover, local anesthetic injections resulted in higher mean reaction scores and longer vocalizations than sham injections during the injections/sham injections but reduced these measures during castration. After castration, the skin temperature was higher in sham-injected piglets than in piglets castrated with local anesthesia, irrespective of the restraining device used.

CONCLUSIONS

Local anesthesia reduced piglets' reaction scores and vocalizations during castration. However, receiving local anesthesia was painful. Based on piglet behavior, restraining in a tubular bench was more aversive than in the castration rack. However, the devices affected the proportions of the different vocalization types during local anesthetic injections and castration in a manner we cannot fully explain. As the restraining method affects piglets' behavior, it may influence pain assessment during local anesthetic injections and evaluation of local anesthesia efficacy during castration. Thus, less stressful methods to handle piglets are needed.

An investigation of the use of ethyl chloride and meloxicam to decrease the pain associated with a single or double incision method of castration in piglets

Castration is a stressful and painful procedure that can impact swine welfare negatively. The objectives of this study were to (1) evaluate the effect of one incision compared to two incisions and the use of a topical vapocoolant (VAPO; ethyl chloride; a topical anesthetic) applied before castration and (2) evaluate the most effective combination in reducing pain in objective 1 and the use of Metacam®; meloxicam before castration on measures of performance, behavior, and physiology. Study 1 consisted of six treatment groups (N = 27 pigs per treatment) and included: nothing (NO); sham castrated (SH); one incision castration (C1); one incision castration plus VAPO (C1V); two incision castration (C2); two incision castration plus VAPO (C2V). Body weights and blood samples were taken at baseline and other time points after castration. Behavior measures were collected for 24 h after castration. Wound scores were collected daily for 10 days. The C1 pigs and C1V pigs were significantly heavier than the other castrated treatment groups but not different from NO and SH pigs. Vocalizations were louder for C1 and C1V pigs (P = 0.0015). Study 2 (N = 40 pigs per treatment) included: nothing (NO); one incision castration (C1); and one incision castration plus meloxicam administered 15 min before castration (C1M). The same measures (performance, behavior, and physiology) were collected as in Study 1. Performance measures and behavior did not differ among treatment groups. Physiological measures were only different for red blood cells (RBC; P = 0.0304). Pigs in C1 and C1M treatment groups had cortisol concentrations that were greater than the NO treatment group at 15 min post-castration (P < 0.05). The data collected give insight into the benefits of one-incision castration compared to 2-incision castration. However, the data only support a lower-level relief from acute pain associated with castration, as it is evident that pigs still experience stress at 15 min post-castration with or without the use of meloxicam. Further research could potentially identify the correct timing, route and dose for the administration of meloxicam.

Efficacy of inguinal buffered lidocaine and intranasal flunixin meglumine on mitigating physiological and behavioral responses to pain in castrated piglets

Managing castration pain on US sow farms is hindered by the lack of Food and Drug Administration (FDA) approved products for mitigating pain. Previous work assessing flunixin meglumine (FM) efficacy in mitigating castration pain has shown the drug to be effective in pigs, meanwhile, results from previous work evaluating lidocaine efficacy are contradictory. Therefore, the objectives of this study were to determine the efficacy of inguinal buffered lidocaine (BL) and FM in mitigating castration pain in piglets. This study was divided into Part I (physiological response) and Part II (behavioral response). For part I piglets were randomly assigned to the following treatments: T1: (C) Castration plus physiological saline; T2: (S) Sham plus physiological saline; T3: (CL) Castration plus BL; T4: (SL) Sham plus BL; T5: (CF) Castration plus FM; T6: (SF) Sham plus FM; T7: (CLF) Castration plus BL and FM; T8: (SLF) Sham plus BL and FM. Blood was collected 24 h prior to castration, 1 h, and 24 h post castration for cortisol quantification. For Part II another cohort of piglets was enrolled and randomly assign to the following treatments: T1: (C) Castration plus physiological saline and T7: (CLF) Castration plus BL and FM. Behavior scoring was obtained in real-time by observing each piglet for 4-min continuously using Unesp-Botucatu pig acute pain scale (UPAPS) at the following timepoints: 1 h before castration (-1 h), immediately post-castration (0 h), and 3 h post-castration (+3 h). Average cortisol concentrations did not differ at -24 h (P > 0.05) or at 24 h post-castration (P > 0.05) between treatments. At 1 h post-castration, castrated piglets (C and CL) demonstrated greater cortisol concentrations (P < 0.05). Castrated piglets in the CF and CLF group had lower cortisol concentrations compared to C and CL-treated pigs (P < 0.05). For behavioral response, there were no differences between treatments on total UPAPS scores (C and CLF, P > 0.05). Intranasal FM was able to effectively reduce the physiological piglet's response immediately post-castration. Inguinal buffered lidocaine had no effect on the either physiological or behavioral response to pain. Long-term research should focus on refining injection techniques for inguinal BL and consider administration frequency and dosing of intranasal FM to control pain for a longer period post-castration.

