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Cushings Syndrome in horses

Dr. Grahame Best, Ranvet Pty Ltd

Horses, like humans, are living longer, healthier lives than ever before. As a consequence, veterinarians, like doctors, are now faced with treating and managing a range of age related diseases which were not diagnosed very often only a few years ago.

With increasing knowledge of Equine Cushing’s Disease there is an increase in the reporting of the disease in horses, and a rapidly escalating requirement for owner education, especially regarding how to manage the valued old family horse or pony.

Long Term Progressive Disease of Older Horses

Right from the beginning, you should understand that Cushing’s Disease is a progressive disease, most commonly of older horses. This discussion is not about curing the disease; it is about understanding the disease enough so that you can manage it and keep the horse comfortable for as long as possible.

What Causes It?

In simple terms, Cushing’s Disease is caused by a hyperplasia (excessive growth) or tumour in the pituitary gland at the base of the brain. The pituitary gland is responsible for the production and regulation of hormones in the body, and while the tumour is benign, it produces excess hormones (particularly ACTH, or adrenocorticotrophic hormone)), creating an imbalance in the body. This pituitary gland hormone excess forces the adrenal glands to produce excessive amounts of the glucocorticoid (cortisone type) hormone cortisol, and this results in Cushing’s Disease.

Equine Cushing’s Disease is considered a disease of horses in their late teens and twenties. It is the most common endocrine (hormone) disorder in horses. It is a disease which is managed; not cured. Animals that are being treated are almost always on medication for the rest of their lives.

The Scientific Stuff

For those who need more scientific detail, Cushing’s Disease is hyperadrenocorticism. That means an excess production of adrenal gland hormones (specifically, the hormone cortisol). Since the pituitary gland controls the entire endocrine (hormone) system of the body, the result in this disease is an excess of the adrenal gland hormone cortisol. The pituitary gland secretes excessive ACTH ( adrenocorticotrophic hormone), which excessively stimulates the adrenal gland to produce cortisol.

Cortisol is an essential hormone which is vital to maintain many normal body functions. It is produced by the adrenal glands normally in response to the hormone ACTH, released by the pituitary gland. Cortisol is a glucocorticoid hormone, which you may recognise as a cortisone, or corticosteroid. Cortisol is released by the adrenal glands at intermittent intervals, and at low levels to help the body respond to short periods of physical, emotional or environmental stress. Excessive secretion of cortisol is what produces many of the clinical signs of Equine Cushing’s Disease.

Recent research suggests that a lack of the inhibitory neurotransmitter, dopamine, leads to an uncontrolled growth of the pituitary gland (hyperplasia), and thus increased pituitary hormone secretions, which then exert effects on the adrenal glands. (Neurotransmitters are present in the brain and nervous system and are involved in transmitting messages between nerves. These messages allow the normal functioning of the body).

Dopamine is released from a specific part of the brain (the hypothalamus), and inhibits the activity of the pars intermedia of the pituitary gland (therefore regulating hormone production). The hypothalamus is vital for controlling many body functions including thirst, hunger, maintenance of body temperature, water balance and blood pressure. Dopamine normally transmits messages by stimulating specific receptor sites in the brain. Therefore, when there is less Dopamine there is less inhibition of the pars intermedia of the pituitary gland, which then increases in size (and produces more excessive hormone).

Cushing’s Disease (or Cushing’s Syndrome) is the term used to describe the clinical effects of increased glucocorticoid (adrenal gland hormone, or cortisol) activity in the body. (Note: glucocorticoids may be known to you as corticosteroids, or cortisone) There is a wide spectrum of disease possible, from mild and intermittent secretion of hormones without any real enlargement of the pituitary gland, to significantly elevated hormone concentrations in the blood associated with an enlarged pituitary gland. In most cases of Equine Cushing’s Disease, the pituitary gland is not initially truly neoplastic (a tumour), but it is enlarged and overactive because of the faulty regulation from the hypothalamus. It usually becomes a tumour at a later stage.

Pituitary tumours do not spread to other parts of the body - the main effects are through the increased secretion of hormones, as stated earlier.

