Abstract
Background
Pigs pose distinct challenges to the anaesthetist due to their temperament, the limitations of manual restraint and species‐specific anatomical features. Despite the importance of minimising stress during clinical procedures, gentle handling techniques tailored for pigs in a clinical setting have yet to be clearly described or systematically investigated.
Methods
Sixteen healthy domesticated pigs were prospectively enrolled in a clinical study evaluating ‘combing’ with a fork as a gentle tactile handling technique in a hospital setting and randomly assigned to either a combing or a control group. Responses were evaluated by a non‐blinded, single observer using a seven‐domain sedation scale and a dichotomous response scale.
Results
Sedation scores were significantly higher for combing than control. The onset of peak sedation time was significantly faster for combing than control (5 vs. 10 minutes). Fewer (12.5%) pigs in the combing group moved away from the injection compared to 75% of control pigs.
Limitations
Limitations included the injectable drug protocol used, the lack of blinding with a single observer aware of group allocation, and more than one individual performing the ‘combing’ protocol.
Conclusions
‘Combing’, a gentle handling technique performed prior to intramuscular sedation, resulted in pigs being less reactive to the injection, and faster sedation onset.
INTRODUCTION
General anaesthesia is always required for pigs undergoing advanced medical procedures such as diagnostic imaging and surgical interventions. Comprehensive physical examination in swine is sometimes limited by the difficulty of effective restraint. Their aversion to handling frequently provokes intense vocalisation, which can obscure auscultatory findings, particularly during cardiopulmonary assessment. Gaining venous access prior to sedation can also be technically challenging due to their temperament and resistance to handling. 1 Additionally, certain pigs are genetically predisposed to malignant hyperthermia, or porcine stress syndrome—a potentially fatal condition triggered by stress or specific anaesthetic agents. 1 Handling‐induced stress may further lead to a range of pathophysiological responses, including hyperthermia and respiratory distress. Therefore, all pre‐sedation handling should be performed as efficiently and calmly as possible to minimise physiological compromise.
Acclimatisation, habituation and training are well‐established techniques used in laboratory pigs to reduce stress associated with handling and experimental procedures. These methods involve repeated, low‐stress exposure to human interaction and routine procedures, allowing pigs to become conditioned to their environment and handlers over time. 2 Proper handling and environmental consistency are critical in minimising stress and anxiety in laboratory animals, with personnel being specifically trained to effectively implement these strategies. 2 However, in clinical settings—where pigs present as patients—such time‐intensive conditioning is often impractical. The urgency of medical care and the unpredictability of patient presentations limit the feasibility of implementing prolonged behavioural training prior to administering sedatives by intramuscular (IM) injection.
In this context, ‘combing’ (also known colloquially as ‘forking’), a gentle tactile intervention, has emerged as a practical and time‐efficient alternative to traditional behavioural conditioning techniques. This method appears to exert a calming effect on pigs, facilitating handling and potentially improving both patient welfare and procedural success in veterinary clinical environments. Anecdotally, ‘combing’ involves the use of a plastic fork applied in a combing‐like motion along the dorsum, neck and trunk with mild to moderate pressure. It has been employed in clinical practice to calm pigs prior to IM injection for sedation or injectable anaesthesia. 3 Most pigs exhibit a relaxed demeanour during ‘combing’, suggesting a potential anxiolytic effect; however, individual variation exists, and some animals may remain apprehensive or resistant to the additional handling. Given the limited tolerance of pigs to IM injections—often evidenced by vocalisation or abrupt withdrawal responses—‘combing’ may offer a valuable, low‐stress strategy to improve compliance and reduce handling‐associated distress in clinical settings. Despite its apparent utility in practice, however, a literature review identified no studies assessing ‘combing’ (or ‘forking’) as a pre‐sedation handling technique in domesticated pigs.
This study aimed to evaluate the effects of a gentle tactile intervention, referred to as ‘combing’, on pigs undergoing clinical procedures in a hospital setting. Specifically, it sought to assess (a) the behavioural response of pigs to an IM injection, (b) the time to onset and depth of sedation, and (c) the need for additional sedative administration to facilitate further handling.
The alternative hypotheses tested were that ‘combing’ would (a) attenuate adverse behavioural responses to IM injection of a sedative combination, (b) reduce the time required to achieve peak sedation, (c) result in higher sedation scores, and (d) decrease the frequency of supplemental sedative administration needed for subsequent handling.
