Seasonal Effects on Texas Cushing's Test Results (AAEP 2011)

Seasonal Effects on Texas Cushing's Test Results (AAEP 2011)

"ACTH concentrations in Texas are affected by season," Buchanan concluded. "Our research was consistent with others findings in the US and abroad that there is a seasonal effect of ACTH in Texas."

Photo: Anne M. Eberhardt/The Horse

In horses with Cushing's disease, veterinarians often measure blood levels of adrenocorticotropin hormone (ACTH) to monitor treatment response. Seasonal variations in these levels--important for test result accuracy and treatment recommendations--have been confirmed in several geographic areas, with no seasonal changes reported in others. Ben Buchanan, DVM, Dipl. ACVIM, Dipl. ACVECC, and colleagues examined seasonal ACTH testing in Texas horses, presenting results at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas.

Buchanan, of Brazos Valley Equine Hospital in Navasota, Texas, reviewed that older horses are subject to developing benign tumors (adenomas) or enlargement of the pituitary gland, often referred to as Cushing's disease or pituitary pars intermedia dysfunction (PPID). Resulting decreases in dopamine production allow the pars intermedia to become overactive, which leads to an elevation in circulating ACTH. Alterations in the pituitary hormones put these horses at risk for developing laminitis along with chronic infections and the inability to absorb nutrients optimally. Current Cushing's disease treatment includes pergolide, a dopamine agonist that mimics the effects of dopamine in PPID horses - this effect suppresses ACTH secretion, which is what veterinarians assess. Buchanan noted that there is a known seasonal increase of ACTH in all horses during late summer and fall, recognized as normal, likely due to the effect of changing hours of daylight on the pituitary gland.

The study consisted of 15 horses: Three had definitive diagnoses of PPID based on their hirsutism (having a long and curly hair coat, now more appropriately termed hypertrichosis) or previous dexamethasone suppression testing; the other dozen were confirmed non-PPID horses. According to Buchanan, fasting blood samples were obtained monthly for one year between 8 and 10 a.m. and shipped to reliable reference labs for ACTH and insulin level analysis. Feed and pasture samples also were collected for analysis.

"We need to establish seasonal normal values or we should avoid testing in autumn months if we are only using a single value for year-round analysis," urged Buchanan. Even the healthy horses had greater than 35 pg/mL of ACTH in late summer and fall--a level usually associated with a PPID diagnosis. Such variations could lead to false positives when testing in autumn months.

Buchanan noted that season did not significantly affect plasma insulin concentrations. However, there were differences throughout the year related to changes in pasture nonstructural carbohydrate content that might exert significant effects that could trigger pasture-associated laminitis. He commented that weather itself likely has more to do with pasture nonstructural carbohydrate content than overall seasonal influences and recommends that researchers conduct further studies to determine the role of pasture and how it impacts insulin resistance and PPID.

"The take-home message is that ACTH concentrations in Texas are affected by season," Buchanan concluded. "Our research was consistent with others' findings in the U.S. and abroad that there is a seasonal effect of ACTH in Texas."

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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