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

The objective of this experiment was to compare the clinical efficacy of various dosages of phenylbutazone (PBZ) with a negative control. The hypothesis was that higher PBZ doses would result in improved efficacy in a dose-dependent manner. Eight horses were shod with adjustable heart bar shoes on one front foot. Grade 4.0/5.0 lameness was induced every 2 to 4 weeks by tightening a set screw against the heart bar. Heart rate (HR) and lameness score (LS) were monitored at rest; every 20 min after lameness induction for 5 h; and hourly for another 8 h. One hour after lameness induction, treatment was administered IV in a randomized blinded manner and included negative control (isotonic saline: SAL) or PBZ at 2.2 (LOW), 4.4 (MOD), or 8.8 (HIGH) mg/kg body weight. Jugular blood samples were obtained at experiment hours 0, 1:05, 3, 5, 9, and 13, and were evaluated for plasma PBZ concentration. Results were compared using repeated measures analysis of variance and Student–Newman–Keul’s test with the level of significance set at p < 0.05. MOD and HIGH dose PBZ reduced HR from 3.0 h through 12.0 h and LS from 1.67 h through 12.0 h post-administration (p < 0.05); there was no difference in response between MOD and HIGH dose PBZ. Compared to SAL, LOW dose PBZ reduced HR from 3.67 h through 5.0 h and LS from 3.67 h through 4.0 h after administration (p < 0.05). The conclusions were that LOW dose PBZ was clinically effective for only a brief period compared to SAL, and HIGH dose was not more effective than MOD dose PBZ.