MATERIALS AND METHODS
All study protocols and procedures were approved by the Ohio State University Institutional Animal Care and Use Committee (IACUC protocol number 2023A00000009). Prior to commencing the study, pigs were randomly allocated to one of two groups using a permuted block technique (www.randomization.com): group combing (n = 8) or group control (n = 8). The study aims and methods were discussed with the pigs’ owners, and informed consent was obtained from all owners who chose to enrol. This was a prospective, randomised, open‐label, clinical trial.
Animals
Sixteen healthy, American Society of Anesthesiologists (ASA) status I‒II, client‐owned pigs of varying breeds (Sus scrofa domesticus) were included in the study. Pigs presented to the Ohio State University Veterinary Medical Center—Farm Animal Medicine and Surgery Service (FAMS) for elective procedures between March and July 2023. Pigs were fasted for approximately 12 hours prior to the start of data collection. Animals were not eligible to participate if they had been sedated or anaesthetised within the previous 24 hours, were feral and/or aggressive, did not tolerate gentle handling, or if they were ASA status III‒V. Previous handling or domestication history was not known and therefore was not considered. Following data collection, animals underwent their scheduled procedures and were subsequently discharged to their owners.
Study design
All pigs were placed in a carrier or transporter to acclimatise to the environment for approximately 30 minutes in a temperature‐controlled low‐noise area immediately after presentation to FAMS. At the end of the acclimatisation period, a baseline sedation score was recorded. ‘Combing’ was then performed for 5 minutes in the combing group, while pigs in the control group were not handled for 5 minutes with the anaesthesia technician standing immediately next to the transporter/carrier. An anaesthesia resident (K.C.B.) performed all scoring, and all ‘combing’ was performed by large animal anaesthesia veterinary technicians (D.L.H. or C.P.O.) who have extensive experience handling and anaesthetising pigs in a clinical setting. In both groups, the pigs were kept in a room where only the person performing the ‘combing’ and injection, as well as the evaluator were quiet and visible to the pig. ‘Combing’ was performed with a commercially available polypropylene tined fork. Several strokes were applied with the fork in a combing‐like fashion along the pig's dorsum and cervical region. The strokes were interspersed with gentle, yet firm staccato tapping motions focused on the injection site (cervical area). After 5 minutes of ‘combing’, a sedation score (T0) was recorded. Subsequently, the individual performing the ‘combing’ administered the IM sedative injection in the cervical region with a 20‐gauge, 1.5‐inch needle. The injection consisted of 2 mg/kg Telazol (tiletamine and zolazepam, Zoetis), 1 mg/kg xylazine (Rompun, Dechra Veterinary Products), 1 mg/kg ketamine (Zetamine, MWI) and 0.1 mg/kg midazolam (Midazolam, Avet Pharmaceuticals) administered in one syringe. With a quick, firm motion, the needle was inserted into the lateral cervical muscles at a 90° angle. The drug mixture was administered via depression of the syringe plunger, and the needle was extracted from the muscle in a smooth motion (close IM injection technique). A sedation score was immediately recorded post‐injection (T1). In the combing group, pigs were ‘combed’ for an additional 3 minutes, whereas pigs in the control group were left undisturbed. Sedation scores were then obtained at 5, 10, 15, 20, 25, and 30 minutes from the time of IM injection (T5, T10, T15, T20, T25, and T30) (Figure 1).
FIGURE 1.
Timeline of a prospective, randomised clinical trial evaluating a pre‐operative gentle handling technique (‘combing’) and intramuscular sedation with Telazol, ketamine, xylazine and midazolam in 16 domestic pigs. Sedation scores were recorded at baseline (BS), 5 minutes after BS (T0), immediately after injection (T1), and every 5 minutes thereafter until 30 minutes after injection had been reached (T5, T10, T15, T20, T25 and T30).
Sedation scoring
The seven‐domain sedation scale was adapted from Lacoste et al. in 2000 4 (Supporting Information). Pigs were evaluated on their posture, motor activity, drowsiness, palpebral droop, head position and vocalisation. The lowest score possible was 0 (no signs of sedation) and the highest score possible was 21 (deep sedation). The collection of physiological data was intentionally not performed to avoid disturbing the animals. Pigs were also scored on their reaction to the IM injection using a ‘yes’ or ‘no’ response to: vocalisation, moving head and/or neck away from the injection, changes in recumbency, attempts to escape the enclosure, and if physical restraint to accomplish the injection was necessary. Additionally, the pigs were scored ‘yes’ or ‘no’ as to whether they would require additional sedation for further handling prior to the clinical procedure.
