AAEP Convention: Peripheral Cushing's

Peripheral Cushing's syndrome (PCS) is a recently named problem seen in middle-aged horses and ponies with obesity-associated laminitis. These horses tend to accumulate fat in the crest of the neck, over the rump, and in the sheath of males. It was first proposed that classic Cushing's disease--a pituitary gland disorder that leads to excessive cortisol levels in the blood--was causing the obesity-associated laminitis. (Cortisol is primarily produced by the adrenal gland; some of its effects include conversion of amino acids to glucose in the liver, elevation of blood sugar levels, and promotion of glycogen storage in the liver.)

A significant, sustained rise in the level of cortisol circulating in the blood can result in clinical signs of classic Cushing's disease, such as a long, curly hair coat and laminitis. However, when tested for classic Cushing's disease using the overnight dexamethasone suppression test (DST), these overweight, laminitic horses/ponies tested normally, i.e., had normal pituitary gland function.

In "Thyroid Function in Horses with Peripheral Cushing's Syndrome," Emily Graves, VMD, of Michigan State University, discussed what researchers have discovered about this disease. In a recent study, researchers at Michigan State evaluated thyroid gland function in horses with PCS to see if thyroid function was normal. They confirmed their hypothesis that the thyroid gland functions normally in PCS-affected horses. Graves said their findings helped to rule out hypothyroidism as a risk factor for development of obesity-associated laminitis.

In addition, preliminary data indicate that oxoreductase activity of 11-beta hydroxysteroid dehydrogenase (HSD) is increased. This enzyme converts inactive cortisone into active cortisol in the peripheral tissues throughout the body. This mimics a human disorder called central obesity or metabolic syndrome, in which elevated HSD action is the cause. The human disorder is serving as a model for further characterization of PCS in horses and ponies.

Additional endocrine testing of PCS-affected horses also found elevated serum insulin concentrations. Graves said that based on the study, serum insulin concentration should be measured and combined with an overnight DST to evaluate and monitor horses with obesity-associated laminitis. An additional goal is to develop a test that measures HSD activity in body tissues. She commented that it is still not known if PCS-affected horses might be at a greater risk for development of classic Cushing's syndrome, or if horses with classic Cushing's syndrome have an elevated oxoreductase activity of laminar tissue HSD.

Graves said that more research is needed since this disease is still not well understood in equine species, and noted that there could be multiple metabolic/endocrine disorders involved. She recommends that PCS-affected horses be treated with appropriate corrective trimming and shoeing, non-steroidal anti-inflammatory drugs for laminitis-associated pain, and that weight should be reduced through diet and exercise once laminitic pain is controlled. She said that several horses treated in this manner have clinically improved and have shown a reduction in serum insulin concentrations.

Philip Johnson, BVSc, and colleagues at the University of Missouri's School of Veterinary Medicine, are working to develop methods of measuring 11-beta HSD activity. As the human and veterinary medical communities gain further knowledge of this disorder, improved treatment options should become available. Until that time, laminitis management and weight reduction where appropriate will be the primary therapies.

More information: See article #4058 at www.TheHorse.com.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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