Administration of procaine-based local anaesthetic prior to surgical castration influences post-operative behaviours of piglets

In an effort to mitigate piglet acute responses to surgical castration, the procedure can be preceded by injections of a local anaesthetic. Regardless of potential benefits at castration, the impact of local anaesthetics on piglet welfare following the procedure remains under-documented. The present paper is based on data collected in two separate castration studies investigating the impact of injection with procaine, using different injection methods, different volumes of injected drug and different timing of injection, on behavioural responses of 3–4 day old piglets, as measured by indicators of social motivation immediately after castration (Study 1 and 2), as well as behaviours recorded continuously for 10 min upon return to the home pen (Study 2 only). Study 1 involved 597 piglets, and 13 treatments: castration without anaesthesia (CC), local anaesthesia followed by castration involving all combinations of two methods of injection (intra-funicular and intra-testicular) and four intervals between injection and castration (2.5, 5, 10 and 30 min), and sham handling separated by the same four intervals (SH). Study 2 involved 290 piglets and 5 treatments: castration without anaesthesia (CC), castration after intra-testicular injections of 0.5 or 0.3 mL of procaine per testis, and sham handling with either one (SH1) or two stays in a castration bench (SH2). Across both studies, piglets injected with procaine showed signs of reduced motivation to approach their siblings in the social motivation test compared to controls castrated without anaesthesia or sham handled. The indicators of social motivation did not differ from the controls in case of castration 30 min after drug injection. In addition, responses shown in the social motivation test were less impacted after injection of 0.3 compared to 0.5 mL of procaine per testis. In Study 2, piglets injected with 0.5 mL of procaine appeared to be less active at the udder, and displayed more huddled up postures, immediately upon return to the home pen, as compared to piglets injected with 0.3 mL of procaine or the controls castrated without anaesthesia or sham handled. Altogether, the results suggest that injections with a procaine-based local anaesthetic negatively impact the responses of piglets in a test of social motivation as well as the home-pen behaviour of piglets in the early post-operative period. The present results call for attention towards the post-surgical phase, but understanding the potential welfare impacts as well as the motivational changes underlying these findings require further study.