Validating the effectiveness of alternative euthanasia techniques using penetrating captive bolt guns in mature swine (Sus scrofa domesticus)

Abstract

Euthanasia of mature swine is challenging. Temporal and behind-the-ear locations are two sites that have been identified as alternatives to the more commonly used frontal placement. In stage one, the effectiveness of two penetrating captive bolt gun styles (cylinder or pistol) was evaluated using frontal, temporal, and behind-the-ear placement in anesthetized mature swine (n = 36; weight: 267 ± 41 kg). For stage one, when evaluating treatment efficacy by sex, the cylinder-style equipment was 100% effective in achieving death when applied to all cranial locations (frontal, temporal, and behind-the-ear) for sows; however, the pistol-style equipment was only 100% effective when applied at the behind-the-ear location for sows. For boars, the cylinder-style equipment was 100% effective when applied to the frontal and behind-the-ear location, but the pistol-style equipment was not effective for any cranial location in boars. Therefore, the pistol–frontal, pistol–temporal, pistol–behind-the-ear, and cylinder–temporal were not included for boars, and pistol–frontal and pistol–temporal were not included for sows in stage two. In stage two, commercial, mixed-breed, mature swine (n = 42; weight: 292 +/− 56 kg) were randomly assigned to one of four treatments based on the inclusion criteria described in stage one. A three-point traumatic brain injury (TBI) score (0 = normal; 1 = some abnormalities; 2 = grossly abnormal, unrecognizable) was used to evaluate six neuroanatomical structures (cerebral cortex, cerebellum, hypothalamus, thalamus, pons, and brain stem), and the presence of hemorrhage was also noted. All treatments were 100% effective in stage two. A significant interaction between gun style and placement was determined on predicting total TBI as the cylinder style produced a higher total TBI score compared with the pistol type of the magnitude of +2.8 (P < 0.01). The cylinder style tended to produce a greater TBI score than the pistol in the temporal location (+1.2; P = 0.08). No difference was noted for TBI score behind-the-ear between the cylinder- and pistol-style gun (P > 0.05). TBI tended to be less in boars compared with sows (−0.6; P = 0.08). Hemorrhage was observed in frontal, parietal, occipital, and temporal lobes. This study demonstrated that the cylinder-style captive bolt gun more effectively resulted in brain trauma and death compared with a pistol-style gun and the behind-the-ear and temporal placement showed promise as an alternative placement site for euthanizing mature pigs on-farm.

Introduction

Euthanasia is the process by which animals are rendered insensible and effectively achieve death through brain function loss and cardiac arrest (Irwin et al., 2009; National Pork Board, 2011; Mullins et al., 2017). Conducting humane euthanasia is a moral obligation for all individuals involved in the swine industry and requires that those working with pigs are appropriately trained to euthanize using techniques that are safe, effective, and reliable (Gemus-Benjamin et al., 2015; Shearer, 2016; Leary et al., 2020). Several euthanasia techniques have been identified and approved for use in swine. On commercial swine farms in the United States, the most commonly used euthanasia techniques to date on mature pigs include penetrating captive bolt gun (PCBG) and gunshot (National Pork Board, 2011; Woods, 2012; Leary et al., 2020).

In mature swine, defined as sexually mature gilts, sows, and boars (National Pork Board, 2019), euthanasia is often achieved through physical disruption of brain activity through the application of a PCBG. The bolt gun is placed firmly in the frontal position against the forehead of the pig at midline, and insensibility and death are achieved by concussive damage and trauma to the cerebral hemisphere and brain stem (Leary et al., 2020). Although this method is typically effective (Woods, 2012), efficacy can be directly impacted by the ability of the bolt to penetrate the skull. As pigs age, the thickness of the skull increases, the frontal sinuses expand, and some breeds may develop a distinct bony ridge on the midline of the forehead (Humane Slaughter Association, 2013; Bjarkam et al., 2017; Anderson et al., 2019). These anatomical variations can make it difficult to inflict sufficient tissue damage using a PCBG if the bolt length is insufficient, the power charge is insufficient, and particularly when the cranial skin to brain distance extends beyond 5 cm (Shridharani et al., 2012; Bjarkam et al., 2017; Anderson et al., 2019). Therefore, mature swine represent a challenging subpopulation of animals when attempting to ensure effective and reliable euthanasia via PCBG (Woods, 2012).

