Physiological, biochemical and haematological effects on horses of a phenylbutazone paste

Five matched pairs of horses were used to investigate the biochemical, haematological and general clinical effects of a new dosage schedule of a phenylbutazone paste administered under controlled feeding conditions. One group of horses received a loading dose (8.8 mg/kg) on day 1, followed by doses of 3.3 mg/kg daily on days 2 to 8, 10 and 12 with no treatment on days 9 and 11. The second group received equivalent doses of a placebo paste. Bodyweight, skin temperature, respiratory rate, glutamate dehydrogenase activity, packed cell volume, mean corpuscular volume and neutrophil count were altered significantly in the drug-treated but not in the placebo-treated animals. From the direction and magnitude of the changes in these variables, it was concluded that they did not reflect toxic actions of phenylbutazone. Several variables were unaffected by either treatment both during and after dosing and others were significantly altered in both groups of horses. These changes were considered to be toxicologically insignificant.