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Equine Cushings Disease

If your horse has difficulty in losing weight, or suffers from bouts of laminitis, they may be suffering from Equine Cushing’s disease and/or Equine Metabolic Syndrome. The following articles will hopefully help you understand more about these relatively common but often misunderstood and confusing conditions.

What is Equine Cushing's disease?

Equine Cushing's disease, also known as Pars Pituitary Intermedia Dysfunction (PPID) is a disorder of the horses’ endocrine system (hormonal system). Equine Cushing’s most commonly affects pony breeds, but larger breeds can be affected and it is primarily seen in older horses above the age of 15 years.

Equine Cushing's is caused by an enlargement in the pars intermedia (middle section) of the pituitary gland. The pituitary gland is located at the base of the brain and is an important endocrine organ that produces a variety of different hormones. This enlargement of the pars intermedia results in the increased release of several hormones, most notably ACTH (adrenocorticotropic hormone). ACTH causes the adrenal glands (located near the kidneys) to increase the production of cortisol, a type of steroid. It is this overproduction of cortisol that causes the majority of clinical signs seen in this condition.

What does Equine Cushing's look like?

There are many clinical signs associated with Cushing’s disease. Recurrent, and often unexplained, bouts of laminitis along with an abnormal hair coat are two of the most common clinical signs of a horse with Cushing's.

Other signs include a pot-bellied appearance, ravenous appetite, increased urination and drinking, lethargy and a decrease in the immune system seen as recurrent skin and respiratory infections, as well as dental disease and an increased susceptibility to worms. Cushing's horses can also get an abnormal deposition of fat above their eye where normal horses would have a depression (this depression can often be seen moving when a horse chews) and can also show wasting of their skeletal muscles.

Abnormal hair coat can vary from mild changes in shedding pattern to the development of a long (up to 10cm) thick and wavy overgrown coat. This strange coat characteristic is known as ‘hirsutism’ and is due to the enlarged pars intermedia of the pituitary gland compressing the hypothalamus, which is a section of the brain located next to the pituitary gland that regulates body temperature, appetite and seasonal cyclic shedding of hair. 

The link between Equine Cushing's disease and laminitis is still not fully understood but it is the focus of extensive research. It is currently believed that the increased levels of circulating cortisol and cortisol-like hormones may play a major role in the development of laminitis, so too might a metabolic state known as insulin resistance, which we will come to later.

How do you diagnose Cushing's disease?

In classic cases (i.e. the older pony with hirsutism and recurrent laminitis) a diagnosis can be made on presentation alone, but not all Cushing's cases have obvious clinical signs - often they are very subtle. When investigating subtle signs, or when a definitive diagnosis is wanted, there are a couple of tests your vet can conduct.

The most common test is to measure resting ACTH. This is a simple once off blood sample that gives fairly reliable results in regards to diagnosis, as well as being used to monitor the progression of the condition once treatment has started.  Another common test performed to achieve a diagnosis of Equine Cushing's disease is the Low Dose Dexamethasone Suppression Test. This test involves taking a blood sample from the horse to get a base-line measure of circulating cortisol. After the blood is taken, a dose of dexamethasone is administered into the muscle. Dexamethasone is a steroid that in normal horses will cause  suppression of circulation cortisol levels. In horses affected by Cushing's disease the body is unable to respond properly to the injection and cortisol levels remain elevated. A second blood sample is taken about 20hrs after the injection of dexamethasone. If the cortisol levels have not decreased then a diagnosis of Cushing's is made.

Treatment and Management of Equine Cushing's disease

Unfortunately there is no cure for Equine Cushing's but we can help reduce the effects of the disease.

Pergolide is the most commonly used Cushing's medication. It comes in tablet form that can be crushed up and fed in feed or hidden inside a treat and is given once per day. Pergolide is a dopamine agonist (dopamine inhibits the pars intermedia and hence helps to reduce its size and thus reduce  over production of cortisol) and has been reported to be up to 80% successful in reducing the severity of signs seen in Cushingoid horses. Other medication such as cyproheptadine and trilostane can also occasionally be considered if results with pergolide are disappointing.

Management is just as important as medicating Cushingoid horses. Frequently inspecting for wounds and the evidence of any kind of infection and ensuring prompt veterinary attention is very important as affected horses may have reduced levels of immunity and often need more intensive and prolonged treatment than other horses. Regular farrier visits and dental checks along with routine vaccinations and a de-worming program involving frequent faecal egg counts are vital to ensure the health of Cushing's affected horses. Simple things like clipping out excessive hair will also go a long way in ensuring the comfort of affected horses especially in hot weather.