Equine Cushing’s Disease: Causes, Symptoms and Treatment
How old is your horse? Is it in its late teens or early 20s? Does your horse drink too much these days? Do you notice any sign of increased urination or sweating? It could be that it’s long curly haired coat doesn’t shed as it should. Your horse is at risk or already has equine Cushing’s disease.
What is Equine Cushing’s Disease?
Equine Cushing’s disease is one of the most common diseases that occur in older horses. This doesn’t rule out the incidence of the diseases in horses as young as 9 or even 7 years old. But it is more common to horses at the age of 15 and above. American physician, Harvey Cushing, first described this disease in 1932.
It is also called pituitary pars intermedia dysfunction (PPID). Equine Cushing’s disease has to do with hormonal disorder. It is a dysfunction of the pituitary gland. This is the small oval gland located at the base of the brain. It influences hormone production as the brain signals the need for it.
Others call it equine Cushing’s syndrome. Some others still refer to it as hyperadrenocorticism. All these are synonymous descriptions of horses with a kind of endocrine disorder. Some people mistake it for other equine diseases like Equine Metabolic Syndrome (EMS). But there are clear differences as you’ll later see.
Dr. Nicholas Frank is the large animal section chief of the University of Tennessee. He identifies the difference in these following words. “Equine metabolic syndrome is a disorder relating to increased metabolic efficiency and obesity that develops in young and middle-aged horses and ponies, whereas Cushing’s is more common in older horses and ponies and is caused by a small tumor developing in the pituitary gland.”
Since research studies like the above have shown the stark differences, it’s no longer known as EMS.
Statistical figures on the prevalence and incidence of this disease are not specific. The specific ones are not harmonious. Still, the available data shows that it affects about 0.1% to 0.5% of all horses. The average age of the diagnosed is 19 or 20. PPID affects all horses and breeds. But it is more common to ponies and Morgan than stallions. Mares and geldings also have gotten positive diagnoses for this disease.
Causes of Equine Cushing’s Disease
The main cause of PPID is the loss of inhibition of the pars intermedia section of the pituitary gland. This results in the enlargement of the gland. The production of certain hormones known as adrenocorticotropin (ACTH) will increase. There can be compression in other areas of the pituitary gland. This is due to the reduction of certain hormones. This also follows compression of certain brain adjacent structures in the pituitary and hypothalamus. This is what veterinarians call pituitary adenoma. It’s the enlargement of this gland resulting in function losses.
Harold Schott is an associate professor at Michigan State University. He puts the above into perspective in his 2001 study, The Michigan Cushing’s Project. He links PPID to oxidative process. He says: “It’s possible that horses who get this disease are more susceptible to oxidative stress in this part of the brain.”
Dianne McFarlane is another assistant professor who makes this point more forceful. She is the pioneer researcher on PPID at Oklahoma State University. She says: “We haven’t proven oxidative stress causes PPID, but there’s been a strong association of oxidative stress to the neurons in these horses compared to normal aged horses.”
Bad nutrition habits can also be responsible for Cushing’s disease. Horses with a history of obesity and metabolic issues are at higher risk of PPID. Environmental changes can also lead to hormone imbalance and cause Cushing’s disease.
Symptoms and Effects of PPID
Symptoms of Cushing’s disease can be vague and variable. Some even confuse it with other diseases in its early manifestation. Not being able to find the right terms, some consider it “old age.”
None of the signs common in horses with PPID can be responsible on their own. So a horse can manifest one or two of those symptoms and yet not have the disease. At the same time, you don’t have to wait until all those signs manifest.
Some horses may experience weight loss and lethargy. This can happen while their eating habit doesn’t change. Some even experience increased appetite, it is common to many of them, and they still don’t gain weight. Instead, they show considerable weight loss.
You can also see some signs over their saddle area and rump. These are where you notice muscle mass loss which is usually caused by the breakdown of protein. In some horses, this will manifest as “potbelly.” It will develop in some horses and ponies when the abdominal muscles are weak and stretched.
There may be fat deposits along the crest of the neck, above and behind the eyes, and above the tail head. Some horses may sweat more than usual, especially in regions where the coat is long. There can also be an increase in thirst and the volume of water intake in many horses. This will lead to increased and more frequent urination. Of course, it may be hard to access this in animals kept in the field.
Some affected horse can become more docile than others. Their response to pain may not be sharp. They may appear to tolerate it. They may have recurrent infections, like dental and respiratory. This is common because Cushing’s disease will suppress their immune system. It may take longer for injuries to heal. Simple bruises or mere mouth ulcers may take more than a usual treatment. Sometimes, the coat of the horse may appear permed. There can also be cases of laminitis that seem to defy all medical explanations.