So, we now know that the pituitary and adrenal glands produce excessive amounts of the specific hormones that play a vital role in the regulation of normal metabolism, inflammatory processes and immune responses.

Interesting Fact: Hyperadrenocorticism, or Cushing’s Disease, produces the opposite changes in hair coat in horses and dogs. In dogs, Cushing’s Disease is due to a primary adrenal gland pathology, while in horses it is usually secondary to an adenoma (tumour) of the pituitary gland. Horses develop shaggy, dull, dry hair coats, while dogs lose hair.


The Practical Stuff

What Clinical Signs Do We See With Equine Cushing’s Disease?

The clinical signs are variable, and any one of the following signs may appear, alone or in combination:

* Often the most notable sign is the growth of a long, shaggy hair coat. This is usually dry, often curly or wavy, longer than normal, usually lighter in colour, and does not shed in summer months. Well over one third of Cushings horses show this characteristic hair growth.

* Laminitis (founder) is perhaps the most common, and devastating, manifestation of Equine Cushing’s Disease. In fact, all Cushings horses will develop laminitis at some period if they live long enough. Many horses diagnosed with Equine Cushing’s Disease will actually initially founder without any apparent cause, or founder after mildly excessive grain consumption. It is often only after the founder that the diagnosis of Equine Cushing’s Disease is actually made, as the horse recovers. The reason that laminitis is important, is that laminitis occurring as a part of Equine Cushing’s Disease can be very difficult for veterinarians to manage because the hormones secreted by the pituitary gland affect the blood flow to the hoof, and weaken the bonds between the hoof wall and the coffin (pedal) bone. (It is well known, for example, that corticosteroids such as dexamethasone will aggravate laminitis).

* Excessive sweating, particularly around the chest and neck, can commonly be seen

* Body condition changes. Early in the course of the disease horses begin to show an abnormal distribution of body fat, where they develop a “cresty” neck (as is frequently seen in fat ponies), and fat deposits over the back and base of the tail. In spite of this altered fat deposition, the horses lose muscle mass, and the ribs are often easily felt or seen. Finally, a pot-bellied, sway-backed appearance develops as the horse loses muscle mass over the topline and rump . This sway-backed, pot bellied appearance is particularly evident if the horse has been starved or dieted in the mistaken belief that the laminitis is due to obesity. (a common assumption in ponies - see later in this article)

* One common sign is weight loss, despite an increased appetite. .
Horses still in normal work often lose muscle mass over the top of the body and rump, and may show unexplained exercise intolerance such as rapid, shallow breathing.

* Interestingly, the normal depression that is easily seen above the horse’s eyes (supra-orbital fossa) is filled with fat, and often bulges out (take a look at a normal horse and notice the deep depression commonly above and behind the eye).

* The hormones secreted by the pituitary gland cause a suppression of the immune system, and persistent or unexplained infections such as dermatitis and respiratory infections, foot infections, mouth ulcers and teeth and gum disease, and even worms, often occur.

* Although the horse may not be obviously lame, laminitis may occur continually. This damages hoof and sole structures repeatedly, allowing bacterial infection in the sole, leading to abscesses under the sole. Horses that have had previous episodes of laminitis are prone to these subsolar infections, and these should be treated very aggressively by your farrier and veterinarian.

* Many horses suddenly develop polydipsia (drinking excess water) and therefore, polyuria (excessive urination), although this may not be obvious in horses in a paddock. In stabled horses you may only notice that the bedding is excessively damp unless you carefully monitor water consumption. Some of these horses may become diabetic as well. An affected horse may drink up to 80 litres of water a day (an average normal horse drinks 20-30 litres daily)

* The pituitary hormones often cause lethargy. This is often confused or ignored, because the horse is considered to be getting older and quieter with advancing age.

* In most cases the progression of this disease is very slow. In most cases, any treatment or management changes occur when it is required to maintain the horse’s quality of life.

How Do You Diagnose Equine Cushing’s Disease?

