According to this article by the Kentucky Equine Research Staff, it is believed that 20-33% of horses will develop Equine Cushing’s Disease in their lifetime. That is an incredibly high percentage! Most horses are over the age of 15 at diagnosis. Although Cushing’s Disease is the most commonly recognized name for this endocrine dysfunction, a more appropriate and recent term is pituitary pars intermedia dysfunction, or PPID.
Many people believe that an abnormal hair coat or lack of shedding in the spring, long “guard hairs”, and a cresty neck are the earliest symptoms of this disease. While these abnormalities are often associated with PPID, they may not be the earliest indicators of a problem…
Years ago, I owned an Arabian gelding named Kari that ultimately received a diagnosis of PPID. The first symptom I experienced with him was extreme lethargy. I was riding with some friends one day and he simply could not keep up. He had no other signs of disease. He was around 16 years old.
I lived in Illinois at that time, and I had never identified an equine veterinarian that I really liked. I called the guy I was using and told him I wanted to work up my gelding. Because Kari had no other clinical signs, I had to browbeat this guy into drawing blood for a workup. However, I was certain there was something wrong with Kari and I wanted answers.
Sure enough, the lab work revealed PPID. Because Kari had no other symptoms at that moment, I wanted to take a few days to process the information and formulate a plan of action. Sadly, my decision to not immediately begin treatment was a poor choice, as Kari foundered within days of the diagnosis. His laminitis was severe and his future was irrevocably impacted.
Ultimately, I did start him on Pergolide, one of the treatment options for PPID. (More about treatment in a future blog.) However, his severely damaged feet ultimately forced the decision to euthanize him.
What causes PPID?
Researchers are still trying to pinpoint what causes so many horses to develop PPID. This scholarly article states:
The current pathophysiological theory for PPID is an age-related oxidative stress-induced neurodegeneration of the dopaminergic neurons of the hypothalamus.
So what does that mean? First, let me explain oxidative stress. During normal cellular function, the cells produce by-products (free radicals) that are typically neutralized by another substance (antioxidants) also produced by the cell. Normally, the body can maintain a balance.
However, as the body ages, that dynamic can change. Too many free radicals affect the neurons in the hypothalamus, resulting in an increase in ACTH hormone (as well as other hormones) produced by the hypothalamus. Scientists are not sure why this oxidative stress occurs.
Why is it called PPID? This article provides a very succinct explanation about the role the pituitary gland plays in this disease, in conjunction with changes in the hypothalamus.
“The pituitary gland drives the production of hormones in the body to maintain homeostasis, or a state of balance in the body. PPID results from the abnormal, but benign, growth of a specific region of the pituitary gland called the pars intermedia,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER). “Growth of the pars intermedia occurs secondary to the degeneration of the hypothalamus, another region of the brain. The hypothalamus links the endocrine and nervous systems, playing key roles in controlling thirst, hunger, sleep, and mood.”
ACTH stands for adrenocorticotropic hormone. This is one of the substances the body produces in response to stress. It is the increase in this stress hormone that causes the symptoms of PPID in horses.
What are the signs of PPID?
The classic clinical signs of PPID in horses include the following:
- Unusual hair coat (no shedding; delayed shedding; wavy, curly, dense, or patchy haircoat; long guard hairs)
- Hoof problems (laminitis, sore feet, persistent abscesses)
- Abnormal fat deposition (cresty neck, fat deposits around the tailhead)
- Lethargy (This was the only sign in my gelding!)
- Changes in water consumption (PPID horses drink more water, and therefore urinate more)
The earlier a horse is diagnosed, the better the prognosis for the future. It was the laminitis and severe rotation that necessitated the euthanasia of my Arabian.
How do you test for PPID?
The most common test for PPID is a blood test to measure the level of ACTH in the blood. Normal levels of ACTH can fluctuate with the time of year. Sometimes, clinical signs and test results can be confusing. This article makes the following suggestions:
Horses and ponies with ambiguous results (suggestive clinical signs with normal ACTH results) or in the gray zone (clinically normal horses with mildly elevated ACTH levels) should either be resampled during autumn or undergo a TRH stimulation test measuring ACTH to improve sensitivity.
TRH stands for Thyroid Releasing Hormone. In this test, blood is drawn, and then TRH is given to the horse. Exactly 10 minutes later, another blood sample is drawn. The level of ACTH is measured in the before and after blood samples. If ACTH levels increase dramatically in response to the administered TRH, the horse is considered to have PPID.
Many veterinarians now consider the TRH stimulation test to be the best approach for an accurate and early diagnosis. Early cases of developing PPID might not be diagnosed if only the ACTH levels are measured.
In next week’s blog, I’ll talk about treatment options and prognosis.
(Feature photo shows two 32-year-old horses diagnosed with both PPID and Insulin Resistance. Both horses receive treatment. The donkey has Insulin Resistance.)