Investigation of the use of PCBG euthanasia at alternative locations to the standard frontal position in live, mature swine is needed. Therefore, the objective of the present study was to evaluate the efficacy of euthanasia at two alternative cranial locations (temporal and behind-the-ear) utilizing two, heavy-duty, extended bolt PCBG equipment styles (cylinder and pistol styles; Table 1).

Table 1.

Cylinder- and pistol-style Power Actuated Stunner parameters (BUNZL Processor Division/Koch Supplies, Riverside, MO)

Device	Bolt extension, cm	Bolt penetration depth, cm	Maximum load power charge, grain
Cylinder style	14.61 (5.75 inches)	11.43–12.70 (4.50–5.00 inches)	6.00 (6.00)
Pistol style	14.43 (5.68 inches)	8.26 (3.25 inches)	4.50 (4.00)

Materials and Methods

The protocol was approved by The Ohio State University Institutional Animal Care and Use Committee (no. 2020A00000031). The present study was carried out in two stages in accordance with the Guide for the Care and Use of Agricultural Animals in Research and Teaching (American Dairy Science Association, American Society of Animal Science and Poultry Science Association, 2010). Considering there were no data available at the time this study was complete on the effectiveness of the treatments examined herein for swine, the sample size for this study was determined using a combination of ethical and financial considerations.

Stage one: verification of techniques’ effectiveness in anesthetized mature swine

Study animals and housing

Thirty-six commercial, mixed-breed, mature sows (n = 18; weight: 239.78 ± 14.74 kg) and boars (n = 18; weight: 344.17 ± 22.94 kg) were purchased from local commercial vendors. Pigs were transported and temporarily held (<24 h) at The Ohio State University Meat Science Laboratory, where they were individually weighed and identified using a permanent marker. The objective of stage one was to verify each technique’s effectiveness in unconscious, anesthetized animals.

Anesthetic protocol

Sows and boars were restrained using sort panels and anesthetized utilizing a single intramuscular injection of three anesthetic agents (xylazine, 4.4 mg/kg, AnaSed LA, VetOne, MWI Boise, ID, USA; ketamine HCl, 2.2 mg/kg, Ketathesia, Henry Schein Animal Health, Dublin, OH, USA; tiletamine-zolazepam HCl, 4.4 mg/kg, Tilzolan, Dechra Veterinary Products, Overland Park, KS, USA; Pairis-Garcia et al., 2014). Ten minutes following the anesthesia onset, individuals were evaluated for consciousness. Once swine achieved a deep plane of anesthesia (defined as a pig exhibiting no evidence of a palpebral, corneal, or deep pain reflex; Pairis-Garcia et al., 2014), euthanasia was administered on the same side of the head at each target location to standardize data collection.

Euthanasia procedure

One trained investigator with over 30 years of experience performing swine euthanasia conducted all euthanasia events.

Euthanasia equipment

Euthanasia was performed using two styles of PCBG. The cylinder and pistol styles are both Super Heavy-Duty Jarvis Power Actuated Stunners (BUNZL Processor Division/Koch Supplies, Riverside, MO). For the present trial, a 6.0- and 4.0-grain cartridge loads were used for the cylinder- and pistol-style equipment, respectively (Table 1).

Cranial location

Each captive bolt style was evaluated at three cranial locations. The frontal location was defined at the intersection of imaginary diagonal lines extending from the top of the attachment of each ear to the medial canthus of the opposite eye at an angle resulting in a bolt path directed toward the brain stem (Figure 1a). The temporal location was defined as 3 cm (1.18 inches) posterior to the lateral canthus of the eye within the plane between the lateral canthus and the base of the ear (Figure 1b). The behind-the-ear location was defined as directly caudal to the pinna of the ear on the same plane of the eyes with the plane targeted toward the middle of the opposite eye (Anderson et al., 2019; Leary et al., 2020; Figure 1c). The frontal location was used as the experimental control, given that frontal is an approved location for penetrating captive bolt euthanasia in swine (Leary et al., 2020). Both cylinder and pistol styles were evaluated at each cranial location, resulting in six treatment combinations. Within stage one, three sows and three boars were randomly assigned and evaluated for each treatment (n = 36 total; Table 2).