Validation of the Unesp-Botucatu pig composite acute pain scale (UPAPS) in piglets undergoing castration

To accurately assess pain and support broadly-based analgesic protocols to mitigate swine pain, it is imperative to develop and validate a species-specific pain scale. The objective of this study was to investigate the clinical validity and reliability of an acute pain scale (UPAPS) adapted for newborn piglets undergoing castration. Thirty-nine male piglets (five days of age, 1.62 ± 0.23 kg BW) served as their own control, were enrolled in the study and underwent castration in conjunction with an injectable analgesic administered one-hour post-castration (flunixin meglumine 2.2 mg/kg IM). An additional 10, non-painful female piglets were included to account for the effect of natural behavioral variation by day on pain scale results. Behavior of each piglet was video recorded continuously at four recording periods (24 h pre-castration, 15 min post-castration, 3 and 24 h post-castration). Pre- and post-operative pain was assessed by using a 4-point scale (score 0-3) including the following six behavioral items: posture, interaction and interest in surroundings, activity, attention to the affected area, nursing, and miscellaneous behavior. Behavior was assessed by two trained blinded observers and statistical analysis was performed using R software. Inter-observer agreement was very good (ICC = 0.81). The scale was unidimensional based on the principal component analysis, all items except for nursing were representative (rs ≥ 0.74) and had excellent internal consistency (Cronbach's alpha ≥ 0.85). The sum of scores were higher in castrated piglets post-procedure compared to pre-procedure, and higher than in non-painful female piglets confirming responsiveness and construct validity, respectively. Scale sensitivity was good when piglets were awake (92.9%) and specificity was moderate (78.6%). The scale had excellent discriminatory ability (area under the curve > 0.92) and the optimal cut-off sum for analgesia was 4 out of 15. The UPAPS scale is a valid and reliable clinical tool to assess acute pain in castrated pre-weaned piglets.

Comparative Study of Pain-Related Responses of Male Piglets up to Seven Days of Age to the Application of Different Local Anaesthetics and Subsequent Castration

Simple Summary

Since 2021, surgical piglet castration must be performed with complete pain elimination according to the Animal Protection Law in Germany. General anaesthesia by isoflurane inhalation, which can be performed by the farmer, or by injection of ketamine and azaperone, which must be performed by a veterinarian, are the options available. At present, local anaesthesia is still under debate because of the lack of proof of complete pain elimination and the pain on injection. We tested three local anaesthetics (procaine, lidocaine, and mepivacaine) at two different doses each. Because pain responses can be masked by reactions caused by handling, the piglets were given superficial isoflurane anaesthesia. The pain on injection to the testes was compared with intramuscular injection, and the effectiveness during castration was compared among the local anaesthetics. Nocifensive movements, respiratory rate, blood pressure, heart rate and its variability as well as electroencephalography (EEG) changes were studied in relation to the painful interventions. Most indicators of nociception point to testicular injection pain being beyond intramuscular injection pain when an effective amount of local anaesthetic was used. However, complete pain elimination could not be achieved during castration under local anaesthesia.

Abstract

To evaluate pain responses to intratesticular and subscrotal injection of three local anaesthetics and their efficacy during castration a randomized controlled study was conducted. In groups of 20 piglets, procaine (2%), lidocaine (2%), or mepivacaine (2%) were administered subscrotal and intratesticularly in two different dosages: 0.5 mL of the original substances or the maximum recommended dosage according to body weight diluted with isotonic saline to a volume of 0.3 mL per each injection site. Two placebo groups received the equivalent volume of isotonic saline. A control group was injected intramuscularly with 0.5 mL isotonic saline for injection pain comparison. Electroencephalographic changes, respiratory rate, heart rate and its variability, blood pressure, and nocifensive movements were assessed in superficial isoflurane anaesthesia. While EEG-changes and linear measures of heart rate variability did not appear conclusive, the low frequency/high frequency (LF/HF) ratio corresponded best with the other pain indicators recorded. The injection of 0.3 mL diluted local anaesthetic per injection site elicited significant fewer signs of pain compared to intramuscular injection of saline. However, pain reduction, but not complete pain elimination, during castration could only be achieved with 0.5 mL of the 2% local anaesthetics per injection site, whereby lidocaine and mepivacaine were the most effective.

