The effect of needle-free administration of local anesthetic on the behavior and physiology of castrated pigs

Pigs are routinely castrated in many countries without giving pain relief, which is an animal welfare concern. The objectives of this study were to evaluate the stress caused by administering local anesthetic (LA) using a needle-free injection system and evaluate if there are any benefits of administering LA immediately before castration on postoperative pain. At 3 days of age, piglets were allocated to 1 of 4 treatment groups. Treatments included sham castration (CON), surgical castration (CAS), castration plus LA administered immediately before castration using a conventional needle and syringe (NED), and castration plus LA administered immediately before castration using a needle-free injection system (NF). In experiment 1, behavior and stress vocalizations were recorded during administration of treatments, and blood samples were collected from all piglets before and 30, 60, 120, and 180 minutes post-treatment to measure complete blood cell counts and cortisol concentrations. In experiment 2, behavior was recorded in the farrowing crate for 180 minutes post-treatment using 1-minute scan sampling (n = 9 pigs/treatment). Change in the percentage of stress vocalizations in response to processing differed among treatments (P = 0.004) with CAS, NED, and NF pigs producing more stress vocalizations than CON. Over the entire processing period, tail movement and the sum of the behavioral scores during processing were greater (P ≤ 0.01) for CAS, NED, and NF compared with CON pigs. During the administration of LA, NF pigs tended to have lower tail flicking (P = 0.062) and sum of behavior scores (P = 0.056) than NED pigs. Cortisol concentrations were elevated (P < 0.05) in all pigs in response to castration regardless of pain relief treatment compared with CON. Change in complete blood cell counts, body weight, and wound healing was not effected by treatment. Pigs castrated without pain relief spent more (P = 0.01) time lying without contact compared with CON pigs, and NED (P = 0.074) and NF (P = 0.032) pigs spent less time lying without contact compared with CAS pigs; however, there was no effect of treatment on other behaviors measured. These results suggest that administering LA using a needle-free injection may cause less stress; however, administering LA immediately before castration did not appear to reduce postoperative pain. More research is needed to find effective practical methods of pain relief for piglets during castration.