In general, you may see more than three or four of the following signs in your horse. Take the horse to a local veterinarian for proper testing for Cushing’s disease.
1) Hypertrichosis, that is, long and curly hair
2) Delayed hair coat shedding
3) Change in body conformation. It can also show as rounded abdomen or a “potbelly”
4) Decreased athletic performance
5) Loss of muscle mass
6) Change in attitude/lethargy
7) Excess fat deposits
9) Increased vulnerability to infections
10) Polydipsia, that is, increased thirst and drinking
11) Abnormal (usually increased) urination also called polyuria
12) Delayed wound healing
13) Recurrent infections
14) Absence of reproductive cycle/infertility
15) Neurologic deficit/blindness
16) Weight loss
Diagnosing Equine Cushing’s Disease
As noted earlier, you are likely to see it with literal eyes when a horse is having PPID. But consider the words of Schott, quoted earlier. “You can look at a horse who has an unnaturally shaggy hair coat and figure it’s probably PPID, but we encourage owners to get a good veterinary examination and evaluation, including bloodwork.” This blood work can be either CBC or serum chemistry or both.
It’s not all straight to diagnose PPID. Several factors usually play roles in the diagnosis. These factors include: clinical signs, medical history, and specific hormone tests among others.
The following can spring up complications in diagnosis:
- The progress of the diseases is slow
- Early tests don’t usually reflect the true state of things
iii. The disease develops in stages; tests in the mature state are more reliable
- The hormone outputs vary with seasons
- There’s no true gold standard of measurement
Dr. Frank, mentioned earlier, speaks further on this. “One limitation of the currently available diagnostic tests is that they may not detect early Cushing’s disease. An experienced veterinarian may, therefore, recommend a six-month trial period on pergolide on the basis of clinical judgment alone.”
A veterinarian will analyze any of the 16 clinical signs listed above. The vet will treat each symptom and place the horse under observation. The horse’s response will give the vet an idea of what’s happening. The next is to weigh the horse’s response against its medical history. If the horse has had a case of insulin resistance or ulcers, the diagnosis may conclude here. Previous issues of obesity and metabolism are also crucial.
Anything beyond that will call for laboratory testing. Actually, a vet may decide to perform a lab test plus the earlier clinical assessment.
To arrive at a conclusive diagnosis, veterinarians can choose any of the following
- Test the level of adrenocorticotropic hormone (ACTH)
A vet will take a single blood sample of a horse through the ACTH test. This will show the level of adrenocorticotropic hormone present in the horse’s blood. The vet will carry out glucose measurement and check insulin concentration levels. The vet should use the same blood sample to do this. This is because a horse with a medical history of EMS may remain insulin-resistant.
A vet should remember that even at this point, false results are not uncommon. Other factors can shoot up the levels of ACTH. Besides improper sample handling, time of the day can distort the hormonal balance.
“You can measure levels at 10 a.m. and come back an hour later and get a completely different reading,” says Schott.
Even the season or month can bring different results. Schott says: “Because ACTH is elevated in normal horses, we try not to test between August and November.”
The horse’s stress level and pain can affect the results. Schott concludes: “This is because ACTH is released in a pulse fashion when sending signals to other glands.”
- Conduct a thyrotropin-releasing hormone (TRH) stimulation test
It is possible to detect PPID with the thyrotropin-releasing hormone (TRH) response test. Veterinarians usually combine this with the dexamethasone suppression test. They analyze the cortisol levels after they have administered dexamethasone injection. They will later introduce TRH and then measure cortisol levels.
The result is usually accurate even though the use is less frequent. It is because it is more costly and requires extra sampling.
iii. Perform the dexamethasone suppression(DST) test
Another one is the low-dose dexamethasone suppression test (DST). Dexamethasone is a synthetic steroid hormonelike to cortisol. A veterinarian performs this test by adding the dexamethasone to the horse’s blood. This will be in a low dose. The vet will inject the horse with its sample. Under normal circumstances, this intramuscular injection will cause a decrease in cortisol levels. But the horse with PPID can’t respond as appropriate to the injection. So the cortisol level will remain high.
This test is also considered a better and accurate test. It doesn’t have a lot of complications. The only challenge is that the vet must carry out the test on two occasions within a 24-hour period. The vet will then compare the result. Another issue is that there are concerns that dexamethasone can induce laminitis.