Statistical analysis
Outcomes included sedation scores, time to peak sedation (the first assessment point at which the pig reached its maximum sedation score), maximum sedation score, need for additional sedation and adverse responses to injection (vocalisation, movement away from the injection, change in recumbency, trying to escape the enclosure, requiring physical restraint). Normal probability plots were inspected to assess the distribution properties of time to peak sedation, maximum sedation score, age and weight. All continuous variables are summarised as median (range), while categorical variables are summarised as counts and percentages. Variables were individually compared between groups using the Wilcoxon rank sum test (time to peak sedation, maximum sedation score, age, and weight) and Fisher's exact test (need for additional sedation and each of the adverse responses to injection). Statistical significance was set to a p‐value of less than 0.05. All analyses were performed using SAS version 9.4.
RESULTS
The animal demographics and procedures are presented in Table 1.
TABLE 1.
Sex, breed and procedure summary of pigs which presented for a prospective, randomised clinical trial (n = 16) to evaluate the effect of using a pre‐operative gentle handling technique (‘combing’) on intramuscular sedation with Telazol (2 mg/kg), ketamine (1 mg/kg), xylazine (1 mg/kg) and midazolam (0.1 mg/kg) in healthy domestic pigs.
| Group | N | Sex | Breeds | Procedures |
|---|---|---|---|---|
| Combing | 8 |
F (6) MN (1) M (1) |
Vietnamese pot‐bellied (5) Spotted swine (1) Mixed breed (1) Miniature (1) |
Ovariohysterectomy (6) Cryptorchid castration (1) Hoof and tusk trim (1) |
| Control | 8 |
F (6) MN (2) |
Vietnamese pot‐bellied (4) Mixed breed (2) Duroc (1) Miniature (1) |
Ovariohysterectomy (4) Skull computed tomography (1) Umbilical herniorrhaphy (1) Hoof and tusk trim (2) |
Abbreviations: F, female intact; M, male; MN, male neutered.
A summary of the age and weight of the pigs, as well as the time to onset of sedation and peak sedation scores, is presented in Table 2.
TABLE 2.
Summary of results for age and weight of pigs (n = 16); time to peak sedation and maximum sedation scores after being randomly assigned to receive ‘combing’ (n = 8) or to remain undisturbed (control) (n = 8) and subsequent intramuscular injection of Telazol (2 mg/kg), ketamine (1 mg/kg), xylazine (1 mg/kg) and midazolam (0.1 mg/kg).
| Treatment | N | Median (range) | p‐Value | |
|---|---|---|---|---|
| Age (months) | Control | 8 | 9.0 (5.0‒96.0) | 0.35657 |
| Combing | 8 | 12.0 (6.0‒96.0) | ||
| Weight (kg) | Control | 8 | 61.3 (10.0‒130.0) | 1.0000 |
| Combing | 8 | 60.5 (10.3‒250.0) | ||
| Time to peak sedation | Control | 7 | 10 (5‒30) | 0.0470 * |
| Combing | 7 | 5 (0‒10) | ||
| Maximum sedation score | Control | 8 | 21.0 (20.0‒21.0) | 0.3112 |
| Combing | 8 | 21.0 (20.0‒21.0) |
Significant difference between groups.
The combing group demonstrated a faster time to peak sedation (at a median time of 5 minutes [0–10 minutes]) compared to the control group (at a median time of 10 minutes [5–30 minutes]) (p = 0.047). Table 3 summarises the pigs’ responses to the IM injection.
TABLE 3.
Summary data for 16 domestic pigs’ responses to an intramuscular injection of Telazol (2 mg/kg), ketamine (1 mg/kg), xylazine (1 mg/kg) and midazolam (0.1 mg/kg) randomly assigned to receive ‘combing’ (n = 8) or to be left undisturbed (control) before the intramuscular injection (n = 8).
| Response | Treatment | N | Variable outcome ‘yes’, N | p‐Value |
|---|---|---|---|---|
| Additional sedation | Control | 8 | 4 | 1.000 |
| Combing | 8 | 4 | ||
| Vocalisation | Control | 8 | 6 | 0.31547 |
| Combing | 8 | 3 | ||
| Moved away | Control | 8 | 6 | 0.04106 * |
| Combing | 8 | 1 | ||
| Change recumbency | Control | 8 | 4 | 1.0000 |
| Combing | 8 | 3 | ||
| Tried to escape | Control | 8 | 4 | 0.07769 |
| Combing | 8 | 0 | ||
| Required physical restraint | Control | 8 | 4 | 0.07769 |
| Combing | 8 | 0 |
Significant difference between groups.
In the combing group, significantly fewer pigs moved away from the injection than in the control group; one of eight (12.5%) pigs compared to six of eight (75.0%) pigs (p = 0.041). Figure 2 presents the sedation scores over time. The sedation scores at T1 (immediately after injection) were significantly higher for the combing group (median [range], 16.5 [7‒21]) than in the control group (4 [0‒20]) (p = 0.047).