If you notice the almost classical long, shaggy, dull hair coat which doesn’t shed in summer months, that’s pretty diagnostic, but many horses don’t show that degree of change.
Some will simply founder with no warning, or obvious cause. Some, especially ponies, may simply have endless laminitis-like problems. Others may just drink and urinate a lot. So the clinical picture varies tremendously. The typical weight loss from the topline, and redistribution of fat deposits is also pretty diagnostic.

Diagnosis of early stages can be more difficult

Hormones of the pituitary gland (ACTH) can be measured in the blood, but the levels of these hormones vary as secretion varies during a normal day. Some horses with obvious signs of very long, curly hair coat may only have small elevations in pituitary hormone such as ACTH.

One problem is that elevated ACTH levels can lead to high blood sugar levels (glucose), and whenever the body has high blood sugar levels it secretes more insulin (another hormone produced by the pancreas). So, both glucose and insulin can also be measured to help in the diagnosis by your vet.

The most effective test is called the dexamethasone suppression test. Dexamethasone is a glucocorticoid, or cortisone, compound often used by vets. This test determines the effect of dexamethasone on the blood cortisol concentration, because dexamethasone blocks ACTH secretion by the healthy pituitary gland and significantly reduces circulating cortisol (because ACTH no longer stumulates the adrenal gland to release cortisol). This drug is given as an intramuscular injection, (at a dose of 0.04mg/kg) and on the following day (15-19 hours later) a blood sample is taken to determine if the pituitary gland has responded appropriately to the presence of this hormone. This test thus tests the functionality of the pituitary gland. One problem is that dexamethasone can exascerbate laminitis in rare cases, so this test isn’t used a lot on any laminitic horses. If laminitis is a worry, the alternative test is for plasma ACTH levels.

In normal horses, administration of dexamethasone stimulates a response that suppresses secretion of cortisol from the adrenal glands, giving a much lower concentration of cortisol in the second blood sample. Horses suffering from Cushing’s Disease however, show less suppression, and thus higher cortisol levels.

There is also a TRH Suppression Test - generally considered to be the test of choice to investigate hyperadrenocorticism in horses. This test is reliable but depends on repeated sampling, and is more expensive than the dexamethasone suppression test. In this test TRH (thyroid releasing hormone) is injected IV after a normal serum sample is taken, then blood is taken 15 minutes, and 60 minutes later
Many horses with Cushings Disease also have low thyroid hormone levels, so if a horse is diagnosed with hypothyroidism (low thyroid levels) then Cushings Disease should also be considered.

It is common for veterinarians to test directly for hormone levels - ACTH, cortisol, insulin and glucose, and these levels can then be monitored at yearly, or other, intervals to determine disease progression. Many affected horses are insulin resistant, and some have significantly high levels of blood glucose. Early recognition of these factors will allow proper nutritional management of the disease in future.
High cortisone levels interfere with the action and activity of the hormone insulin, causing elevated insulin levels in horses with Cushing’s Disease

What Factors Are Important in Managing Equine Cushing’s Disease?

Nutrition is critical. Provision of a low soluble carbohydrate diet is highly important. Any excessive carbohydrate consumption should be avoided (oats, sweet feed, lush grass) - see detailed notes later.

Treat any infection promptly and aggressively - remember that Cushing’s Disease suppresses immune responses

Treat regularly for worms - the horses have little immunity to resist heavy worm burdens.

Clip long hair coats before the hot summer months

Do not allow horses to become obese, as this increases the risk of laminitis.

Pay particular attention to diet - feed low soluble carbohydrate levels at all costs

The long term prognosis for horses with Cushing’s Disease depends on the severity of the complications observed. For example, laminitis is the most severe and life- threatening complication commonly seen, and is also the most common reason for euthanasia of effected horses.

Chronic laminitis (you will see the characteristic rings on the horse’s hooves) will require strict attention to hoof trimming or shoeing, plus supplementation of essential nutrients. The rings you are interested in are the ones that are closer together near the toe and farther apart at the heel, as with all laminitis rings.

In general terms the prognosis for most Cushing’s Disease horses is quite good, as the progression of the disease is very slow. If, however, the horse has chronic laminitis, the prognosis is not so good, as outbreaks of founder are likely to re-occur.

