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

Abstract

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.