Cushing’s Disease in Horses

You have just discovered that your horse has Cushing’s disease?  You want to know as much about it so that you can extend the usefulness and quality of life for your horse?

Equine Cushing’s Disease is also known as Cushing’s Syndrome and also referred to as Pituitary Pars Intermedia Dysfunction (or PPID).  It is normally considered a condition of people and old dogs but has recently been accepted as a hormonal (endocrine) disorder in horses.  While the condition is relatively rare, the recently developed tests have allowed it to become more easily identified, which has created a lot of interest.

cushings horse

This is Waverly, a prize dressage horse, showing the heavy hair growth of Cushings Disease. Photo courtesy of S.D., Queensland Australia.

It may be caused by a tumour or other dysfunction of the pituitary gland, or endocrine system in general, which in turn leads to over-secretion of cortisol by the adrenal gland.  The pituitary gland is located near the base of the brain and it controls most of the body systems by secreting hormones (made in other parts of the body).  With Equine Cushing’s Disease, the part of the pituitary gland known as the pars intermedia, becomes enlarged and secretes large amounts of a variety of hormones.  The most notable of these is the stress-hormone cortisol which is produced in the adrenal glands.

Persistent and abnormally high levels of hormone secretion causes a wide range of detrimental health effects for your horse, such as high blood sugar and the suppression of the immune system.

Equine Cushing’s Disease is more common in older horses, although it has been known to occur in younger horses.

The tragedy about Cushing’s Disease in both horses and dogs, is that because it occurs in later years, owners often mistake the disease for a general decline due to age.  As a result in the past, many undiagnosed animals didn’t receive appropriate treatment and sadly, were eventually euthanized.

In recent times it has become more commonly diagnosed due to better awareness of the disease, convenient diagnostic testing and an increase in the number of older horses.

The good news is that once it has been diagnosed, treatment is available, if long term, and in many cases allows the horse to return to normal health.  Treatments include drugs and alternative methods, like photonic therapy.

Horses and ponies with Cushing’s Disease can lead very happy and active lives. There are veterinary case history reports of horses and ponies returning from subclinical arthritis and laminitis, being pain-free and living in retirement quite happily without having to be retired prematurely.

The important thing to know about Cushing’s Disease is that it is treatable  and if your horse has it, it is possible for it to live many more happy and useful years.

Signs of Equine Cushing’s Disease

There are many signs of Cushing’s Diseases in horses.  Often the signs increase overtime.  They include:

  • heavy hair growth that may be curly and doesn’t shed normally
  • excessive sweating (including heat stress in hot humid conditions)
  • increased thirst and urination
  • muscle wasting
  • lethargy and poor performance
  • an increased susceptibility to infection (particularly sinusitis, teeth and hoof infections)
  • slow healing wounds
  • pot bellied
  • increased appetite
  • weight loss (which may be hard to detect under the curly coat)
  • Mucky eyes and sheath
  • Fat pads around the eyes, along crest of the neck, above the tail and in sheath area
  • Abnormal season or infertility in mares

A horse with Equine Cushing’s Disease or Cushing’s Syndrome will show one or more signs and not every affected horse will show every sign.

Diagnosis

While early diagnosis and treatment are desirable in counteracting the effects of Cushing’s Disease and helping your horse to be more comfortable, the condition may be treated at any stage under certain guidelines.

Diagnosis includes observation for the signs of the disease and formal testing.

Even though the outward signs are often very obvious, a number of tests are available to make a positive diagnosis.

Tests commonly include a dexamethasone test and adrenocorticotropic hormone stimulation. In addition, a test which combines these DST with a thyroid stimulating hormone release test will eliminate the overlap of the values of normal horses with those with pituitary tumors.  The Overnight DST involves taking a sample of blood in the late afternoon then a synthetic form of cortisol is given immediately after, and the second blood sample is taken between 8am-12pm the next day.   Cortisol levels are compared in the two samples.

