The objectives of this project were to evaluate the effectiveness and quality of gas euthanasia applied to suckling and nursery piglets at different flow rates, and with the use of different gas mixtures. Pigs were euthanized with the Smartbox™ system, which allowed for precise control of gas types, mixtures, flow rates and delivery times. Four flow rates were examined: slow, medium, fast and prefill (20%, 35%, 50%, and prefilled + 20%; chamber exchange rates per minute, respectively). The gas mixtures examined included 100% CO2 and a 50:50 CO2:Argon gas blend. Piglets were categorized and examined as two age groups: neonates (less than 72 hours old) and weaned (16 to 24 days of age). A control group was also included in each age group. The control piglets were allowed to remain in the euthanizing chamber for 10 minutes with ambient air passing through, after which blunt force trauma was applied. Effectiveness and quality of gas euthanasia were examined and analyzed using a variety of techniques, including behavior observations collected directly and from video, as well as physiologic (pulse, respiration and body temperature) and endocrine (cortisol, epinephrine and norepinephrine) responses. Live observation data was collected primarily to assess the effectiveness of the process including parameters such as latency to open mouth breathing, loss of posture and last movement (cessation of respiration). Live observation also included the occurrence of behavioral measures assumed to be associated with sensation and distress, including licking and chewing, nasal discharge, defecation, and urination. The video was utilized to collect behaviors in detail, specifically duration of behaviors, and added the ability to capture accurately, quickly occurring behaviors of interest. Video was scored primarily for behavioral traits which are difficult to collect live and mostly related to the quality of the euthanizing process, and sensation by the pig. These traits included duration of ataxia, duration of open mouth breathing, escape attempts and others.
As expected, piglets succumb to the effects of the gas quicker with the flow rates that create a rapid buildup of CO2 concentrations within the chamber. To achieve loss of posture in the weaned piglet, 100% CO2 at the slow flow rate took 3.33 minutes, whereas 100% CO2 at a fast or prefilled configuration took approximately 1.7 minutes. For the weaned piglet, addition of Argon increased the time to loss of posture by about 0.8 minute at the fast flow rate. Within the slow rate, this effect is even more dramatic, taking an additional 1.25 minutes. This same trend is seen for last movement with the addition of Argon increasing the latency time by 2 minutes for the fast flow rate and over 4 minutes for the slow flow rate. Between the age groups, it was observed that neonate piglets succumb to the effects of the gas as quickly as, or quicker than, weaned piglets, as judged by latency to open mouth breathing, loss of posture, and last movement. For example in the slow CO2 treatment, weaned piglets took approximately 3.33 minutes, while neonates averaged around 2 minutes for loss of posture. Differences were not observed between treatments for percentage of piglets displaying indicators of sensation or distress, but fast and prefill, 100% CO2 treatments resulted in decreased durations of these behaviors. Thus based on the data collected in this experiment, inclusion of 50% Argon with CO2 was not associated with benefits in terms of reduced distress, and is not recommended for gas euthanasia of pigs. Furthermore, slow fill rate (25% chamber volume exchange per minute) should be avoided due to prolonged distress. Additionally, differences were found between age groups, with neonate piglets succumbing to the effects of the gas as quickly for more quickly than weaned piglets.