Piglets' acute responses to local anesthetic injection and surgical castration: Effects of the injection method and interval between injection and castration

Although applied in some countries, efficacy of local anesthetics based on procaine to mitigate acute responses to piglet castration remains questioned. This paper presents results from a factorial study examining the effects of two methods of injection of a procaine-based drug (intra-funicular, IF, vs. intra-testicular, IT), and four intervals between drug injection and castration (2.5, 5, 10, and 30 min) on acute responses of 3–4 day old piglets. The study involved 597 male piglets, and 13 treatments: surgical castration without anesthesia (CC), local anesthesia followed by castration involving all combinations of injection method and interval, and sham handling separated by the same four intervals (SH). Responses of piglets to drug injection, castration and sham handling were evaluated based on quantification of intra-procedural vocalizations and leg movements, as well as saliva cortisol concentration in samples taken before and after castration. No differences were found between IF and the simpler IT injection method. Intervals of 2.5 or 30 min led to stronger piglet responses than the other intervals. Overall, treatments involving anesthesia led to significantly stronger responses than sham handling, during both injection and castration. All treatments, even sham handling, led to a significant increase in saliva cortisol, with no differences between anesthesia treatments and controls. Based on these results, castration 5–10 min after intra-testicular injection of procaine seems to be preferable as compared to the other treatments tested. However, piglets still showed measurable signs of pain and stress during both injection and castration, while handling alone (including the use of a castration bench) triggered a noticeable stress response. In light of these findings, the overall benefit of the procedure in terms of piglet welfare remains arguable.

Role of Topical Anaesthesia in Pain Management of Farm Animals, a Changing Paradigm

Field evidence indicates that livestock producers are motivated by access to products that readily deliver pain management during husbandry interventions and, more recently, viral epidermal infectious diseases, including FMD. There has been impressive adoption in Australia of a farmer-applied spray-on topical anaesthetic wound formulation (TAF; Tri-Solfen®, Medical Ethics, Australia), initially for managing pain of the breech modification ‘mulesing’ procedure that reduces susceptibility of sheep to flystrike. Over 120 million lambs have now received pain relief and cattle producers have commenced using the TAF for a range of husbandry procedures. This product has demonstrated efficacy for surgical castration and tail docking of lambs, surgical castration and dehorning of calves, surgical castration of piglets, debridement of lesions of the hoof for lame cattle and, importantly, treatment of clinical FMD lesions, including decubitus ulcerations occurring from prolonged recumbency. Multimodal use of an NSAID for improved pain management is advocated, particularly meloxicam, available by prescription from veterinarians for injection and as an oral formulation (Ilium Buccalgesic®, Troy Laboratories, Australia), with current work assessing the potential for prolonged delivery in molasses blocks. Increased use of TAF with NSAIDs significantly reduces pain and suffering in livestock, with enhanced healing of FMD lesions, reduced viral loads from Orf infections in lambs and diminished necessity of ‘antibiotic cover’, assisting antimicrobial-resistance (AMR) stewardship.

Piglets' acute responses to procaine-based local anesthetic injection and surgical castration: Effects of two volumes of anesthetic

Surgical castration of piglets is painful, but practiced routinely in commercial pig production. Procaine-based local anesthetics are used to mitigate piglet pain during castration in some countries. Yet, effects of the volume of anesthetic injected remain under-studied. The volume of drug administered may modulate the pain mitigating effect via variation in intra-testicular pressure at injection, potentially leading to pain or discomfort, as well as variation in the dose of active ingredient administered. The present study investigated the effects of injection with two volumes of a procaine-based local anesthetic, 0.3 vs. 0.5 mL per testis, on acute responses of 3-4 day old piglets. A total of 290 piglets were divided into 5 treatment groups: castration without anesthesia, castration after intra-testicular injection of 0.5 or 0.3 mL of drug per testis, and sham handling with one or two stays in a castration bench. Acute responses to drug injection, castration and sham handling were evaluated based on quantification of intra-procedural vocalizations and foreleg movements, as well as saliva cortisol concentrations before and after castration. Regardless of the volume, injection of anesthetic as well as castration led to significantly stronger responses than sham handling. Responses to the two drug volumes did not differ significantly, and responses to castration following injection of 0.3 mL did not differ from piglets castrated without anesthesia. All treatments, including sham handling, led to a significant increase in saliva cortisol, and no difference was found between anesthesia treatments and sham handling. Overall, the results suggested that injection of 0.5 mL led to better pain mitigation at castration compared to injection of 0.3 mL, but even when the local anesthetic was used, the combined procedures of injection and castration led to intra-procedural signs of pain and stress.