Figure 1.

The images represent the recommended placement of a PCBG for (a) frontal, (b) temporal, and (c) behind-the-ear approaches for euthanasia procedures in mature swine. Red dots indicate the placement of the PCBG muzzle. Blue dashed lines represent reference landmarks (a) and the trajectory of the penetrating captive bolt (c).

Table 2.

Equipment style (cylinder and pistol style) and cranial location (frontal, temporal, and behind-the-ear) assigned by trial stage

		Pig number
Equipment style	Cranial location	Female	Male
Cylinder	Frontal	3	3
	Temporal	3	3
	Behind-the-ear	3	3
Pistol	Frontal	3	3
	Temporal	3	3
	Behind-the-ear	3	3
		Total n = 36
Cylinder	Frontal	7	7
	Temporal	7	—
	Behind-the-ear	7	7
Pistol	Behind-the-ear	7	—
		Total n = 42

Evaluation of treatment efficacy

Upon application of euthanasia treatment, the following measures were used to assess treatment efficacy:

Confirmation of death

Confirmation of death was defined as any pig that achieved cardiac and respiratory arrest within 10 min of treatment application. Cardiac arrest was confirmed by the lack of a detectable rhythmic heart rate using a pulse oximeter (SurgiVet Advisor Vital Signs Monitor Code V9204, Smiths Medical ASD, Inc., St. Paul, MN), and respiratory arrest was confirmed by visual assessment of respiratory cessation of the chest cavity. Any pig that did not achieve a confirmed death 10 min after treatment administration was euthanized using a commercial-grade mobile electrocution stunner (TBG 96/N, Hubert HAAS, Neuler, Germany) applied across the head behind the eyes for 10 s and from head to heart for 10 s.

Following confirmation of death, carcasses were shackled, hoisted, suspended by a hind leg, and exsanguinated prior to removal of the head from the carcass. The skull was split along the midline into two sagittal sections using a band saw to conduct skull thickness measurements and to score brain hemorrhage and traumatic brain injury (TBI).

Skull thickness

Skull thickness was collected by measuring the distance from the edge of the cranial cavity (skull to skin interface of the parietal bone) to the inner bone lamina encompassing the cerebrum.

Hemorrhage and TBI scoring

Hemorrhage scores were assigned to four regions of the brain (Figure 2a) as a 0 (no hemorrhage) or 1 (hemorrhage present) score for each region. TBI scores were assigned to each of six anatomical landmarks in the brain (Figure 2b) using a 3-point scale (0 = normal; 1 = some abnormalities; and 2 = grossly abnormal, unrecognizable). Scoring systems were adapted from Woods (2012).

Figure 2.

The illustrations depict a midsagittal view of the porcine brain noting anatomical regions assessed for (a) damage due to captive bolt penetration and hemorrhage and (b) TBI score.

Upon completion of stage one, each equipment style by cranial location treatment was verified for efficacy within sex. Efficacy was defined as all three animals within a subclass meeting the confirmation of death requirements within 10 min post-euthanasia application. Treatments that did not achieve this verification were excluded from stage two of the trial. This evaluation was done independently for each sex; therefore, one treatment could have been considered efficacious for one sex but not for the other.

Stage two: validation of alternative euthanasia techniques in conscious mature swine

Study animals and housing

Forty-two commercial, mixed-breed, mature sows (n = 28; weight: 247.15 ± 16.96 kg) and boars (n = 14; weight: 307.94 ± 46.32 kg) were purchased from commercial vendors. Pigs were transported and temporarily held (<24 h) at The Ohio State University Meat Science Laboratory and managed as described in stage one. The objective of stage two was to evaluate the efficacy of each verified technique in conscious swine.

Euthanasia procedure

The same trained investigator from stage one completed all euthanasia events for stage two. Conscious swine were randomly assigned a treatment, weighed, and restrained using a common pig snare prior to the implementation of the euthanasia treatment. A total of four treatments (cylinder–frontal, cylinder–temporal, cylinder–behind-the-ear, and pistol–behind-the-ear) were evaluated for sows (7 sows/treatment) and two treatments (cylinder–frontal and cylinder–behind-the-ear) were evaluated for boars (7 boars/treatment) for a total of 42 animals (Table 2).

