Clinical Efficacy of Varying Dosages of Intravenous Phenylbutazone in a Reversible Model of Equine Foot Lameness.

Overview

• This study evaluated how different intravenous doses of phenylbutazone (a pain-relieving drug) affected lameness in horses, using a controlled and reversible model of induced foot pain.
• The research aimed to determine if higher doses of phenylbutazone would provide greater clinical improvement in lameness symptoms compared to a placebo control.

Study Design and Methods

• Subjects: Eight horses were used, each fitted with adjustable heart bar shoes on one front foot to simulate lameness.
• Lameness Induction: Grade 4 out of 5 lameness was induced by tightening a set screw that applied pressure to the heart bar, simulating foot pain.
• Timing: Lameness was induced repeatedly every 2 to 4 weeks for multiple trial sessions.
• Monitoring: Heart rate (HR) and lameness score (LS) were recorded:
  • At rest (baseline),
  • Every 20 minutes for 5 hours after lameness induction,
  • Then hourly for an additional 8 hours.
• Treatment Administration: One hour after inducing lameness, horses were administered intravenously:
  • Negative control (isotonic saline, SAL),
  • Low dose phenylbutazone (PBZ) at 2.2 mg/kg (LOW),
  • Moderate dose PBZ at 4.4 mg/kg (MOD),
  • High dose PBZ at 8.8 mg/kg (HIGH).

  Treatment assignment was randomized and blinded.

• Blood Sampling: Jugular blood was collected at six time points (0, 1h5min, 3h, 5h, 9h, 13h) to measure plasma phenylbutazone concentrations.
• Data Analysis: Repeated measures ANOVA and Student-Newman-Keuls test were used to evaluate heart rate and lameness scores, with significance set at p < 0.05.

Findings

• Effect on Heart Rate (HR):
  • Both moderate and high PBZ doses significantly lowered heart rate from 3 hours to 12 hours post-treatment compared to control.
  • The low PBZ dose reduced heart rate only between 3.67 and 5 hours post-treatment.
• Effect on Lameness Score (LS):
  • MOD and HIGH PBZ doses significantly improved lameness scores from approximately 1.67 hours through 12 hours after treatment.
  • LOW dose PBZ improved lameness score, but only briefly, from about 3.67 to 4 hours post-injection compared to saline.
• Dose Comparison:
  • No statistical difference in efficacy was found between MOD and HIGH doses for either heart rate or lameness score.
  • This indicates that doubling the PBZ dose from moderate to high did not provide additional clinical benefit.

Conclusions

• Low-dose phenylbutazone provides a short-lived clinical effect on equine lameness compared to no treatment.
• Moderate dosing offers significant and sustained clinical improvement lasting up to 12 hours.
• Increasing the dose beyond the moderate level does not enhance clinical efficacy, suggesting a ceiling effect for PBZ in this model.
• The study supports using moderate doses of phenylbutazone for effective pain management in horses with foot lameness and suggests higher doses may be unnecessary.