Equine Cushing's Disease

Equine Cushing’s disease (ECD) is one of the most commonly diagnosed endocrine (hormone) disorders in horses.  In recent years it has become more recognised for two reasons: horses are living longer and there has been more research into the condition due to its link with laminitis, leading to increased owner awareness.

It is not a new condition; horses have been diagnosed and successfully treated with the same medication for over twenty years.  However, it can be confused and even co-exist with equine metabolic syndrome (EMS) which is a relatively recently diagnosed disorder relating to increased metabolic efficiency and obesity that develops in young to middle age horses often resulting in laminitis.  Cushing’s disease tends to be seen more commonly in animals over the age of 15, although it can occur in the younger age group. Various studies show differing results on its prevalence in horses and ponies and there is no confirmed sex predisposition.

Aetiology (cause)

Cushing’s disease is caused by an enlargement of the region of the pituitary gland known as the pars intermedia which is located at the base of the brain.  Hence the alternative name of pituitary pars intermedia dysfunction (PPID).  The pituitary gland secretes and/or stores many of the hormones that are responsible either directly or indirectly for the body to function.  It is thought that age related oxidative damage to the nerves regulating the pituitary gland leads to overproduction of its derivatives including excess production of adrenocorticotropic hormone (ACTH) that in simple terms regulates cortisol (the stress hormone).  Many predisposing factors have been implicated more recently including obesity and EMS earlier in life and environmental factors such as diet, although there is little data to support this at present.

Clinical signs 

A wide variety of clinical signs have been reported but it is important to recognise that the horse or pony may only show one of the signs (such as laminitis in the absence of hair coat changes) as the disease has a long and slow onset:

Hair /coat changes – long shaggy coat (hirsutism) Fig 1, curly coat, failure to shed or grow a coat at the appropriate time (the horse may shed normally in the spring but then regrow another coat in the summer).  The coat quality may become coarse and discoloured.  Weight loss/abnormal fat redistribution – potbellied and loss of topline due to muscle catabolism sometimes with polyphagia (increased appetite).

• Laminitis – (Fig 4) recurrent, non-responsive cases, sometimes the hind feet are severely affected
• Recurrent infections – sinusitis/tooth root infections
• Delayed wound healing and skin infections
• Polyuria (increased urination) and polydipsia (increased thirst)
• Excessive sweating
• Periorbital fat pad bulging (Fig 5)
• Lethargy and poor performance
• Increased susceptibility to parasites among others

Diagnosis

“Classic” clinical signs such as hair coat changes can be diagnostic for the condition.  However, laboratory diagnosis by your vet is important in confirming suspect cases and assessing the response to treatment.

There have been several laboratory tests used over the years, some involving the assessment of cortisol levels following the injection of specific drugs.  The current most commonly used diagnostic test involves taking a single blood sample to assess the resting plasma ACTH concentration.  False positives and negatives can occur and there is a natural variation of ACTH throughout the year so seasonal adjustments are necessary. 

The measurement of insulin and possibly glucose levels is also important as many cases will show evidence of insulin resistance which leads to high levels of circulating insulin in the blood.  Several studies have shown that raised insulin levels can induce laminitis.

Development of a more reliable diagnostic test, especially to detect the early stages of Cushing’s, is the researchers challenge.

Management

Supportive care is important in the management of Cushing’s cases, these include:

• Clipping of excess hair
• Prompt and thorough treatment of all wounds and infections
• Routine farriery
• Routine dental checks
• Routine vaccination
• Appropriate worming programme
• Appropriate dietary management to maintain correct body condition

Treatment

There is no cure for Cushing’s disease but it can often be managed effectively for many years.  The treatment of choice is oral administration of pergolide.  Pergolide acts by inhibiting hormone production within the pars intermedia of the pituitary gland which in turn lowers circulating ACTH and other hormone levels.  The main clinical side effect is a transient anorexia and depression when the drug is first introduced in some animals.  However, this can be overcome by reducing the dose rate and gradually reintroducing it over a period of time in most cases.

Equine Metabolic Syndrome

The term Equine Metabolic Disease (EMS) was first used in 2002 to describe horses with a history of laminitis, insulin resistance, obesity and a characteristic body type of a cresty neck with increased fat deposits along the topline.  Fat can also accumulate in the sheath and near the mammary glands.

Both environmental and genetic factors all contribute to the development of EMS.  Ponies’ natural adaptation to survive adverse conditions, together with overfeeding and insufficient exercise all contribute to obesity.

It is now thought that EMS is a contributing factor in the development of Cushings disease later in life and the two conditions may co-exist.  The similarities and differences are summarised in Table 1.

Cushings and EMS have marked similarities especially in the cases of chronic laminitis.  However, the underlying pathology is different and it is important to differentiate between the two conditions as the management is different in each case, especially in terms of nutrition and exercise.

Table 1 - Similarities and differences between Cushings Disease and EMS