A Note on Laminitis
Trials at Penn State University in 2004 by Assistant Professor Mark Donaldson confirmed that Equine Cushing’s Disease is the most common cause of laminitis among horses treated by vets. In these trials, 28 out of 40 horses with laminitis were diagnosed with Equine Cushing’s Disease by ACTH determination.


Are There Any Treatments?

The UK and American literature discusses several treatments, and we will look at these below.

When deciding whether to medicate a horse with Cushing’s Disease, the cost of treatment and the condition of the horse should first be considered. Because the horse will invariably be older, any decision as to the horse’s future will usually be associated with a high level of emotional input. This is understandable when you may be dealing with a horse or pony which has taught the whole family, and possibly even half of the local pony club, to ride.

Horses with relatively mild symptoms generally respond best to medical treatment, and the treatment may extend the useful life of the horse by several years. It may also improve quality of life significantly.

However, any horse that is already suffering from chronic founder (laminitis) and/or recurrent infections as a result of immune system failure will likely obtain little benefit from treatment.

While the treatments below do treat the symptoms of Cushing’s Disease, they do not treat the pituitary tumour, which will continue to grow until it compromises the horse’s quality of life. There is currently no method of removing a pituitary adenoma surgically or medically.

Remember that in some horses the pituitary gland changes may result in very little disability other than the change in hair coat, and many cases with typical hair coat changes can live happily for years before they become laminitic, diabetic, or begin to lose too much bodyweight.

If the horse has suffered laminitis, it may be possible to keep the horse active and healthy simply by careful attention to regular hoof trimming and shoeing.

Making the treat / do not treat decision is one which should always involve your veterinarian.

Combine medication, supportive care and strict attention to diet
Optimal management of Cushing’s Disease involves varying combinations of both specific medication to normalise pituitary gland function, plus supportive care to manage and prevent any complications associated with this disease. In all cases, management will be for life, as there is no way to reverse the disease process.

In the early stages specific medications may not be required, and simple measures including clipping body hair, strict attention to the diet, and very careful ongoing attention to teeth, hooves, worm control and preventive care may be enough to allow a good quality of life. Remember that affected horses are often insulin resistant, so sweet feeds, grains and concentrates, and even lush pasture growth, all high in soluble carbohydrate (or nonstructural carbohydrate - see later) should be avoided at all costs, in favour of diets incorporating fibre and fat. Avoid any feed high in sugar or molasses as well.

For those who like herbal treatments where possible, the Laminitis Trust, UK (www.laminitis.org/cushings.html) has reviewed trials on an extract of the plant Monkspepper or Chasteberry (Vitex agnus castus), after The UK Horse Journal ran a field trial including 10 horses and ponies aged between 13 and 25 years, reporting encouraging results. The Horse Journal reported that typical response was “rapid and dramatic”. With hair shedding beginning within 3 weeks, and energy level quickly improving. High blood glucose and insulin levels dropped in some cases within 4-6 weeks. These claims were later argued against by the University of Pennsylvania, as well as the Laminitis Trust.

While it would be wonderful to report that this herbal treatment was of value, the results were very negative, as reported in the following peer reviewed paper:

Jill Beech, Mark T. Donaldson, and Sue Lindbergh. Comparison of Vitex agnus castus extract and Pergolide in Treatment of Equine Cushing’s Syndrome. AAEP Proceedings, Vol 48. 2002. pp175- 177.

This study on 14 horses with confirmed Cushing’s Disease reported that the herbal extract “did not have a beneficial effect on horses”, and further that “the dexamethasone suppression test remained abnormal in the horses tested”. Discuss this option with your vet after you read the website details if you wish to try it.

The most effective treatment for horses is the drug Pergolide mesylate, used in human medicine to treat Parkinson’s Disease. It is the drug of choice for the treatment of pituitary pars intermedia hyperplasia, or Equine Cushing’s Syndrome. This drug is usually given orally, and works by binding with drug receptors in the brain that control the production of dopamine. The same trial above which evaluated the herbal extract, compared the results of the herb to those of pergolide, reporting “in contrast, with the exception of one horse, pergolide had a beneficial effect”.

