Assessment of Animal Welfare and Acid-Base Imbalance in Pigs that Recovered Consciousness after Co2 Stunning

During slaughtering procedure, every animal should be unconscious chiefly to avoid inflicting undue pain during bleeding. Exsanguination without prior stunning is an extremely controversial practice from the standpoint of animal welfare because some animals take a long time to lose brain function. CO₂ stunning can be reversible or irreversible; pigs may recover consciousness before dying, such that the time between stunning and exsanguination is a determining factor for the efficiency of the stunning process. Some researchers have described that pigs should be exposed rapidly to 90% CO₂; though it has been shown that exposure to CO₂ at 90% for 120 s is more effective than exposure for 90 s in abolishing consciousness reflexes. The objective of the present study was to evaluate the effect of different concentrations of CO₂ on the stunning of pigs by measuring physiometabolic blood profiles.

A total of 1336 pigs were stunned in a CO₂ chamber for approximately 90 s. The pigs were classified into four groups: the first group rested in pens and was sampled 3 h before slaughtering (reference values, RV). The rest of the pigs were assigned to 3 groups according to the CO₂ concentration used for stunning: 85, 90 or 95%. Each group was then further divided into 2 sub-groups: (A) pigs exsanguinated during the first 60 s after leaving the chamber without recovering consciousness (WRC), and (B) pigs that were exsanguinated after more than 60 s and did recover consciousness (RC).

Blood pH of the RC pigs was below 7.08, while blood Ca²⁺ (> 1.59 mmol/L), glucose (> 159.79 mg/dL) and lactate (> 103.52 mg/dL) levels all increased compared to the control group (P< 0.05). All pigs exposed to CO₂, regardless of concentration, presented changes in critical blood variables, and exposure to the gas also affected their acid-base balance, producing a process of acidosis, hyperglycemia, hyperlactatemia, hypercapnia and hyperpotasemia. This physiological disequilibrium was greater when the animals recovered consciousness seconds after leaving the stunning chamber.

Therefore, it is necessary to reduce waiting times between removals of the pigs from the stunning chamber and performing exsanguination. Under no circumstances should this interval exceed 60 s, otherwise the pigs may recover sensitivity. The study recommends maintaining strict control of entry into, and removal from, the CO₂ chamber in order to avoid backlogs in the slaughtering area and so ensure that the pigs do not regain consciousness.

Disclosure of Interest: None Declared.