Benefits of a multimodal analgesia compared to local anesthesia alone to alleviate pain following castration in sheep: a multiparametric approach

Castration of male ruminants is a common livestock management practice, but induces pain. However, little is known about the effectiveness of multimodal analgesia compared to local anesthesia (LA) alone in reducing pain associated with burdizzo castration in sheep. This study aimed to monitor the pain response induced by castration in sheep and to assess the efficacy of analgesia strategies. Twenty-four 12-month-old male Texel sheep were burdizzo-castrated after administration of physiological serum (Burd), local anesthetic (Burd+LA) or LA plus non-steroidal anti-inflammatory drug (Burd+LA+NSAID). Sheep responses were monitored using behavioral and physiological indices of pain. Sampling occurred from 24 h pre-castration to 78 h post-castration, split into four periods based on the duration of analgesia: P0 (T-24 to T-1 h), P1 (T0 to T+2 h), P2 (T+3 to T+32 h) and P3 (T+36 to T+78 h). Behavioral indices were attention and head position, ear position, position of the eyelid, other facial expression, standing/lying postures, postures of the legs, clinical signs and abnormal activities. Physiological indices consisted in indicators of inflammation (haptoglobin, serum amyloid A (SAA), body temperature), hypothalamo–pituitary–adrenal axis (cortisol, non-esterified fatty acids, glucose), autonomous nervous system (heart rate variability (HRV)) and oxidative stress. The variables contributing most to discrimination of the period×treatment groups were analyzed by factorial discrimination analysis. Pre-castration (P0), there was no significant difference between treatments for all indicators (P > 0.05). Post-castration, eight indicators varied significantly according to period and treatment: cortisol, clinical signs, ratio of low frequency/high frequency (LF/HF) bands of the HRV, attention and head position, SAA, haptoglobin, body temperature and glucose. The treatment×periods groups were well discriminated by the 23 indicators. Burd in P0, Burd+LA in P0 and Burd+LA+NSAID in P0, P1 and P2 had low values for all indicators, likely reflecting absence of pain and discomfort. Burd in P1 and P2 and Burd+LA in P2 showed clinical signs and reduced attention, high LF/HF and high cortisol levels, reflecting acute pain. Burd and Burd+LA in P3 had high temperature, high haptoglobin, high glucose and high SAA, but no response from other pathways. These results suggest that (i) behavioral signs of pain were apparent up to 32 h post-castration, (ii) LA was partially effective, but only during its time of action (2 h) and (iii). multimodal analgesia (LA and NSAID) was effective for up to 3 days post-castration. These findings, and especially those related to sheep behavior, can help veterinarians and farmers better detect pain and refine their pain alleviation methods.