What Is Cushing’s Disease?
You may have heard of Cushing’s Disease in humans, however it is not quite the same disease in horses and is more correctly named Pituitary Pars Intermediary Dysfunction (PPID). It is the most common endocrine (hormone) disease affecting horses.
PPID is typically a condition of older horses, usually over the age of 15 years old, however it has also been diagnosed in horses as young as 7 years of age. It is certainly a possibility for any horses in their ‘teens’, and furthermore 30% of older horses will have the condition. PPID does seem to be more common in ponies however horses can certainly develop it too.
What Causes PPID?
The condition is due to over activity, or the development of a benign enlargement, of one part of the pituitary gland. The pituitary gland is located below the brain and it receives it’s instructions from nerves from the brain. Damage to these nerves cause the pituitary gland to enlarge and produce excessive amounts of certain proteins and a hormone called adrenocortical hormone (ACTH). Essentially the pituitary gland is ‘working overtime’ which has many flow on effects to the rest of the horse. It is the increase in these hormones that cause the clinical signs of PPID, although the complete mechanism of how these signs develop is not yet fully understood.
The clinical signs can be variable, with some cases only having vague signs and others having multiple definitive signs. These can vary from mild to severe and not all signs are seen in every horse. This can mean that early signs that your horse has PPID often go unrecognised.
The majority of laminitic cases have a hormonal basis, either PPID or Equine Metabolic Syndrome (EMS). Laminitis is a very serious and painful consequence of PPID and prompt intervention and management is the key. You can find further information on laminitis in our laminitis article on our website.
Infectious disease occurs more commonly in PPID cases because there is a suppression of the immune system. PPID cases are more prone to parasitic disease, skin infections (greasy heel, rain scald), respiratory infections (including sinus infections and pneumonia) and foot abscesses. Any traumatic wounds or injuries may be slower to heal.
Your horse may seem dull and flat or become more docile. If the horse is ridden, a lack of performance may be noticed.
Loss Of Muscle Mass
This occurs particularly over the back and hindquarters.
Abnormal Fat Distribution And Insulin Resistance
Abnormal fat deposition may occur in up to 60% of horses with PPID. The abnormal fat deposits are typically recognised around the eyes, crest of the neck, base of the tail and around the sheath. Insulin resistance is best discussed in combination with Equine Metabolic Syndrome (EMS) and will be discussed in a future article.
Increased Drinking And Urination
This may occur in approximately 30% of horses with PPID.
Interestingly it is not only those horses with hirsutism that can have this clinical sign and this may even be seen in horses that don’t have excessive hair growth.
Seizures, Weakness, Blindness And Collapse
These are rare and seen in the very advanced cases, most likely due to the enlarged pituitary gland causing pressure on other parts of the brain.
This may be seen in the older breeding mare or stallion and is the result of altered hormone production.
If a horse shows some of the more typical signs a vet may be able to make a presumptive diagnosis of PPID from the history and clinical picture. Definitive diagnosis is made by a blood test to evaluate the level of ACTH. Occasionally in more complicated cases further testing may be required.
There is a medication specifically licensed for horses to manage PPID. Pergolide (trade name Prascend) stimulates receptors in the brain and helps support the damaged nerves to the pituitary gland. This results in reducing the production of hormones to a more normal level and therefore improves the clinical signs. A suitable dose is selected by monitoring the clinical signs and reduction in ACTH levels. It is a tablet which is administered orally and it dissolves very easily and can be syringed into the mouth. Pergolide is considered a safe drug in the horse and has a wide dose range. Owners usually recognise an improvement within 3 months of use.
As stated above, the mechanism by which the increase in hormones causes the clinical signs is not fully understood, meaning the best way to manage any clinical signs is to treat the initiating cause, i.e. the over active pituitary gland, rather than only focusing on treating the symptoms of the disease.
Early recognition, diagnosis and treatment of PPID minimizes the progression of disease and improves quality of life and prolongs the life of the older horse.