“We are not trying to get a behavio change as are people who give hoses cocaine.”
to increase in the fall. He has taken off some horses off the medication permanently. “Twenty years ago, when we were calling it Cushing’s, we said the horse would be on medication the rest of its life, and we would probably have to keep increasing the medicine,” Mike continues to explain. “Now I tell people to keep with this initial dose for a few months to make sure the horse is better. Then let’s cut it in half and go two more months and see if we notice any difference. I believe that many of these horses can improve when we give them pergolide to allow the body to catch up on its dopamine production and be- come healthy. Then as the body gets back into balance, we can decrease the dosage. I also know of horses that go off it seasonally. I know horses that were treated and since then have been off it the rest of their lives.”
Competition Regulations Mike also believes strongly that these horses need to get back to a normal lifestyle that includes regular exercise. With
horses living longer today, people can enjoy them for many more years. Organizations that govern competition are atten- tive to this fact when evaluating their drug and medication policies regarding pergolide. Pergolide is a “forbidden substance” for both USEF and
FEI competitions. However, USEF and FEI placed it in the sub- category of a controlled substance because of the increased number of horses that otherwise could not be active in the show world if not for the medication. According to Mike, there are some very high level competitors with PPID. How- ever, even as a controlled substance, participation in compe- tition becomes tricky. Mike explains by asking, “Do you think it’s okay to give
a horse cocaine and methamphetamine?” His answer is, of course, no. “Cocaine and methamphetamine produces ex- treme amounts of dopamine and produces behavior chang- es. Pergolide is a dopamine mimic. The body thinks pergolide is dopamine,” he continues. “If given in very large doses, pergolide is similar to giving a horse cocaine. However, we
The Science of PPID PPID is the acronym for pituitary pars intermedia dysfunction. The pituitary gland, located at the base of the brain, is often designated the “master gland” because it controls the production and disbursement of various hormones. Of the three lobes making up the gland, the middle lobe or pars intermedia pro- duces adrenocorticotropin hormone (ACTH). The hypothalamus in the brain produces a neurotransmitter called dopa-
mine that inhibits overproduction of hormones, including ACTH. A healthy horse produces enough dopamine for this system to work. A PPID horse does not. By the overproduction of the hormone cortisol (caused by ACTH’s stimula- tion of the adrenal gland), the pituitary gland can become enlarged and fur- ther exerts pressure on the hypothalamus, causing it to further reduce dopamine secretion and debilitate the horse.
Testing The dexamethasone suppression test works by introducing dexamethasone, a synthetic form of the hormone cortisol, into the horse’s system and then testing his blood: If he is healthy, his pituitary gland will respond to the dexamethasone by stopping ACTH production and blood levels of cortisol will return to normal quickly. But if the horse has PPID, his mal- functioning pituitary will continue producing ACTH, which causes the adrenal gland to produce the hormone cortisol unabated, and his blood will contain elevated levels of the hormone. The ACTH left to its own devices can cause tissue to become insulin resistant or unable to control blood sugar level with the normal amounts of the hormone insulin. As the insulin level rises in the bloodstream, the result is often laminitis. The ACTH blood level test determines the level of ACTH at the moment of testing.
How the Medication Works Pergolide is a dopamine agonist (a substance that acts like another substance and therefore stimulates an action) that mimics the naturally occurring dopamine and acts to replace the reduced level of dopamine in horses with PPID. It binds with and activates dopamine receptors, signaling pathways through the dopamine receptor and decreasing the secretion of hormones.
Warmbloods Today 27
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