Confirmation of death and postmortem pathology (skull thickness, hemorrhage, and TBI scores) were conducted in the same manner as described for stage one.

Statistical analysis

Statistical analysis was conducted using STATA 14.2 (College Station, TX). The proportion of attempts resulting in confirmation of death was calculated for stages one and two.

Data from all animals (78 animals: 32 boars and 46 sows) were combined in order to investigate the association between treatment and hemorrhage and TBI scores separately. As such, two separate multivariable linear regression models were built using the total sum of hemorrhage scores (across the four regions examined) and the total sum for TBI scores (across the six regions examined) as outcomes (response variables). The main predictors considered for both outcomes were the treatments, specified by the equipment style (cylinder or pistol) and cranial location (frontal, temporal, and behind-the-ear). Sex (boar or sow), weight (kg), and whether the animal was anesthetized or not were offered to the model as covariates.

Model-building steps included first checking for linearity between the continuous variable (weight) and the outcome, checking for collinearity between predictors, and construction of univariable models. The cutoff used to declare two variables as highly collinear was 0.80; the cutoff value for variables from univariable models to proceed to the full model was P < 0.20. Sex and weight were perfectly collinear and, therefore, only one of those variables (sex) was carried forward during model construction. Ultimately, the two final multivariable models were built using a backward stepwise approach and using a parsimonious approach. Variables were deemed as confounders if they changed any coefficient in the model by 20% or more and, in this case, were retained in the final models. The interaction between cranial location and equipment type was added to the final model. Statistical significance was declared at P ≤ 0.05, and a trend was declared at 0.05 < P ≤ 0.10.

Results

For stage one, four treatments were successful in sows and two treatments were successful in boars (defined as any pig that achieved cardiac and respiratory arrest within 10 min after treatment application). When evaluating treatment efficacy by sex, the cylinder-style equipment was 100% effective in achieving death when applied to all cranial locations (frontal, temporal, and behind-the-ear) for sows; however, the pistol-style equipment was only 100% effective when applied at the behind-the-ear location for sows. For boars, the cylinder-style equipment was 100% effective when applied to the frontal and behind-the-ear locations, but the pistol-style equipment was not effective for any cranial location in boars. For stage-two treatments, pistol–frontal, pistol–temporal, pistol–behind-the-ear, and cylinder–temporal were not included for boars, and pistol–frontal and pistol–temporal were not included for sows as they did not achieve >95% efficacy in stage one. All treatments included in stage two were 100% effective in achieving death in conscious animals.

Skull thickness

Average skull thickness for sows was numerically less than boars with a mean skull thickness of 7.12 ± 0.87 cm (range: 4.5 to 9.5 cm; n = 35) when compared with 9.1 ± 0.89 cm (range: 7.0 to 10.5 cm; n = 22) in boars.

Hemorrhage score

Mean (SD) hemorrhage sum values by cranial location and equipment are presented in Table 3. The final model for hemorrhage scores showed a significant interaction between cranial location and equipment type. When contrasting cranial locations by equipment, the pistol-style gun type yielded a greater total hemorrhage score at the behind-the-ear location when compared with the temporal location (P = 0.009), whereas the hemorrhage score was not different between locations for the cylinder-style type. Furthermore, when contrasting equipment styles, the cylinder style produced a greater hemorrhage score compared with the pistol style type at the temporal location (P = 0.019), whereas there was no difference between equipment types for the behind-the-ear or frontal location. Lastly, sex and having had anesthesia were not significant covariates in the model.

Table 3.

Mean ± SD (total number of animals) sum of hemorrhage scores¹ and across four brain regions² by equipment type style (cylinder and pistol) and cranial location (frontal, temporal, and behind-the-ear)

	Cranial location
Equipment style	Frontal	Temporal	Behind-the-ear
Cylinder	3.90 ± 0.31 (n = 20)	3.85 ± 0.55 (n = 13)x	4.00 ± 0.00 (n = 20)
Pistol	3.67 ± 0.52 (n = 6)ab	3.17 ± 1.60 (n = 6)by	3.92 ± 0.28 (n = 13)a

¹Hemorrhage scores were assigned as a 0 (no hemorrhage) or 1 (hemorrhage present) score for each brain region.

²Brain regions assessed for hemorrhage score included frontal, parietal, temporal, and occipital regions.

