Equine Cushing's Disease (ECD) is more correctly known as Pituitary Pars Intermedia Dysfunction (PPID).  This condition results in the pituitary gland, located at the base of the brain, releasing hormones in an abnormal manner.  This causes loss of control of a type of cell called melanotropes.  This will result in unregulated release of a hormone called adrenocorticotrophic hormone (ACTH).  In advanced cases a benign tumour called a pituitary adenoma will have developed.  Increased ACTH will stimulate the adrenal glands to release an excessive amount of the hormone cortisol into the blood stream and resulting in a number of characteristic symptoms. Cortisol influences a wide range of body functions and metabolism. As Cortisol, and the other hormones controlled by the pituitary gland, have such a wide range of actions, the symptoms can be many and varied. 

PPID is a common condition in older horses and ponies.  30% of 15 year old horses and 80% of 30 year old horses will be affected.  There will be a wide range of (clinical) signs.

Symptoms

  • Usually older horses (more than 15 years usually but can be as young as 7 years old).
  • Long, curly, thick hair coat.
  • Failure to shed winter coat in spring.
  • Lethargy, dullness and muscle weakness.
  • Excessive thirst (normal intake about 20-30 litres a day).
  • Excessive urination – bedding soaked.
  • Chronic recurrent LAMINITIS that often responds poorly to treatment. (see more information on our laminitis information sheet)
  • Patchy sweating.
  • Suppressed immune system resulting in poor, slow healing and recurrent infections (especially foot abscess).
  • Bulging fat pads above the eyes.
  • Depression.
  • Lack of facial expression.
  • Reduced performance under saddle.
  • Weight loss and muscle wastage, often despite increased appetite.
  • Swaybacked appearance with a potbelly.
  • Reproductive cycle in mares interrupted or abnormal.

Diagnosis

Often the clinical signs of Cushing's disease will be quite obvious but usually further tests will be needed to confirm the diagnosis. Unfortunately simply measuring the levels of Cortisol in the blood is not very reliable as there is quite a large variation in levels in normal horses, and levels fluctuate throughout the day. However there are other blood tests we can carry out :– 

  • Levels of ACTH in the blood will be raised in 90% of horses with PPID. The blood sample is spun down and separated, and then it has to be chilled before posting to the laboratory on ice.
  • Some horses with Cushing’s will have high levels of glucose in their blood and urine.
  • PPID can cause insulin resistance so insulin levels in the blood are raised in 90% of cases (but insulin can also be raised in other conditions).
  • TRH Stimulation test is used for horses with suspicious signs but have a normal ACTH blood test.  Blood sample 1 is taken then the horse is given an injection of TRH (another hormone usually controlled by the pituitary gland) and after 10 minutes, blood sample 2 is taken.

Ideally, we would test blood levels of ACTH, glucose, and insulin to give us the best picture of what is going on in each case. ACTH levels are used for monitoring response to treatment.

In laminitics, we will also test for Equine Metabolic Syndrome 

NB It is also important to run routine blood tests (haematology and biochemistry) to check other body systems e.g. the liver which may be compromised in older horses or to identify underlying infection. Underlying disease may affect the choice and dose of drugs to be used for treatment.

Treatment

Siskin after treatment

Siskin after treatment

Unfortunately, due to the nature of the condition, we can not cure horses with PPID, but we can manage them very successfully improving their quality of life (massively in some cases). Management is usually based around administration of drugs that eliminate clinical signs, combined with a healthy diet and regular preventative medicine e.g. worming and dentistry.

The drug of choice is called Pergolide (Prascend), which is a dopamine agonist.  This drug is very effective,with a wide dose range and is licensed for use in horses.  It will need to be given daily for the rest of the horse’s life.  The rate of improvement after starting treatment is variable, depression and lethargy are usually lost first and the horse is brighter and more energetic after a few weeks. Most importantly, there is a reduction in the painful bouts of laminitis. The hairy coat may take a little longer to improve. Treatment can be very successful and returns many horses to normal health for several years. (Our longest patient under treatment currently has been under control for >7 years.) Regular blood testing for ACTH levels are required to monitor the response to treatment and to see if adjustments to drug dosage are required.

Horses suffering with ECD are immunosuppressed so preventative health care is very important and seemingly mild ailments such as a touch of dermatitis or snotty nose should be treated as a matter of urgency. It is important that ECD horses are vaccinated and wormed appropriately.

Successful management of ECD sufferers requires the following;

  • Higher plane of nutrition, - feed a diet high in digestible fibre but low in starch and sugar to avoid laminitis (Cushcare for thin horses).
  • Appropriate foot care.
  • Regular worming, vaccination and dentistry.
  • Frequent hair clipping, especially during warm weather, (+/- rugging).
  • Access to sufficient water.
  • Prompt detection and treatment of infections.

Updated August 2021