Further research by Michigan State University researchers in cooperation with Michigan veterinarians (Harold Schott, Cynthia Coursen, Susan Eberhart, Raymond Nachreiner, Kent Refsal, Susan Ewart, Judy Marteniuk. The Michigan Cushing’s Project. AAEP Proceedings, Vol 47. 2001. pp 22-24) advocated pergolide therapy as the “best” treatment for Equine Cushing’s Disease, and that “clinical improvement and reversal of abnormal endocrine tests is better with pergolide treatment than with cyproheptadine treatment

Pergolide is called a dopamine agonist. Pergolide has dopamine-like properties and thus suppresses secretion of hormones from the pituitary gland. So pergolide works by stimulating the same receptor sites in the brain as dopamine. It acts effectively as a dopamine substitute. Although it controls secretions it does not completely normalise the function of the pituitary gland, so ACTH and blood glucose levels need to be monitored after 3-4 weeks to ensure that an adequate dose of pergolide is delivered.

0.5 to 1mg pergolide is given orally once daily to an average 450kg horse to commence. The average response time was close to 3 weeks, and improvement in most horses continued until 21 weeks. Many horses respond well to this dose and are maintained indefinitely, however others require 1.5-2mg daily. If treatment is abandoned, symptoms can redevelop within as little as 2-3 weeks. The dose may be increased if clinical improvement fails too occur after 1-2 months of therapy.

Pergolide is generally accepted as the best treatment option, getting dexamethasone suppression tests to return to normal in most, but not all, cases. A decreased appetite during the first week of treatment is the most common side effect reported.

The other drug frequently mentioned is Cyproheptadine, which does not appear as effective as pergolide. This is a serotonin blocker used in humans, started at a dose of about 0.13mg/kg (58mg for an average 450kg horse). This is effective in about 75% of Cushing’s Disease cases, so is not accepted as well as pergolide.

A more recent drug in the UK, called trilostane, is currently being researched after one Australian report of efficacy in horses (McGowan, CM, Neiger, R. Efficacy of trilostane for the treatment of equine Cushing’s syndrome. Equine Vet. J. 2003 Jun;35(4):414-8) reported that trilostane caused “improvement in clinical signs in horses”, and that further work was warranted, to compare it with pergolide . At this time little further information is available. Trilostane works a different way to pergolide. It is a competitive inhibitor of an enzyme that works at the level of the adrenal gland to block normal steroid formation (of cortisol).

In addition to these medications, there are several nutritional supplements advocated, including supplementation of magnesium (to achieve a target calcium : magnesium ratio of 2:1), and chromium picolinate. These nutrients are helpful in management of type 2 diabetes and insulin resistance in humans, and may help in horses, although there is no confirming research at this stage.

What, Specifically, Should I Feed, (or not feed), a Horse With Equine Cushing’s Disease?

In simple terms, Cushing’s Disease horses should be fed a diet adequate in fibre, protein, vitamins and minerals, but low in soluble carbohydrates. This means very little or no grain, concentrate mixes, or even lush pasture growth, which is very high in soluble carbohydrates.

You will notice that this advice is almost identical to the feeding story for laminitis.

You will also now be aware that this story is so similar to what is seen with many horses (especially ponies) with laminitis, that in fact many chronic laminitis cases (especially fat ponies with crested necks and fat deposits at the base of the tail) may actually be because of Equine Cushing’s Disease. Of interest is the fact that horse owners with these fat ponies and horses often report that it is extremely difficult to reduce the weight of these horses by restricting the diet alone. (think of all the poor fat ponies on starvation diets because the owners think they are doing the right thing in preventing laminitis!)

Explain Carbohydrate Rations in More Detail, Please?

This is a common question. It is easy to state that carbohydrate levels in the ration of a laminitic or Cushing’s Disease horse should be reduced.

But what, exactly, does that mean?

When considering a low carbohydrate ration, it is important to differentiate nonstructural carbohydrates (NSC) from fibre (or structural carbohydrates):.

Nonstructural carbohydrates make up the majority of the carbohydrates found in grains such as oats, barley and corn, and are commonly referred to as starch or sugar.

