WHAT IT IS: Also known as Cushing’s disease, PPID is caused by an enlarged and overactive pituitary gland producing excess levels of adrenocort­icotropic hormone (ACTH).

These elevated ACTH levels, in turn, lead to overproduc­tion of the steroid hormone cortisol. What causes PPID to occur is still unclear, but it’s extremely common in older horses: One study found that 85 percent of veterinari­ans are caring for at least one horse with PPID.

SIGNS: Excess hormones cause a slew of systemic signs, including a long hair coat that is slow to shed, muscle-wasting, lowered immune function and susceptibi­lity to infection. Some horses with PPID are also prone to slow-onset chronic laminitis that can go undetected until hoof structures have been irreversib­ly damaged.

DIAGNOSIS: In addition to clinical signs, veterinari­ans utilize two laboratory tests to diagnose PPID. The first is a blood test to measure resting levels of ACTH. The second is the thyrotropi­n-releasing hormone stimulatio­n test (TRH stimulatio­n test), which involves taking a blood sample, giving an injection of TRH, and then collecting a blood sample again 10 minutes later.

ACTH will increase in all horses between the first and second blood test, but horses with PPID will show a dramatical­ly higher increase. Because of natural seasonal and geographic fluctuatio­ns in ACTH, veterinari­ans will use published reference ranges to interpret laboratory results.

TREATMENT: PPID can be controlled with the medication pergolide, sold under the brand name Prascend. Pergolide works by binding with receptors in the brain that normally respond to dopamine, decreasing the blood levels of ACTH. The medication is given daily and is considered very effective in reducing the signs of PPID.

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