FIGURE 2.
Sedation scores of domestic pigs after receiving intramuscular sedation with Telazol, ketamine, xylazine and midazolam (control group, n = 8) or after performing a pre‐operative gentle handling technique (‘combing’) and receiving intramuscular sedation with Telazol, ketamine, xylazine and midazolam (combing group, n = 8) at baseline, T0, T1, T5, T10, T15, T20, T25 and T30 for combing and control groups. *Significantly different between groups.
DISCUSSION
In this study, we demonstrated that the gentle handling technique of ‘combing’ prior to IM injection decreased the time to peak sedation and the number of pigs that moved away from the injection compared to the control group. ‘Combing’ reduced median time to peak sedation by 50% when compared to the control group (5 vs. 10 minutes after injection, respectively). Similarly, a report in domestic cats found that a low‐stress handling technique reduced the time to peak sedation by 50% compared to the control group. 5 Additionally, in that study, the median induction dose of propofol was significantly reduced by low‐stress handling. 5
Sedation scores at T1 (immediately after the IM injection) were significantly higher in the combing group than the control group, further showing that ‘combing’ had an impact on pigs’ sedation and subsequently diminished reaction to the injection. This decrease in reactivity likely resulted in a more successful injection, such as better needle positioning into the muscle. Optimal needle position during IM injection reduces the risk of inadvertent subcutaneous deposition of drug, as well as loss of drug upon removal of the needle from the animal after injection.
Different techniques are available for rapid IM injection. When administering a close IM injection, the operator is next to the animal and the needle is introduced into the muscle while connected directly to the syringe. Another common injection technique is the remote IM injection, where the needle is connected to an extension set connected to the syringe. 3 This technique allows for greater distance between the operator and the animal, and lower risk of premature needle dislodgement when compared to the close method. However, as pigs do not tolerate physical restraint well, there is always a risk of intra‐adipose tissue injection as well as loss of drug if the pig moves excessively during injection. In our study, a close‐contact IM injection technique was used consistently across both treatment and control groups. Notably, pigs that received the combing intervention were generally compliant, and the injection could be administered without the need for physical restraint (0 pigs in the combing group vs. 4 in the control group required physical restraint for the injection). In contrast, several pigs in the control group exhibited avoidance behaviours, including attempts to escape the enclosure following injection. While not statistically significant, none of the pigs that received the ‘combing’ treatment attempted to escape the enclosure immediately following IM injection, compared to four out of eight pigs in the control group. This observation may reflect a calming effect of the combing intervention and suggests that the difference could have reached statistical significance with a larger sample size. Additionally, pigs in the combing group were less likely to move away in response to the injection itself. It is possible that the control pigs, having had no prior tactile interaction with the handler, perceived the injection as a sudden and aversive stimulus, prompting escape behaviours. In contrast, the brief period of gentle tactile stimulation provided by combing may have served to reduce arousal or anxiety, thereby improving tolerance to handling and injection. Although not used in this study, a remote injection technique can also be employed in swine medicine to minimise restraint and reduce stress, particularly in untrained or fractious animals. These methods can be beneficial in situations where close handling is impractical or unsafe. However, remote injection also presents limitations: the lack of prior tactile interaction means that the injection itself remains the first and only physical contact, which may still provoke a strong avoidance response. In the context of this study, the use of ‘combing’ appeared to bridge the gap between the need for effective sedation and the desire to minimise stress and restraint. It enabled the use of a close‐contact injection technique without the adverse behavioural responses typically associated with restraint, highlighting its potential as a low‐resource, welfare‐friendly handling strategy in clinical swine practice.
Drugs that have been evaluated for their sedative effects in pigs include benzodiazepines, 6 alpha‐2 adrenergic agonists, 7 dissociatives (ketamine, tiletamine), 8 opioids, 9 phenothiazines, 10 alfaxalone 11 , 12 and azaperone. 13 Most sedatives used in swine are more effective when administered in combination with other agents, which can enhance sedation depth, reduce individual drug dosages, and improve overall safety and predictability. 3 Consequently, multimodal sedation protocols are the preferred approach in swine practice. The sedative combination of Telazol (a 1:1 combination of tiletamine and zolazepam), ketamine, xylazine and midazolam (TKXM) has been used in pigs to reliably produce sedation and/or injectable anaesthesia. 14 , 15 , 16 This combination has a small injection volume compared to some other combinations, which is advantageous for successful IM injection in medium to large pigs. In swine, Telazol‒ketamine‒xylazine anaesthesia has a time to onset of action of approximately 3 minutes when injected IM. 15 Additionally, the onset of sedation with midazolam administered IM in swine has been described as approximately 3 minutes. 6 After T1, sedation scores were not different between the two groups. This finding could be attributed to the rapid onset of action of the heavy sedative protocol used. In addition, although both groups achieved profound sedation, as indicated by sedation scores at T5 and beyond, the same number of pigs (four out of eight in each group) required additional sedation following the 30‐minute evaluation period to proceed with the medical procedures. This suggests that the calming effect of ‘combing’ does not extend beyond the initial minutes surrounding the procedure itself.