Structural carbohydrates are common in forage products such as hay. These make up the bulk of the structure of plants, and contain little available starch or glucose. You know this as fibre.

NSC values (nonstructural carbohydrate values) provide a measure of the amount of starch and sugar contained in a forage, grain or mixed feed. When thinking in terms of a “low carbohydrate” feed for use in equine diets, a low NSC (NSC=starch+sugar) is what is actually being considered.

Some NSC levels in feedstuffs are shown below;

Lucerne Hay 11.394
Oaten Hay 23.087
Barley 63.08
Corn 75.589
Cottonseed meal 10.669
Linseed meal 15.038
Molasses 58.406
Oats 50.667
Rice Bran 19.881
Soyabean Hulls 7.169
Soyabean meal 16.319

All good veterinary reference material agrees that laminitic, Cushing’s Syndrome or insulin resistant horses should be placed on a ration low in NSC levels - That’s not in question.

Starvation Rations are Not Sufficient
The problem that arises is that many owners, and some vets, put the horse on “starvation” rations to force the horse to lose weight, and to ensure that no excess soluble carbohydrates are being fed. These horses and ponies are often locked in a yard and fed rough, dry hay and water. This almost invariably makes conditions worse for the horse, as all horses still need some energy to maintain body functions, as well as all of the essential nutrients such as amino acids, vitamins, minerals and trace elements to allow normal body functions to continue, as well as to allow tissue healing and repair.

If a horse is not receiving adequate nutrients on a daily basis it has no opportunity to heal any damage. This is particularly important in horses with laminitis, as they need to heal tissue damage. You should also remember that Cushing’s Disease horses all already have suppressed immune responses, so they need all of the nutritional help they can get. They certainly don’t need starvation!

Laminitis horses, for example, will need additional supplementation with biotin and methionine, as well as a proven adequate diet in calcium, to ensure new horn formation and sound foot structures. In most cases a good quality multi-vitamin & mineral supplement will be essential for hoof repair and maintenance in laminitic horses.

Antioxidants such as Vitamin E may be of significant value in many horses with tissue damage and stress.

The essential amino acids (needed to form protein) must always be available from the diet - particularly lysine, threonine and arginine (one of the most potent vasodilators the horse’s body has access to).

Omega 3 fatty acids are very important because of their anti-inflammatory activity. They also provide readily available, safe, energy. Ranvet’s Racing Oil has been of particular value in providing the Omega fatty acids

So, how do we put all of this together to form a good diet for a compromised horse?

The NSC values listed above make it clear that feed components such as oats, barley, corn, molasses, and even oaten hay provide too much soluble carbohydrate (or nonstructural carbohydrate), and may aggravate a laminitis or Cushing’s Disease condition. So, we can’t use them in a practical diet.

If we look for the low NSC levels we find feeds including lucerne hay, cottonseed meal, soyabean hulls, soyabean meal as the most attractive to form a diet for these horses

What we need to do then is to add a few supplements, as required, and we have a safe, nutritious diet:

Ranvet 500 is a protein supplement, originally produced for racehorses, but of tremendous value for laminitic horses and ponies and Cushing’s Disease horses because it has a very low NSC value. It is also very high in the amino acid arginine, compared to other protein meals. By maintaining protein availability we may have a chance to keep the topline on some of these horses while providing the essential amino acids.

Add lucerne chaff to the Ranvet 500, and use Ranvet Racing Oil daily. Racing Oil was again developed for performance horses, but it is equally applicable to these horses as it contains relatively high levels of Omega 3 and Omega 6 fatty acids in the correct balance of 1:5 to provide energy as well as anti-inflammatory support. (many other oils such as sunflower oil have low levels or distinct imbalances of omega 3-6 (0.5:66).
Canola Oil is also a suitable oil to feed for these horses

Add a balanced vitamin & mineral supplement such as Ranvet Aminovite Plus to provide all of the essential nutrients and minerals the horse requires daily

If necessary, add Hoof Food to supplement biotin if the horse has hoof or laminitis problems. This supplements the other critical hoof repair nutrients, magnesium, zinc and sulfur, as well.

