Pituitary pars intermedia dysfunction (PPID), more commonly known as Equine Cushing’s disease, is an incredibly common age-related endocrine disease in horses. It affects between 15-30% of horses over the age of 15 years old. Any horse can develop this condition, but certain breeds like ponies and Morgans seem to be predisposed.
PPID affects hormone production pathways in the brain, specifically from the hypothalamus and pituitary glands. The release of hormones from the pituitary gland is normally controlled by the hypothalamus through the neurotransmitter dopamine. With PPID, the degeneration of neurons that produce dopamine results in an inability to control the hormones produced by the middle lobe of the pituitary gland (pars intermedia). This leads to an increase in the hormones ACTH and cortisol. High levels of these hormones affect many processes throughout the body.
The classic sign of PPID is a long, curly, retained hair coat. Horse owners often notice that their horse is slower to shed out their winter coat come spring. Another common clinical sign is a change in body condition. This includes developing a cresty neck, losing weight, and loss of muscle, especially along the topline. Another significant clinical signs of PPID is laminitis and chronic hoof abscesses. . Other more subtle and nonspecific clinical signs include decreased performance, recurrent infection, soft tissue injury, infertility, increased drinking and urinating, and abnormal sweating.
A diagnosis is made based on clinical signs and bloodwork. The two most common types of testing are a resting ACTH level and a TRH stimulation test. The resting ACTH measures the level of ACTH in your horse's blood. The TRH stimulation also tests ACTH levels but is helpful in finding earlier stages of disease that may have a false negative result on the resting ACTH. ACTH is affected by the time of year so talk to your veterinarian about the best time of year to test and how time of year will affect the interpretation of the results.
The current gold standard for treatment is a medication called pergolide- commonly known as Prascend. This medication does not cure PPID, but it manages the symptoms by decreasing the levels of circulating cortisol. It is a life long medication and the dose may need to be adjusted as your horse ages. ACTH levels can be rechecked after treating for 1 month to guide dose recommendations. Management of symptoms may also be required during treatment such as body clipping a long hair coat or managing episodes of laminitis. Your veterinarian can guide you through the best management of your horse and their clinical signs.
Written by Dr. Madeline Gehin, 12/19/24
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