In a normal horse the second sample should be 8-10 times lower then that of the first sample, as the synthetic cortisol suppresses normal cortisol release. In affected horses the second cortisol sample is either only partially suppressed or not suppressed at all and some have even higher levels then the night before.

These results in conjunction with suggestive clinical signs should confirm the diagnosis of Cushing’s Disease.

Some people say that if the horse has a history of a long hair coat that doesn’t shed properly, this may be as reliable a sign of the condition as specific blood tests.

Horses with Cushing’s Disease are at  risk of having laminitis, particularly if blood sugar levels are high.  Diseases related to and sometimes mistaken for Cushing’s Disease include Equine Metabolic Syndrome and Laminitis.

Treatment and Management

There is no conventional cure for Cushing’s Disease or Cushing’s Syndrome.  It is progressive and any drug treatment to control the symptoms is likely to increase as the horse ages.  Some horses may only need drugs during the seasonal rise or in the early stages of the disease.   Others may need increased levels of drugs as the disease progresses.

There are several types of drugs that have been used in treatment, including hormone mimickers, hormone inhibitors or blockers and hormone stimulators.  The drugs will not cure the disease but will help to reduce the risk of life-threatening complications and will make life more comfortable for your horse.

The correct dose of any drug is the lowest effective dose – that is as much or as little as necessary to control the symptoms.  Most, if not all, drugs will have side effects.

After examination of the horse and in consultation with you, your vet should diagnose your horse and prescribe an appropriate course of conventional or complementary treatment as not all horses respond to drugs and will show no clinical improvements no matter how high the dose.

There is an alternative or complementary means of treatment that has been known to have positive results.  Photonic therapy is the scientific application of light, to particular areas of the skin to produce particular physiological results such as pain relief, increased immune response and improved healing.   Photonic therapy has been shown to be successful in treating Cushing’s Disease.     By understanding the science of how acupuncture works through the autonomic nervous system as well as the spinal nerve transmission from skin stimulation, photonic therapy offers a whole new paradigm and hope of long term relief for Cushing’s Disease sufferers.  Read more about photonic therapy and its application here.

The other aspect of taking care of a horse with Cushing’s Disease is management. These horses are susceptible to worm burdens, infections, weight loss and laminitis, especially if they are not being treated medically. Therefore on a routine and regular basis, your horse will need clipping of excessive hair, checking for wounds and infections, worming, dental checks and vaccinations kept up to date.  While it is acceptable to shoe a horse every 6-8 weeks,  it is imperative that every horse should spend at least 6 weeks every year without shoes.

Horses and ponies also need to be on a good quality diet of fresh grassy hay with some lucerne (alfalfa) and, depending on the work load, some oats and cracked corn.  Pelleted feeds that have been heat processed or steam extruded should be kept to an absolutel minimum as such feeds normally use wheat as the grain to form a binding agent.  Wheat and, to a lesser extent, barley are poisonous to horses and should be restricted as they break down in the stomach rapidly and are the main cause of laminitis.

It is important to note that while drugs may treat the symptoms, if the condition has been caused by a pituitary tumour, which is not always the cause, the drugs will not treat the pituitary tumor itself. Horses with a mild condition may be returned to good health for a number of years by conventional treatment, but eventually the tumour condition will compromise the horse’s life.

Prognosis

If your horse has Cushing’s Disease, or you suspect it, don’t despair.   If your horse has been with you for one year or many years, chances are you will want to care for them through the rest of their lives.

Keeping a horse with Cushing’s Disease can be costly and time consuming.  Veterinary drugs are expensive and even treated animals may get bouts of laminitis, skin conditions, insulin resistance and other health problems.   Using alternative means of treatment such as photonic therapy may prove effective and less costly in the long term.

There is often a special bond between horse and humans, particularly if the relationship has developed over time.   Taking care of your horse into old age is often a very rewarding experience.

Remember, with good management and treatment horses with Cushing’s Disease can live comfortable and useful lives.