That forms the basis of a very healthy, safe diet for these horses. It’s all based on low NSC values provided safely by Ranvet 500 protein supplement

A Note on Old Brood Mares
Brood mares with any degree of Cushing’s Disease have normal oestrus cycles, but are notoriously difficult to breed. They also respond well to the type of diet we have just formulated above.

The real message when feeding is to provide a balanced, low soluble carbohydrate diet - not to simply starve the horse.

A Similar Condition - “Equine Metabolic Syndrome” - the fat, foundered pony

Now that you are thoroughly confused about the technical details of Equine Cushing’s Disease, it is important to make you aware that recent US research has identified a similar equine condition called “Equine Metabolic Syndrome”, or Peripheral Cushing’s Disease Syndrome by various researchers. To us, it is probably better understood if we relate it to the commonly seen fat foundered pony.

This is important, because many clinical signs are identical to Equine Cushing’s Disease, and veterinarians for a long time have recognised a related syndrome of obesity, insulin resistance and chronic founder (laminitis), usually in a younger age group of horses and ponies.

All of these horses and ponies are grossly obese, especially involving fat accumulation in the abdomen, and all show insulin resistance and high blood glucose levels. This is identical with the human disease.

These horses do not show the long, shaggy hair growth seen in some Equine Cushing’s Disease cases - they are just usually grossly fat and foundered. Obesity in these horses is the main problem - you have all seen the grossly overweight pony somewhere in the past. Body fat is stored in the abdomen, liver and skeletal muscle, and this fat is very active hormonally. It can trigger a cascade of metabolic disturbances that lead to insulin resistance and high blood sugar, just as in obese humans. These problems exert serious effects on cardiovascular function, cartilage and bone, and one of the effects is an increased formation and release of cortisol, just as in equine Cushing’s Disease. This increased cortisol is responsible for the almost certainty that these affected horses will develop laminitis.

Now, picture the typical fat pony with chronic laminitis that we have all seen. You see the excessive fat in the crest of the neck, over the rump and base of the tail, and in the sheath of male horses, and you almost always see rings on the hooves, and chronic laminitis.)

Here lies the problem - the veterinarian now faces a diagnostic problem to determine if the affected horse or pony is actually an Equine Cushing’s Disease sufferer, or if it has Equine Metabolic Disease. This can be difficult simply by observation, as both diseases cause almost identical clinical signs.

The only true differentiation is by clinical tests: there is no single test that is diagnostic for these fat, foundered horses, but measurements of insulin and blood glucose help (but remember they are also used in Cushing’s diagnosis) after a period of fasting. It is very important to differentiate equine metabolic disease from early cases of Cushing’s Disease because Cushing’s Disease horses will be expected to respond to medical treatment as outlined, while these metabolic disease horses do not respond to pergolide or other medical therapy.

In these metabolic disease horses, fasting levels of insulin are almost always elevated, and blood glucose levels are frequently elevated.

These horses do not respond to medical treatment, so treatment is confined to reversing the obesity and insulin resistance through a very strict program of dietary modification, plus exercise, if the horse is able to exercise due to the recurrent laminitis. Diet evaluation , modification, and monitoring are critical, as are all of the diet related issues we mentioned above for Cushing’s Disease horses. Strict limitation of soluble carbohydrate intake (including complete elimination of grain, sweet feed, carrots, apples and fresh pasture), the use of exclusively fibre-based feedstuffs at a level of about 1%-1.5% of bodyweight per day. Any levels of soluble carbohydrates in the diet are certain to sustain the insulin resistance these horses suffer. If greater dietary energy is required once obesity has been controlled and exercise has been commenced, the use of fats such as Ranvet Racing Oil, and rice brans will provide sufficient energy without influencing insulin resistance and blood sugar levels negatively.

As you now appreciate, it is not possible, or effective, to just put these fat, foundered ponies in a yard on a starvation diet. They need more thought and attention than simple starvation, and they need careful long term management. Constant diet evaluation and manipulation will be critical to success.
Initially, they do need to be diagnosed, as the ACTH levels and dexamethasone suppression tests in these horses are usually normal, but the cortisol levels are usually elevated. This defines the ongoing management and treatment of each horse.

