Characterizing candidate decompression rates for hypobaric hypoxic stunning of pigs. Part 2: Pathological consequences

Abstract

Pigs are commonly stunned pre-slaughter by exposure to carbon dioxide (CO2), but this approach is associated with significant welfare concerns. Hypobaric hypoxia, achieved with gradual decompression (also known as Low Atmospheric Pressure Stunning or LAPS) may be an alternative, allowing the retention of welfare friendly handling approaches and group stunning. Although validated in poultry, the feasibility and welfare consequences of gradual decompression for pigs are unknown. Here, we characterize pathological changes in 60 pigs resulting from exposure to a range of candidate decompression curves (ranging from 40 to 100 ms−1 ascent equivalent, with two cycle durations 480 and 720 s). To protect welfare, we worked on unconscious, terminally anesthetized pigs which were subject to detailed post-mortem examinations by a specialized porcine veterinary pathologist. All pigs were killed as a result of exposure to decompression, irrespective of cycle rate or length. Pigs showed no external injuries during ante-mortem inspections. Exposing pigs to decompression and the unavoidable subsequent recompression resulted in generalized congestion of the carcass, organs and body cavities including the ears, oral cavity, conjunctivae and sclera, mucosa of other external orifices (anus and vulva), nasal planum, nasal cavities including nasal conchae, frontal sinuses, cranium, meninges, brain, larynx, trachea, lungs, heart, parietal pleura of the thoracic cavity, peritoneum of the abdominal cavity, stomach, small intestine, caecum, colon, liver, spleen and kidneys and representative joint cavities in the limbs (stifles and elbows). Various severities of hemorrhage were observed in the conjunctivae and sclera, mucosa of other external orifices (anus and vulva), nasal cavities including nasal conchae, frontal sinuses, cranium, meninges, brain, larynx, tracheal lumen, lungs, parietal pleura of the thoracic cavity, liver, spleen and kidneys and representative joint cavities in the limbs (stifles and elbows). In general, faster decompression rates produced higher scores, but in the conjunctivae, sclera and kidneys, faster decompression rates were associated with marginally lower congestion scores. There was considerable individual variation in pathological scores across all body regions. The congestion and hemorrhage observed could translate into welfare harms in conscious pigs undergoing this type of stunning, depending when in the cycle the damage is occurring, but no welfare related conclusions can be drawn from the responses of unconscious pigs. Since recompression is always required, its effects cannot be separated from decompression, however cessation of cardiac activity several minutes before recompression should have eliminated any haemodynamic effects relating to cardiac function and blood pressure. This study represents the first systematic attempt to identify candidate rate profiles to underpin future explorations of decompression as a stunning method for pigs. These pathological findings also inform discussions about the likely carcass quality implications of this novel stunning method.

Characterizing candidate decompression rates for hypobaric hypoxic stunning of pigs. Part 1: Reflexive behavior and physiological responses

“…Our work confirms the potential of decompression for stunning purposes and justifies subsequent study in conscious pigs. The results presented here should be considered in conjunction with the pathological assessments presented as a companion paper (41).…”

“…There were two unexpected incidents during two decompression cycles, both caused by a malfunction with the anesthesia equipment (syringe driver malfunction) external to the chamber, which resulted in temporary aspiration of air and rapid administration of propofol to three pigs [cycle run 16 (80 ms −1 ), both pigs and cycle run 28 (80 ms −1 ), a single pig]. These occurred late in the cycle, therefore the physiology and behavior data up to the incidents were included but these animals were excluded for pathology [reported in ( 41 )].…”

“…When considering the potential effects of gradual decompression for slaughter purposes, time to result in a non-recovery state is not the only important factor; carcass quality must also be minimally affected by any stunning method for it to be commercially viable. Therefore, pathological assessment ( 41 ) and the number of whole-body movements (possibly reflecting convulsive activity) are important. We observed greater repeated and prolonged whole-body movements in more pigs undergoing the slowest rate (40 ms −1 ) compared to all other rates, with the lowest number in the intermediate 80 ms −1 rate, again suggesting that it should not be pursued.…”

“…The latencies for swelling onset occurred approximately between 110 and 320 s post cardiac arrest, suggesting that the risk to animal welfare is minimal as the pigs had been in a sustained non-recovery state for >2 min by this time. Its impacts may be more relevant to pathology outcomes and potential carcass and meat quality effects, as the mild swelling was maintained until recompression was initiated, therefore the increased length of a cycle would result in a longer period of distention, although this did not translate to differences in pathological outcomes reported in ( 41 ).…”

“…As part of a systematic investigation of whether hypobaric hypoxia could be the basis of a humane, reliable, and efficient method of stunning for commercial pigs, we investigated four candidate decompression rates, all achieving the same final pressure. This study is Part 1 of a pair, with pathological findings from the same experiments published in Part 2 ( 41 ).…”