Efficacy of a single central injection of lignocaine at the scrotal neck on the behavioural response of lambs to castration

Objective: To compare the behavioural responses of lambs to ring castration with or without local anaesthetic administration by single midline injection at the scrotal neck.

Study design: Blinded, randomized block design pen study.

Animals: A group of 60 male Merino lambs; 6-8 weeks of age; average body weight of 18.5 ± 0.33 (mean ± standard error of the mean) kg at the time of castration.

Methods: Lambs were randomly assigned to three groups of 20: 1) NUMNUTS - lamb placed in a marking cradle and ring castrated with 1.5 mL lignocaine 2%; 2) SHAM - lamb placed in marking cradle and the scrotum manipulated as though an elastrator castration ring were applied; and 3) RING - lamb placed in the marking cradle, ring castration performed using an elastrator. Pain-related behaviours annotated for 1 full minute every 5 minutes post-procedure, from video footage collected for 2 hours post-procedure, according to a defined ethogram.

Results: Lignocaine administration led to significant (p < 0.05) reductions in restless behaviour at 5 and 10 minutes in NUMNUTS lambs compared with RING (mean = 0.2 ± 0.4 versus 1.0 ± 0.4, respectively) post castration. A significant (p = 0.01) reduction in foot stamping and kicking behaviour at 5 minutes post castration (mean = 1.5 ± 0.4, for RING versus 0.4 ± 0.4 for NUMNUTS) and significant (p = 0.001) reduction in the sum of all acute pain-related behaviours at 5 minutes post castration.

Conclusion and clinical relevance: A single midline injection of local anaesthetic at the scrotal neck can alleviate the behavioural responses to ring castration in the immediate post-procedure period. However, the duration of effect was limited, which may be a result of the drug we used. Development of a longer-lasting local anaesthetic formulation is imperative to optimize pain mitigation for ring castration.