^a,bindicates a significant difference within the equipment style (P < 0.05).

^x,yindicates a significant difference between equipment styles (P < 0.05).

TBI score

Mean (SD) TBI score values by cranial location and equipment are presented in Table 4. For the final TBI statistical model, boars tended to have a lesser total TBI score compared with sows by approximately 0.61 (P = 0.08). The interaction between cranial location and equipment type was significant. When contrasting cranial locations by equipment, the pistol-style equipment produced a greater TBI score at the behind-the-ear location compared with both the frontal (P = 0.04) and temporal locations (P = 0.01). Similarly, the cylinder style produced a greater TBI score at the behind-the-ear location compared with the temporal location (P = 0.03) but was not different from the frontal location (P > 0.05).

Table 4.

Mean ± SD (number of animals) sum of TBI scores¹ across six anatomical brain locations² by equipment type style (cylinder and pistol) and cranial location (frontal-temporal, and behind-the-ear)

	Cranial location
Equipment style	Frontal	Temporal	Behind-the-ear
Cylinder	6.95 ± 1.14abx (n = 20)	5.23 ± 1.58a (n = 13)	6.21 ± 1.23b (n = 19)
Pistol	4.17 ± 1.72ay* (n = 6)	3.83 ± 1.94a* (n = 6)	5.77 ± 1.59b (n = 13)

¹TBI scores were assigned a 0 (normal), 1 (some abnormalities), or 2 (grossly abnormal, unrecognizable). Scoring systems were adapted from Woods (2012).

²Anatomical brain locations assessed for TBI score included brain stem, cerebellum, cerebral cortex, hypothalamus, pons, and thalamus.

^a,bindicates a significant difference within the equipment style (P < 0.05).

^x,y indicates a significant difference between equipment styles (P < 0.05).

*indicates a treatment that did not achieve >95 efficacy as defined as any pig that achieved cardiac and respiratory arrest within 10 min of treatment application.

When contrasting equipment styles by cranial location, the cylinder style produced a greater overall TBI score compared with the pistol style at the frontal location (P = 0.0001). The TBI score for the cylinder-style equipment tended to be greater at the temporal location compared with the pistol style (P = 0.08). There was no difference in TBI score between equipment styles at the behind-the-ear location.

Discussion

The present study aimed to evaluate the efficacy of euthanasia at two alternative cranial locations utilizing two PCBG equipment styles in mature swine. To date, the present study is the first to evaluate such euthanasia techniques in mature swine of this weight and age. Despite the limited sample size, the results from the present study have provided critical information in identifying alternative euthanasia techniques that can be effectively applied on-farm and produce a consistent and reliable death in mature pigs.

In the U.S. swine industry, mature sows and boars remain a challenging subpopulation of animals to euthanize (Woods, 2012; Humane Slaughter Association, 2013; Anderson et al., 2019). Previous work has identified that age-related changes in skull thickness and shape may directly impact the bolt’s ability to sufficiently penetrate the cranial cavity and create enough brain damage to result in death (Woods, 2012). To date, the only approved location for the use of a PCBG in pigs is at the frontal location (National Pork Board, 2011; Leary et al., 2020). The frontal location is defined at the intersection of imaginary diagonal lines extending from the top of the attachment of each ear to the medial canthus of the opposite eye with a directed bolt path toward the brainstem. Although this method is approved, previous research validating this location is limited (Woods, 2012), and no studies to date have evaluated this method on pigs greater than 220 kg. Therefore, there is a critical need not only to ensure the frontal location is effective in mature swine but also to identify alternative locations that may be more suitable for the skull size and shape of mature breeding stock.

Based on the results from stage one, four treatments failed to achieve death in boars and two treatments failed in sows. These results are likely due to inadequate bolt penetration due to a combination of increased skull thickness and ineffective bolt gun force. When comparing sex, boars in the present study had an average approximate skull thickness of 2 cm greater than that of sows and demonstrated consistently thicker skulls than that of the sows. In this study, skull thickness for sows ranged between 4.5 and 9.5 cm with an average skull thickness of 7.1 cm, whereas boars consistently measured between 7 and 10.5 cm with an average skull thickness of 9.1 cm. In order for a PCBG to be effective, it must produce enough force onto the skull to create a percussive effect on the brain and create skull fractures and associated intracranial bone fragments as a byproduct (Woods, 2012; Humane Slaughter Association, 2013). This shockwave of kinetic energy results in the immediate loss of consciousness followed by physical trauma to the brain tissue (Adams and Sheridan, 2008). The increased skull thickness in boars likely prevented the bolt from effectively damaging the brain, and this idea is supported by the trend that boars demonstrated an overall lower total TBI score compared with sows. Given that sow skull thickness encompassed a wider range, efficacy issues may not have been as dramatic as those in which was demonstrated with the boars.

