Oral Prednisolone Administration in Horses

Oral prednisolone is frequently used in dermatological and respiratory treatments. Aims were to study the effects of oral prednisolone on endogenous cortisol concentrations, adrenal suppression and detection time in urine (‘doping- positive’).

Six horses (3 mares, 3 geldings; 527±94 kg) received oral prednisolone in capsules (1 mg/kg bwt) for 5 days (Day0 - Day4) at 07:30h. Prednisone, prednisolone and cortisol concentrations were estimated for each horse in 65 blood samples collected from 2 days before treatment until Day8.

At 09:00h on Day8 ACTH and cortisol concentrations were determined before 0.25 mg β1-24 corticotrophin (Synacten®) was given i.v. Cortisol concentration was determined again at 11:00h (ACTH-stimulation test). Morning-urine samples were collected at 5 and 7 days after the last day of treatment (Day4) and analysed by an official FEI-laboratory.

Maximal mean prednisolone concentration (271±188 nmol/l) was achieved 105 min. after administration (range 15-150 min.) and showed a clear day-night rhythm.

Endogenous cortisol concentrations returned to normal morning values on the 4th morning after the end of the treatment. ACTH-stimulation test was normal in all horses (ACTH concentration 21.3±15.8 pg/ml and cortisol concentrations 143±56 and 394±54 nmol/l respectively).

Prednisone and prednisolone concentrations in the urine 5 and 7 days after the end of treatment were all below limit of detection (LOD). In all horses 20-dihydro-prednisolone and 20-dihydro-prednisone urine- concentrations were above LOD on 5th day but below on 7th day. In 3 out of 6 horses 20-β-dihydrocortisone urine-concentration was still above LOD (‘doping positive’) on 7th day.

Five-day oral administration of prednisolone only caused temporary disturbance of the endogenous cortisol production; no adrenal suppression was detected. Withdrawal-times in sport horses need further investigation.