Evaluation of Electroencephalography, Behaviour and Eye Temperature in Response to Surgical Castration in Sheep

Abstract

Castration has been demonstrated to cause pain in sheep. However, it is routinely performed for management purposes. Electroencephalography (EEG) has been used successfully to measure pain in lambs in response to castration and other husbandry procedures in livestock. The aim of this study was to evaluate the use of EEG as a measure of pain and analgesia in conscious lambs undergoing castration on farm over a 24 h period. EEG responses were compared to behavioural observations and changes in ocular temperature via infrared thermography. Twenty-four merino ram lambs (18.63 ± 2.06 kg) were used in this study. Lambs were randomly allocated to one of the following treatment groups: (1) castration with pre-surgical administration of meloxicam (0.5 mg/kg Metacam 20 mg/mL injected subcutaneously into the skin of the neck 15 min prior to recording) and lignocaine (applied via intra-testicular injection five minutes prior to castration, 2 mL lignocaine hydrochloride 20 mg/mL, Troy/Ilium) (CML, n = 8); (2) castration only (C, n = 8); (3) sham castration, handling only (SC, n = 8). EEG was recorded for 5 mins pre-procedure (prior to any intervention), and for 5 mins post-procedure at 0, 1, 4 and 24 h. Behavioural reactions to the procedure were scored, and behaviours were scan sampled at 5 min intervals at the above time points, by blinded observers. Eye temperature was measured for five-minute intervals at each time point. EEG decreased from baseline to 0 h for CML and C groups (p < 0.001), C group values returned similar to baseline at 24 h. Eye temperature increased post-castration at 0 h for C group, no initial change was seen for CML or SC groups. CML and C groups were more likely to have higher reaction scores and showed more abnormal behaviours (p = 0.017). CML and C groups had similar results, indicating minimal effect of analgesic intervention. Lambs in SC group showed significant EEG changes, suggesting that stress from handling also impacted EEG results.

The Neuroimmune Interface and Chronic Pain Through the Lens of Production Animals

“…In conscious lambs that were surgically castrated EEG was able to differentiate the effect of castration and the use of the local anesthetic, lidocaine ( Harris et al, 2020 ). However, in a later study of surgical castration in conscious lambs on farm there was no significant effect of treatment on EEG outcomes between surgical castration without analgesia, surgical castration with lidocaine and meloxicam, and sham castration ( Harris et al, 2021 ). The difference in findings were attributed to the effect of the controlled research environment used for the first trial and the more stressful farm environment in the second trial which may have made the results difficult to interpret.…”

“…Response to stress and pain can be modulated by the sympathetic-adrenomedullary system and the hypothalamo-pituitary-adrenocortical (HPA) system. Changes in the sympathetic system can be measured through autonomic responses including heart rate, heart rate variability ( von Borell et al, 2007 ; Stubsjøen et al, 2009 ), blood pressure ( Peers et al, 2002 ), respiratory rate and ocular temperature ( Harris et al, 2021 ; Stock et al, 2021 ). These measures are more sensitive for mild pain compared to behavioral assessment and HPA axis changes ( Peers et al, 2002 ), particularly in prey species ( Weary et al, 2006 ; Sneddon et al, 2014 ).…”

Assessment of Pain and Inflammation in Domestic Animals Using Infrared Thermography: A Narrative Review

“…Electroencephalography and ocular IRT were used to assess pain in another castration study on merino ram lambs [ 111 ]. Animals were allocated to [ 111 ] groups of castration with meloxicam and lignocaine, castration untreated and sham [ 111 ].…”

Application of Electroencephalography in Preslaughter Management: A Review

“…The EEG has been used to successfully to measure pain in animals during various animal husbandry practices in livestock, such as during castration in lambs [ 102 ]. Lamb castrated with presurgical medication and control groups (lamb castrated without medication) demonstrated higher reaction scores and abnormal behavior ( p = 0.017) than the sham control group, thus indicating a minimal effect of analgesic interventions.…”

“…Lamb castrated with presurgical medication and control groups (lamb castrated without medication) demonstrated higher reaction scores and abnormal behavior ( p = 0.017) than the sham control group, thus indicating a minimal effect of analgesic interventions. The authors [ 102 ] also noted significant higher EEG changes in sham control groups, thus indicating that stress during handling also has a notable impact on EEG results.…”