I feel your pain: Individual differences in welfare indicators after castration in horses.

Overview

• This study investigated how castration affects horse welfare and whether horses’ personalities influence their pain responses.
• The research also evaluated the reliability of the Horse Grimace Scale (HGS) as an indicator of pain across different horses.

Research Purpose and Context

• The impact of castration on horse welfare is not well understood, despite castration being a common and inherently painful procedure.
• Individual horses may experience and express pain differently due to variations in personality traits.
• The study aimed to assess the welfare impact of castration, consistency of individual pain responses, personality influences, and the effectiveness of HGS in measuring pain.

Methodology

• Subjects: 19 horses undergoing standard castration procedures.
• Measurements taken at multiple time points before, during, and after castration included:
  • Infrared Thermography (IRT) of eye temperature as a physiological pain indicator.
  • Salivary cortisol, a hormonal stress marker.
  • Horse Grimace Scale (HGS), which scores facial expressions linked to pain.
  • A pain ethogram, documenting pain-related behaviors.
• Personality traits of horses were assessed to understand their influence on pain and stress responses.

Key Findings

• Significant changes from baseline during castration were found in:
  • Eye temperature (IRT) (p < 0.005)
  • Salivary cortisol levels (p < 0.024)
  • Horse Grimace Scale scores (p < 0.03)
  • Maintenance behaviors such as resting or normal activities (p < 0.004)
• Physiological and behavioral responses varied widely between horses but were not consistent within individuals across time.
• The veterinarian performing the castration influenced pain responses, likely reflecting variation in surgical skill affecting horse welfare.
• Personality traits influenced cortisol responses:
  • Neuroticism was associated with lower cortisol response (estimate = -0.275; p = 0.035).
  • Extroversion was associated with higher cortisol response (estimate = 0.406; p = 0.001).
• HGS scores were not affected by personality differences, suggesting HGS may reliably reflect pain regardless of individual behavioral tendencies.
• HGS correlated significantly with cortisol levels (r = 0.568, p = 0.027), indicating it reflects real underlying affective (emotional) pain states.

Implications and Conclusions

• This study indicates that castration does negatively affect horse welfare, as evidenced by physiological, behavioral, and facial expression changes.
• Personality traits influence stress hormone responses but not facial pain expression as measured by HGS.
• HGS appears to be a promising and robust tool for detecting pain at the individual horse level, less biased by personality differences.
• Variation in veterinary technique underscores the importance of clinician skill in minimizing pain and welfare impacts.
• The study recommends further research with larger sample sizes and standardized procedures to explore:
  • The role of baseline welfare and personality on resilience to pain.
  • The sensitivity and validity of Grimace Scales in diverse painful contexts.

Overall Significance

• Understanding individual differences in pain perception helps improve animal welfare by tailoring pain management practices.
• Facial expression tools like the Horse Grimace Scale have potential as objective, non-invasive methods to evaluate pain in horses.
• Such tools can assist veterinarians and caretakers in timely and accurate recognition of suffering, thereby guiding better post-operative care.