Limitations of our study included the injectable drug protocol used, as it resulted in heavy sedation, potentially masking the effects of ‘combing’. Future studies using a lighter sedation protocol should be considered. Having two individuals performing the ‘combing’ technique could have introduced some variability in the application of fork pressure, and positioning of the fork. However, the veterinary technicians who performed ‘combing’ have extensive experience in anaesthetising and handling porcine patients, and great variations in techniques are unlikely. Given the design of the study with data points immediately after ‘combing’ (T0), immediately after the IM injection (T1), and ‘combing’ resuming immediately following injection, it was not feasible to have a blinded evaluator score the pigs. Although we acknowledge that bias is possible, the two parameters that reached statistical significance in this study (time to peak sedation and movement away from injection) represent objective outcomes for analysis in this study. Lack of an interactive component during scoring may have falsely increased the animals’ score post‐injection. It is important to acknowledge that the ‘combing’ technique as performed here, sequestering the pig in a quiet, temperature‐controlled room, and performing the combing/tapping motions prior to and after the injection, adds time to the procedure, and additional planning is needed when multiple cases are scheduled on the same day. Finally, it is important to note that only relatively calm and easily manageable pigs were included in this investigation, and additional studies with aggressive/feral or undomesticated pigs are warranted.
This study is the first to evaluate a gentle handling technique prior to IM injection of a sedative combination in pigs in a clinical hospital setting. ‘Combing’ is a practical, low‐cost technique that can be easily implemented in clinical veterinary practice to reduce stress and improve sedation outcomes in pigs. Unlike habituation protocols used in research, this method is immediately applicable to pigs presenting for clinical care.
In conclusion, ‘combing’ is a viable technique for the gentle handling of pigs in clinical practice. When performed as described here, ‘combing’ decreased pigs’ avoidance response to an IM injection, resulted in increased sedation scores immediately following an IM injection, and decreased the time to peak sedation with an IM‐administered combination of TKXM. These findings support further investigation into its use prior to sedation, anaesthesia or repeated handling in a broader clinical porcine population.
AUTHOR CONTRIBUTIONS
Conceptualisation: Kaitlynn C. Ban, Carolina H. Ricco Pereira and Phillip Lerche. Methodology: Kaitlynn C. Ban, Carolina H. Ricco Pereira, Chiara E. Hampton and Phillip Lerche. Investigation: Kaitlynn C. Ban and Carolina H. Ricco Pereira. Data curation: Kaitlynn C. Ban, Carolina H. Ricco Pereira and Phillip Lerche. Writing—original draft preparation: Kaitlynn C. Ban, Carolina H. Ricco Pereira and Phillip Lerche. Writing—review and editing: Kaitlynn C. Ban, Carolina H. Ricco Pereira, Kirk A. Muñoz, Chiara E. Hampton, Gianluca Bini and Phillip Lerche. All the authors have read and agreed to the published version of the manuscript.
CONFLICT OF INTEREST STATEMENT
The authors declare they have no conflicts of interest.
ETHICS STATEMENT
All study protocols and procedures were approved by the Ohio State University Institutional Animal Care and Use Committee (IACUC protocol number 2023A00000009).
INFORMED CONSENT
Informed consent was obtained from the owner or legal guardian of all animals included in this work for the procedures undertaken. No animals are identifiable within this publication; therefore, additional informed consent for publication was not required.
Supporting information
Supporting Information
ACKNOWLEDGEMENTS
The authors would like to thank Devin L. Heilman and Carl P. O'Brien for their assistance with this study. This work was financially supported by the Ohio State University College of Veterinary Medicine.
DATA AVAILABILITY STATEMENT
Data supporting the central findings of this research study are contained within the article. Additional research data may be available upon request and are subjected to evaluation on a case‐by‐case basis, respecting the Ohio State University regulations and policies on data handling.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Supporting Information
Data Availability Statement
Data supporting the central findings of this research study are contained within the article. Additional research data may be available upon request and are subjected to evaluation on a case‐by‐case basis, respecting the Ohio State University regulations and policies on data handling.