The Neuroimmune Interface and Chronic Pain Through the Lens of Production Animals

Communication between the central nervous system (CNS) and the immune system has gained much attention for its fundamental role in the development of chronic and pathological pain in humans and rodent models. Following peripheral nerve injury, neuroimmune signaling within the CNS plays an important role in the pathophysiological changes in pain sensitivity that lead to chronic pain. In production animals, routine husbandry procedures such as tail docking and castration, often involve some degree of inflammation and peripheral nerve injury and consequently may lead to chronic pain. Our understanding of chronic pain in animals is limited by the difficulty in measuring this pathological pain state. In light of this, we have reviewed the current understanding of chronic pain in production animals. We discuss our ability to measure pain and the implications this has on animal welfare and production outcomes. Further research into the neuroimmune interface in production animals will improve our fundamental understanding of chronic pain and better inform human clinical pain management and animal husbandry practices and interventions.

Proposed multidimensional pain outcome methodology to demonstrate analgesic drug efficacy and facilitate future drug approval for piglet castration

Castration of male piglets in the United States is conducted without analgesics because no Food and Drug Administration (FDA) approved products are labeled for pain control in swine. The absence of approved products is primarily due to a wide variation in how pain is measured in suckling piglets and the lack of validated pain-specific outcomes individually indistinct from other biological responses, such as general stress or inflammation responses with cortisol. Simply put, to measure pain mitigation, measurement of pain must be specific, quantifiable, and defined. Therefore, given the need for mitigating castration pain, a consortium of researchers, veterinarians, industry, and regulatory agencies was formed to identify potential animal-based outcomes and develop a methodology, based on the known scientific research, to measure pain and the efficacy of mitigation strategies. The outcome-based measures included physiological, neuroendocrine, behavioral, and production parameters. Ultimately, this consortium aims to provide a validated multimodal methodology to demonstrate analgesic drug efficacy for piglet castration.Measurable outcomes were selected based on published studies suggesting their validity, reliability, and sensitivity for the direct or indirect measurement of pain associated with surgical castration in piglets. Outcomes to be considered are observation of pain behaviors (i.e. ethogram defined behaviors and piglet grimace scale), gait parameters measured with a pressure mat, infrared thermography of skin temperature of the cranium and periphery of the eye, and blood biomarkers. Other measures include body weight and mortality rate.This standardized measurement of the outcome variable's primary goal is to facilitate consistency and rigor by developing a research methodology utilizing endpoints that are well-defined and reliably measure pain in piglets. The resulting methodology will facilitate and guide the evaluation of the effectiveness of comprehensive analgesic interventions for 3- to 5-day-old piglets following surgical castration.

Efficacy of Intra-Operative Topical Wound Anaesthesia to Mitigate Piglet Castration Pain-A Large, Multi-Centred Field Trial

Simple Summary

Piglet castration causes pain and stress to the animal. Although desperately needed, there are complexities developing safe and effective methods of pain alleviation applicable for on-farm use. Topical anaesthesia, instilled to the wound during surgery, is a newly evolving on-farm method to mitigate castration pain. In the current study, we investigated the use of Tri-Solfen^® (Animal Ethics Pty Ltd, Melbourne, Australia), a topical local anaesthetic and antiseptic formulation, instilled to the wound during the procedure, to alleviate subsequent castration-related pain experienced in piglets. We performed a large, blind, multi-centred trial comparing pain in piglets castrated with or without Tri-Solfen^®. Piglets treated with wound instillation of Tri-Solfen^®, 30 s prior to subsequent castration, showed significantly lower pain-induced motor and vocal responses during the procedure. Acute post-operative pain-related behaviours, evident in piglets in the first 30 min following castration, were also significantly reduced in treated piglets compared with untreated piglets. Using this method, Tri-Solfen^® provides an effective on-farm method to mitigate acute castration-related pain in young piglets.

Abstract

Piglet castration results in acute pain and stress to the animal. There is a critical need for effective on-farm methods of pain mitigation. Local anaesthesia using Tri-Solfen^® (Animal Ethics Pty Ltd., Melbourne, Australia), a topical local anaesthetic and antiseptic formulation instilled to the wound during surgery, is a newly evolving on-farm method to mitigate castration pain. To investigate the efficacy of Tri-Solfen^®, instilled to the wound during the procedure, to alleviate subsequent castration-related pain in neonatal piglets, we performed a large, negatively controlled, randomised field trial in two commercial pig farms in Europe. Piglets (173) were enrolled and randomised to undergo castration with or without Tri-Solfen^®, instilled to the wound immediately following skin incision. A 30 s wait period was then observed prior to completing castration. Efficacy was investigated by measuring pain-induced motor and vocal responses during the subsequent procedure and post-operative pain-related behaviour in treated versus untreated piglets. There was a significant reduction in nociceptive motor and vocal response during castration and in the post-operative pain-related behaviour response in Tri-Solfen^®-treated compared to untreated piglets, in the first 30 min following castration. Although not addressing pain of skin incision, Tri-Solfen^® is effective to mitigate subsequent acute castration-related pain in piglets under commercial production conditions.