Finally, a short note on the use of corticosteroids in animals

You will all have noticed a reluctance by your veterinarian to use corticosteroids (cortisones) for medium or long term use in your horses. In almost all cases, your vet may use cortisones for immediate, short term, potent anti-inflammatory effect, and then switch to other drugs to continue anti-inflammatory treatment. Usually a veterinarian will then revert to a NSAID (non-steroidal anti-inflammatory drug) for longer term relief. These drugs are anti-inflammatory, but are not corticosteroids or cortisones.

The reason veterinarians, and human doctors, have this attitude (and the reason why you can’t buy strong cortisone products over the counter), is that medium to longer term use of potent cortisone products produces exactly the same condition in animals and man as Equine Cushing’s Disease.

When you think about it, everything written here about Equine Cushing’s Disease is because the elderly horse has a problem which makes its body produce too much cortisol. Cortisol is a glucocorticoid, or cortisone drug. It just happens to be made by the body. The body can’t tell the difference between cortisones produced within the body and cortisones injected or orally administered by people - it simply reacts to the presence of excess cortisone.

Anytime you use cortisone products to treat an animal, you risk producing symptoms exactly the same as Equine Cushing’s Disease, with all the same problems.

While cortisone products are immensely potent and valuable therapeutic drugs, they are also very dangerous in the wrong hands. Not only are they potent anti-inflammatory drugs to reduce inflammation, pain, tissue swelling, etc, they seriously restrict the body’s ability to fight infections by reducing the body’s immune responses, and they can create conditions such as diabetes simply by being in excess.

Drugs such as dexamethasone, prednisolone, methylprednisolone, and triamcinolone are all corticosteroids.

Dexamethasone, for example, is a commonly used potent, short acting corticosteroid which has roughly 25 times the anti-inflammatory potency of naturally occurring cortisol. Corticosteroids such as dexamethasone are important in normal protein, carbohydrate and fat metabolism in normal healthy animals, as well as for their role in controlling inflammation. Dexamethasone is able to be given orally, by injection, by inhalation and topically. It is routinely used systemically in high doses in emergency situations (including acute laminitis) to treat anaphylactic (allergic) reactions, severe trauma or shock, as well as in treating immune mediated diseases, asthma, itching, skin and eye diseases, and some nerve diseases. In horses dexamethasone is used to treat allergic reactions including Chronic Obstructive Pulmonary Disease, itching and arthritis, as well as inflammation. So it is commonly relied upon by veterinarians in carefully controlled situations.

Be aware that chronic or inappropriate use of any corticosteroids can cause life threatening hormonal and metabolic changes (just as you see with Equine Cushing’s Disease), especially when long term, or high doses are used. To reduce the potential risk, veterinarians will often use alternate day therapy with short acting preparations. Any animals which have received long term therapy under veterinary control are withdrawn from the drug slowly.

The use of corticosteroids in horses can lead to the formation of gastric ulceration quite rapidly, and corticosteroids are commonly implicated as a cause of laminitis in horses and ponies. Some corticosteroids are more likely to cause laminitis than others, and dexamethasone drugs are not historically considered to be in the high laminitis risk category, but they still need to be very carefully monitored by veterinarians.

Corticosteroids should be avoided during pregnancy and lactation. Large doses early in pregnancy may deform the foetus. Large doses can induce labour in late pregnant cattle, and has been used to terminate pregnancy in bitches.

Of interest, many old brood mares may have a degree of Cushing’s Disease. These mares usually cycle normally, but are notoriously difficult to get into foal. In large part, this is due to the excess cortisol produced by the Equine Cushing’s Disease.

If you are unfortunate enough to have a valued horse or pony with either chronic laminitis or Equine Cushing’s Disease, the important thing to remember is that there is help;

The most important issue you must approach is how to safely feed these horses - and if you have any questions on how to formulate a safe, balanced, nutritious diet, you are invited to contact Dr. Priscilla Spendlove at Ranvet for assistance with diet evaluation and formulation. priscilla will be happy to help you continue to monitor the diet for maximum health.

Priscilla's email is priscilla@ranvet.com.au