In addition to increased skull thickness, euthanasia device type was also a significant factor. When comparing devices, the pistol-style device has a smaller bolt penetration depth and cartridge load compared with the cylinder-style device. The pistol-style device was ineffective in achieving death when used in combination with any location in boars and both the frontal and temporal locations in sows. Individuals responsible for euthanizing large sows and boars should utilize the most powerful equipment available with the highest cartridge load to ensure maximum impact, as these equipment properties may be more effective in overcoming the anatomical challenges and variations that we see in mature pigs.

It should be noted that the equipment used to confirm cardiac arrest and death may have impacted efficacy outcomes. In a standard commercial farm, cardiac arrest is determined by the lack of physical detection of heartbeat by palpation of the chest cavity. The pulse oximeter equipment used in the present study is a much more sensitive indicator of cardiac activity. Given this, it is possible that death may have been achieved in some of the treatments if different equipment or a longer timeline criterion (i.e., 20 min) was utilized and should be taken into consideration for future research.

In stage two of the present study, four treatments were 100% successful in achieving death. When comparing between equipment device styles, the cylinder style demonstrated an overall greater hemorrhage and TBI score compared with the pistol style equipment. This supports the efficacy results from stage one and suggests that the length, depth, and cartridge load of the cylinder-style gun ensured a fatal TBI and effective euthanasia (Finnie et al., 2003; Andriessen et al., 2010; Xiong et al., 2013).

In addition to the equipment, the cranial location of the bolt gun directly impacted the extent of brain damage and thereby treatment efficacy. In the present study, the behind-the-ear location consistently resulted in greater TBI scores compared with the frontal and temporal locations. This is likely due to the fact that, in avoiding the frontal sinus, the brain stem and cerebellum, two critical areas responsible for consciousness, are more vulnerable to the concussive and destructive force of the PCBG (Ackerman, 1992; Leary et al., 2020).

These results are in contrast to work conducted by Anderson et al. (2019) that reported behind-the-ear location of a pistol-style bolt gun to be less reliable than the frontal location based on bolt path and tissue damage in six-mo-old cadaver heads (average market weight swine: 136 kg). Given the Anderson study was conducted on cadavers, these studies cannot be truly compared given the treatment outcomes for the present study were based on achieving death.

The results reported in the present study highlight the challenges associated with the euthanasia of mature swine but shed light on areas to move forward with this work. This study demonstrated that the cylinder-style captive bolt gun more effectively resulted in brain trauma and death for mature sows and boars compared with a pistol-style gun. This work suggests that when producers are faced with euthanizing large mature animals, a gun style like the cylinder style may be a more effective tool to ensure death. Given the increased skull thickness of mature sows and boars, the extended length of the bolt and higher cartridge load offered by the cylinder-style gun will produce a greater force and more traumatic injury to the brain. In addition, this work demonstrated that the behind-the-ear and temporal placement showed promise as an alternative placement site for euthanizing mature pigs on-farm and may be an advantageous option for producers when access to the front of the pig’s head is prevented due to pig location or facility design. However, given the small sample size in this study, additional work validating these placement locations is needed prior to implementing these alternative cranial locations and techniques in a commercial farm setting.

Glossary

Abbreviations

PCBG

penetrating captive bolt gun

TBI

traumatic brain injury

Conflict of interest statement

S.A.K. is currently a Veterinary Medical Officer at the U.S. Department of Agriculture’s (USDA) Animal Plant Health Inspection Service. He is writing in a personal capacity and his activity, opinions, and views expressed herein are personal and not official. Disclaimer: The views expressed in this manuscript do not necessarily represent the views of the USDA Animal and Plant Health Inspection Service or the United States.