You’ve noticed that your older horse seems to be hanging onto his winter haircoat for a little longer than his younger companions. His haircoat looks a little different from the others as well – his hair is long and shaggy with noticeable guard hairs, and it may have a bit of a wave. Other things may be changing in your old guy too: he seems to be drinking more water, has begun getting hoof abscesses (or other infections), putting on weight in weird places, and/or has lost some muscle along his topline. He may have even had a bout of laminitis (founder).
What could be going on?
From a veterinarian’s perspective, when we see an old shaggy horse with these types of symptoms, our first three thoughts are Cushing’s disease, Cushing’s disease, and then Cushing’s disease! You may have heard of Cushing’s disease by its other newer name, Pituitary Pars Intermedia Dysfunction (PPID), or heard it discussed by your vet, farrier, feed store or trainer.
Cushing’s is Common! We recommend testing and treatment for this disease on a weekly if not daily basis. Most horses with Cushing’s will be over 20 years old, but it has been diagnosed in horses as young as seven.
Variations of this disease are also seen in people and dogs. A long, shaggy haircoat is the most classic symptom.
What is Cushing’s Disease/PPID?
In it’s most basic form, Cushing’s disease can be described as a hormonal problem affecting the body’s normal production of steroids. Most often, Cushing’s disease is caused by the development of a benign tumor called an adenoma in the pituitary gland.
This tumor, by itself, is benign – we don’t see signs of spreading or metastasis throughout the body like other cancers. Normally, the pituitary gland is responsible for producing regulatory hormones like dopamine, oxytocin, melatonin and others.
With Cushing’s disease, a certain part of the pituitary gland (the “pars intermedia”) develops this tumor, which ends up producing more dopamine than a normal-sized pituitary. Increased levels of dopamine will cause the production of other hormones to shut off, leading to a buildup of other hormones (“POMC”s, or proopiomelanocortins) which then leads to an overproduction of cortisol, or “steroid”. Increased levels of cortisol in the bloodstream cause all sorts of problems!
Why did my horse get a tumor?
The cause of Cushing’s disease (and therefore the tumor) is still unknown, but there are several theories out there. One thought is that the pituitary gland is “prone” to developing tumors, especially in older horses, and therefore Cushing’s is a common side effect of aging. Another theory discusses the possibility of degeneration between the neurons in the pituitary gland, which is similar to the cause of Parkinson's disease in people.
Why is this a problem?
A horse with an elevated level of steroid circulating throughout the body is a bit like a person trying to function under a constant level of extreme stress. Both situations cause elevated cortisol levels in the bloodstream, which can lead to a whole host of problems:
Decreased immune function
Hoof abscesses, tooth root abscesses, sinusitis, increased susceptibility to viruses, skin infections, poor wound healing
Fat deposits (can be present in skinny horses too)
Hair growth/hirsutism – not always present
Long guard hairs, wavy hair, failure to shed out fully in summer
Some horses will fully shed out, but grow thicker hair and/or take longer
Less common signs:
How do I know for sure if my horse has Cushing’s?
Cushing’s disease can be challenging definitively diagnose, and no test is 100% accurate. There are three widely-used testing methods out there that are recommended depending on the horse and time of year.
Resting ACTH+/- insulin panel: This is one of the most common tests offered, as it is the simplest to obtain in the field. The veterinarian will take one sample of blood, which is then processed before being shipped to an endocrine laboratory. These levels fluctuate with time of day and season, which can affect results.
Of course, horses don’t always read the veterinary textbooks, so results from an ACTH test may be in a grey-zone or even normal. If the vet is still suspicious of Cushing’s, he or she may either recommend starting treatment anyway, or pursuing additional testing.
TRH Stimulation Test (Thyrotropin-Releasing Hormone): This test involves multiple blood draws over a 30 minute period. The veterinarian will take a “baseline” blood sample for ACTH, then inject a drug that, in horses with Cushing’s, will usually cause an over-production of ACTH. A second blood sample is taken 10 minutes after administration, and a third blood sample may be taken 30 minutes after administration.
This test can be tedious, time-sensitive and expensive, which is why it is not usually recommended as a first-time test.
DS Test (Dexamethasone Suppression): This test also involves multiple blood draws. The veterinarian will take a baseline blood sample, and then inject your horse with a specific dose of dexamethasone (a steroid). A second blood sample will need to be taken the next day. A horse with Cushing’s is usually unable to regulate the way they respond to steroids, and comparison of the two blood samples will show this.
While this test is often touted as being more specific, many veterinarians use it with trepidation as laminitis has been anecdotally seen after administration of dexamethasone.
Is there a treatment for Cushing’s?
While Cushing’s can’t be cured, there are treatments!
Pergolide (brand name: Prascend) is a once-daily pill used to treat horses with Cushing’s. This drug is a dopamine agonist originally formulated to treat people with Parkinson’s disease, and helps regulate the pituitary gland to work in a more normal fashion. Most horses will show a noticeable change in attitude and quality of life during treatment. Few side effects are seen apart from a transient (short-acting) anorexia and lethargy when medication is first started – this usually resolves within a few days.
Most veterinarians recommend Prascend (the brand name product) as it is the only formulation that is FDA-approved and clinically proven to work. Other compounded forms of pergolide are available and while they may be less expensive, the amount of active ingredient can vary significantly.
Cyproheptadine is another drug sometimes added as an adjunct therapy to horses already on pergolide. This medication is a serotonin blocker and has been shown to be effective but less consistent in modulating symptoms of Cushing’s.
Good preventative care – this is the most important aspect! Since Cushing’s can predispose to all sorts of problems, it is always recommended that your horse be up to date on all vaccines and undergo a good examination with dental care on a routine basis.
Cushing’s disease or PPID is very common in older horses and can often be treated, providing a better quality of life for our old friends. Catching this disease in the early stages can often avoid further problems down the road. Contact us or your regular veterinarian for any questions or if you think your horse has Cushing’s disease.
Dr. Katy Brandes