Effects of rectal examination on intraocular pressure and behavioral responses in pregnant and non-pregnant mares.

Overview

• This study investigated how rectal examination affects intraocular pressure (IOP) and behavioral responses in pregnant and non-pregnant mares.
• It aimed to determine if rectal exams cause acute stress measurable by changes in IOP and behavior, and if pregnant mares respond differently than non-pregnant ones.

Background

• Rectal examination is a common reproductive management procedure in horses but may induce stress, impacting animal welfare and safety for handlers.
• Intraocular pressure (IOP) is influenced by autonomic nervous system activity and can quickly reflect physiological stress responses.
• Behavioral reactions during veterinary procedures provide visible indicators of stress or discomfort.

Objectives

• To evaluate whether rectal examination causes acute changes in IOP in mares.
• To measure behavioral responses during the rectal exam as an indicator of stress.
• To assess if pregnant mares differ from non-pregnant mares in physiological and behavioral responses during the exam.

Methods

• Subjects: 20 Rahvan mares, divided into two groups:
  • 10 pregnant mares (30-40 days gestation)
  • 10 non-pregnant mares (aged 4-7 years)
• Procedure: All mares underwent a transrectal examination without sedation.
• Measurements:
  • IOP was measured in both eyes using rebound tonometry at six time points:
    • T1: 2 minutes before exam
    • T2: During exam
    • T3-T6: 1, 5, 10, and 15 minutes after exam respectively
  • Behavior during T2 was scored on a scale of 1 to 5 to assess resistance.
• Analysis:
  • Linear mixed-effects models assessed changes in IOP over time.
  • Ordinal mixed models analyzed behavioral scores.
  • Spearman’s rank correlation assessed the relationship between IOP change (ΔIOP) and behavioral scores.

Results

• Behavioral responses:
  • Pregnant mares showed higher behavioral resistance compared to non-pregnant mares. Median scores were 4 (with interquartile range 3-4) versus 2 (2-3) respectively.
  • Odds ratio (OR) for higher resistance in pregnant mares was 3.8 (95% CI 1.6-9.2), statistically significant with p=0.003.
• Intraocular pressure changes:
  • IOP increased during rectal examination in both groups.
  • Pregnant mares’ IOP rose by an average of +4.2 mmHg from baseline (T2 vs T1), whereas non-pregnant mares had a smaller increase of +1.3 mmHg.
  • This resulted in higher IOP readings in pregnant mares at T2 (34.8 ± 2.1 mmHg) compared to non-pregnant mares (31.6 ± 1.7 mmHg); this difference was statistically significant (p < 0.001).
  • Elevated IOP in pregnant mares remained at T3 (1 minute post-exam) but returned to baseline levels at later points (T4-T6).
• Correlation between physiological and behavioral responses:
  • The change in IOP during examination (ΔIOP) was positively correlated with behavioral resistance scores (Spearman’s ρ = 0.62; p = 0.004), indicating that higher stress behaviors matched higher IOP increases.
• Area under the IOP-time curve (reflecting overall IOP exposure over time) did not differ significantly between groups (p = 0.348), suggesting the total physiological stress impact was similar over the 15 min post-examination period.

Conclusions

• Rectal examination induces transient stress responses in mares, evidenced by temporary increases in intraocular pressure and behavioral resistance.
• These stress responses are more pronounced in pregnant mares, indicating reproductive status influences sensitivity to handling procedures.
• IOP measurement is a rapid and sensitive tool for detecting acute stress and can complement behavioral observations in welfare assessments during veterinary procedures.
• The findings underscore the importance of careful handling and stress mitigation during reproductive exams to enhance horse welfare and safety for handlers.