Cushing's Disease and Laminitis--Not Just Old Horses

Researchers recently revealed that Cushing's disease is a major contributing factor to laminitis among horses in a primary care veterinary practice, and that the median age of laminitic Cushing's horses was 15 1/2 years--disproving that this problem is just an "old-horse" disease. Along with his co-authors, Mark T. Donaldson, VMD, Dipl. ACVIM, a field service veterinarian at the University of Pennsylvania's New Bolton Center, also said that some horses with laminitis might have Cushing's, also called pituitary pars intermedia dysfunction, without displaying conspicuous characteristics typically associated with the disorder.

Courtesy Mark T. Donaldson, VMD, Dipl. ACVIM

A Cushing's horse with bulging supraorbital fossas, or excess fat over the eyes. Click here to see other common features of Cushing's horses.

Cushing's disease is caused by degeneration of the hypothalamic dopaminergic neurons, which are those in the hypothalamus (which controls metabolism by exerting influence on the pituitary gland) using dopamine as their neurotransmitter. The pituitary and adrenal glands in turn produce abnormal amounts of hormones that play a vital role in regulating metabolism and inflammatory and immune responses.

Donaldson began this study in 1996, after diagnosing and confirming (post-mortem) Cushing's in a 4-year-old pony. The finding struck Donaldson as unusual, because Cushing's is often linked to older animals. "That's what prompted my initial interest, because I saw that it could occur in relatively young or middle aged horses," he said. "Most laminitis research is done in a hospital setting, where many cases develop from severe diarrhea, retained placentas, and as a complication of colic, while in an ambulatory population, laminitis cases often are of unknown origin."

Beginning with that case, Donaldson tested every horse he diagnosed with laminitis for Cushing's and found that 28 of of 40 (70%) laminitis cases were horses suspected of having Cushing's as defined by high concentration levels of the hormone plasma adrenocorticotropin (ACTH). He chose the ACTH test over the dexamethasone suppression test (DST) as the main diagnostic test for the study because there has been a reported association between laminitis and corticosteroid administration, and that test would require the administration of the corticosteroid dexamethasone. The incidence could be even higher, because at least one horse showed unremarkable findings on the ACTH test, later on the DST, and the thyrotropin releasing hormone stimulation test (yet another useful test for Cushing's). However, this horse had Cushing's confirmed post-mortem when a pituitary pars intermedia tumor was found.

Chronic laminitis was found to be significantly more common in the Cushing's horses (22, or 78.6%) than those with ACTH levels in normal range (five, or 41.7%).

While Cushing's was highly evident in the study population, "It's not the sole cause of laminitis--there are many horses with Cushing's that do not have laminitis. It's a contributing factor to laminitis, but not the sole cause," said Donaldson.

Management and Clinical Features
Access to lush pasture was reported in seven of the 28 Cushing's horses (25%), and seven of 12 horses (58.3%) without Cushing's, so excessive consumption of lush pasture was considered a possible cause of laminitis. September was the most common time of laminitis onset in the Cushing's horses; May was the month when most of the horses without Cushing's had laminitis appear. The study reported that it was possible that seasonal variation in the nutrient quality of the grass was associated with the onset of laminitis, since most of the horses were maintained on pasture.

Hirsutism, or abnormal hairiness, is a common clinical observation in Cushing's horses, but Donaldson emphasized from the findings of this study that horses with normal coats can be affected. "People look at a horse and say ‘he has a normal coat, so he can't have Cushing's,' " which isn't true. Only five (17.9%) of the Cushing's horses had a long hair coat. Previous retrospective studies of Cushing's cases have shown 94% hirsutism occurrence, but retrospective studies might have selection bias for horses with hirsutism since it is a sign often attributed to the disease.

Another notable finding was the median age of Cushing's horses with laminitis. "We were surprised to find that the median age was 15.5 years (ages ranged from three to 28 years). Most people think of horses with Cushing's in their 20s. This finding is probably because of the nature of the study--we were testing all these horses that didn't have other signs of Cushing's." In fact, 1/5 of the horses showed no visible clinical signs of Cushing's at all.

Donaldson found that the most common clinical signs in the Cushing's horses were abnormal body fat distribution, cresty necks, bulging supraorbital fossas (excess fat above the eyes), and fat deposits over the hindquarters. Click here to see images of horses with these features.

Treatment and Outcome
Out of the 28 Cushing's horses, eight were euthanized (28.6%), mainly because of severe laminitis. According to Donaldson, it was difficult to make assessments about treatment according to outcome of cases in this study since he wasn't examining treatment methods, but pergolide treatment seemed to have very positive results.

"It's important that people recognize that you have to adjust the dose of pergolide based on an endocrine function test--in some cases the dose might be increased to control Cushing's disease. "We usually start at 1 mg. per thousand pounds, but that frequently that only seems to cause normal endocrine function tests in about 35% of horses. If you increase the dose to 1.5 mg, (the normal function tests are driven) up to 65%."

Future Research
Donaldson next plans to examine the seasonal aspect of laminitis, look at Cushing's treatments, and evaluate better diagnostic techniques for Cushing's.

"At least one other study has shown that horses are more likely to develop laminitis in the fall," said Donaldson. He explained that there are a couple of reasons why--horses at pasture reach their peak body weight in the fall, and seasonal changes could affect endocrine function.

Additionally, Donaldson would like to look at obesity and Cushing's. "So many of the horses that didn't have Cushing's had high insulin concentrations and may have actually had Cushing's but the (ACTH) test didn't show it. We'd like to figure out how obesity is related to laminitis. We are currently looking at the endocrinology of obesity in horses since some of the hormones secreted by the pituitary gland also control body weight."

Donaldson stresses to horse owner, "Test any horse for Cushing's that develops laminitis for an unknown reason, especially cases that were blamed on consuming lush grass.

"I'd also warn that obesity might be a contributing factor (to laminitis). So if you suspect your horse has Cushing's disease, make sure it does not become overweight. If you treat (Cushing's) with pergolide, follow up with work with your veterinarian that confirms your horse is getting an adequate dose of the drug."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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