Other testing options include:
- ACTH stimulation test
- Urine cortisol to creatinine ratio test
- Test that measures resting glucose levels
vii. Combination of any of the above
According to Dr. Frank, “The most practical test is the resting plasma ACTH concentration test.” Why? “Because it is easily performed and can be included in annual spring wellness examinations for older horses.”
Treatment and Management of Cushing’s Disease
It’s quite unfortunate that Cushing’s disease is irreversible. The usual thing is to divert efforts toward managing it. Worse still, if it is not well managed, it can deteriorate. But the horse owner can provide supportive care and medication.
Supportive care includes dietary changes. There are diets fortified with the right vitamins and minerals. Such diets should also contain high levels of antioxidants. They should be low in non-structural carbohydrates (NSC). This is because horses infected with PPID are insulin-resistant. They are also prone to high blood sugar.
So, don’t let such horses feed on forages with high NSCs. Since NSCs are what account for the horse’s blood sugars and starches, they should be less than 20%. In critical cases, it should be about 10%.
Triple Crown has feeds manufactured to fill that need. They include:
- Triple Crown Low Starch which is corn pelleted feed with 13.5% NSC level
- Triple Crown Lite which is a pelleted feed with of 9.3% NSC level
- Triple Crown Senior which textured feed with 11.7% NSC level
- Triple Crown Safe Starch Forage, a pasture grass with less than 10% NSC level
Feeds like these will support the weakened immune system. If the management starts early enough, the horse can still enjoy a comfortable life.
Management of EMS focuses on dietary changes and weight control. But most of Cushing’s disease management efforts need medication. The drug pergolide is generally accepted as management medication for Cushing’s disease. That’s what most vets prescribe.
This drug is also known as dopamine agonists. Its oral administration suppresses the activity circulating ACTH and lowers other hormone levels. It replaces lost inhibition that initially caused the disease. Its side effects are minimal. These side effects include: anorexia, colic, diarrhea, and depression. The medication has reported 65-85% effectiveness.
In case pergolide fails to control it, you can add serotonin antagonists. It also can replace the lost inhibition of the pituitary. It may increase dopamine effectiveness. Cortisol antagonists are another option. Veterinarians are in the best position to work on your horses with these medications.
Routine vet examinations are also an important part of the management for PPID.
Vets have another option besides dietary changes and medication. They can clip the excessive hair of the horse. They also examine the affected horse for injuries and infections. They will ensure prompt treatment of these. So, they know when they should add antibiotics to the medication. They need this to enhance the quick healing of wounds.
You should also schedule routine appointments with a farrier and dentist. You must follow the schedules for vaccination, and deworming exercise.
Area of Confusion
If care is not taken, it’s all too easy to confuse equine Cushing disease with equine metabolic syndrome. Dr. Frank also admits this. “Cushing’s and EMS feature striking clinical similarities, most notably chronic laminitis.” Dr. Frank then adds “But the underlying disease biology is quite different.”
The confusion arises because of the similarity in symptoms. Horses and ponies with EMS are prone to PPID. When they develop PPID later in life, the insulin resistance becomes worse.
So if you want to manage PPID in your horse and get good results, you must understand the difference.
The table below may help bring those differences to the forefront:
|Equine Cushing’s Disease Versus Equine Metabolic Syndrome|
|1. Susceptibility||Horses above the age of 15||Any mature horse. It usually manifests between the ages of 5-15.|
|Any ponies and Morgans||Only “easy keeper” horse and pony breeds|
|2. Cause||Enlargement of the pars intermedia which results in abnormal hormone production||Genetics, insulin resistance, metabolic deficiency, obesity|
|3. Clinical signs||
Delayed shedding, in the early stages;
Long wavy hair coat in an advanced stage;
Fat accumulation, etc
|Obesity or fat deposits in certain regions, e.g. “cresty neck.”|
|4. Diagnosis||Clinical signs||Clinical signs|
|Increased adrenocorticotropic hormone levels||Screening test to show an increased blood insulin level with normal glucose level. Detection of hyperglycemia, especially in diabetes mellitus cases.|
|Abnormal dexamethasone suppression test results. (Cortisol will not suppress)||Glucose tolerance test results are abnormal in advanced tests.|
|5. Treatment and management||Pergolide||Management|
|Serotonin antagonists||Levothyroxine or metformin, in extreme cases only|
|Other supportive cares|
Is it possible to avoid PPID in a horse? Yes, it is possible. Horse owners need to carry out supportive care as a preventative measure against it. But, if you notice your horse is suffering from it, your focus should shift towards effective management measures.