Glucocorticoid cream applied at normal doses onto healthy equine skin will be quickly absorbed into the body's system and can generate adverse systemic effects, said German researchers in a new study report. As a result, it can also cause a horse to test positive in performance drug tests within two days.
When used to treat allergic skin reactions, glucocorticoids are usually given topically instead of orally to reduce systemic side effects. However, this new research suggests that the drug can cause such effects regardless of how it is administered, said Getu Abraham, DVM, PhD, researcher at the Institute of Pharmacology, Pharmacy, and Toxicology at the Faculty of Veterinary Medicine of Leipzig University in Leipzig, primary author of the study.
Researchers found that glucocorticoid cream applied to the skin can affect the horse systemically.
In the study, normal therapeutic doses (8.5 mg) of a relatively mild glucocorticoid (dexamethasone) were applied to a shaved area of skin on the necks of 10 horses twice daily for ten days. By the second day of the test, blood analyses were already revealing signs that the steroid was circulating throughout the body. Specifically, the normal activity of the hypothalamus-pituitary-adrenal (HPA) axis was lessened, causing significant adrenal suppression which lasted a week after treatment ended. This is a red flag not only for doping tests, but for stress crises as well.
"When the adrenal gland is suppressed, the normal stress-relieving hormone, cortisol, is not being released," Abraham said. "The horses are at risk of greater fatigue and metabolic disturbances when faced with performance, separation, training, traveling, illness, or any other kind of stress because they are deprived of the cortisol."
White blood cell counts were also significantly reduced, indicating that the immune system was weakened.
"We were really surprised to see such massive systemic changes in such a short time," Abraham said. "And this was in healthy horses. In horses being treated for dermatitis, where the skin is irritated, the effects might even be more pronounced; but that is yet to determine."
Even so, he said, systemic adverse effects do appear to be less extreme with topical administration, compared to oral and intravenous administration of glucocorticoids.
The study, "Effects of dermal dexamethasone application on ACTH and both basal and ACTH-stimulated cortisol concentration in normal horses," was published in the August 2009 Journal of Veterinary Pharmacology and Therapeutics. The abstract is available on PubMed.
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