Equine Cushing's Disease (ECD) is caused by a benign tumour called a pituitary adenoma developing in the pituitary gland at the base of the brain, causing an excessive amount of the hormone cortisol to circulate in the blood and resulting in a number of characteristic symptoms. The pituitary gland regulates many hormones, but in particular, the level of hormone Cortisol released by the adrenal glands into the blood. Presence of the tumour causes the pituitary gland to release too much of the hormone ACTH (adrenocorticotrophic hormone) which in turn tells the adrenal glands to release too much cortisol. 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.

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.
  • Reduced performance under saddle.
  • Weight loss and muscle wastage, often despite increased appetite.
  • Swaybacked appearance with a potbelly.
  • 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).
  • Reproductive cycle in mares interrupted or abnormal.
  • Bulging fat pads above the eyes.
  • Depression.
  • Lack of facial expression.

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 ECD. The blood sample is spun down and separated, and then it has to be frozen before posting to the lab.
  • Some horses with Cushing’s will have high levels of glucose in their blood and urine.
  • ECD causes 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 equivocal cases.

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, it is not possible to remove the pituitary tumour, so we can not cure horses with Cushings disease, but we can manage them medically instead. Management is usually based around administration of drugs that suppress the release of the hormone ACTH combined with a healthy diet and regular preventative medicine e.g. worming and dentistry.

The most commonly used drugs are Pergolide (Prascend) and Cyproheptadine (Periactin), which 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)

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.