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Cushings Disease (Pituitary Pars Intermedia Dysfunction – PPID)

Introduction to PPID Cushing’s disease occurs when a horse’s body makes too much cortisol.  The condition is named after Harvey Cushing, a prominent, human, American, neurosurgeon, whom first described the condition in his human patients in 1912.

Cortisol is a natural steroid hormone made by the adrenal glands, which are two small glands located near each kidney (image).  Cortisol has several functions vital for life, including:

  • Regulating blood pressure (increases blood pressure through vasoconstriction)

  • Regulating the immune system (decreases the inflammatory response)

  • Balances the effect of insulin to keep blood sugar normal (increases blood sugar)

 

In horses, Cushing’s disease occurs most commonly in older horses (over 7) and especially ponies.

Clinical Signs of PPID

The most common clinical sign in horses is hirsutism (abnormally long hair coat, or failure to shed their winter hair coat).  This sign occurs in 60-80% of horses.  In the early stages of diseases, hair coat abnormalities tend to be subtle and regional. Your horse may shed his winter coat everywhere except in small patches—usually around the jawline and base of the neck, and along the back of the front and hind legs. The summer coat may grow longer and lighter in colour in these areas.

 

In more advanced cases, hair coat abnormalities are more noticeable and generalized; your horse does not shed out his winter coat until well into spring or summer, or in some cases, not at all. The hair may also appear abnormal—lighter in colour, longer, and/or curly.

The second most common clinic sign is laminitis, inflammation of the sensitive laminae within the foot.  Up to 50% of horses, in both early and advanced stages of disease, can be affected.  

 

Other common clinical signs include:

  • Polyuria/Polydipsia (urinating and drinking more frequently),

  • Weight loss despite eating well,

  • Lethargy (slow and calm),

  • Secondary infections of skin, respiratory tract, teeth and gums,

  • Excessive sweating (hyperhidrosis) or failure to sweat (anhidrosis),

  • Redistribution of fat deposits (ie. Patchy fat deposits on neck and around tail head),

  • Pendulous abdomen (pot belly),

  • Sway back, and loss of muscle mass,

  • Infertility – abnormal or absent reproductive cycles, development of allergies and hypersensitivities (ie.vaccinations, flies), neurologic signs, such as seizures or narcolepsy (sleepy), blindness in advanced cases.

 

The Pathophysiology of Cushing’s Disease/PPID

 

In brief, the pathophysiology of this disease is quite complex.  For a more thorough explanation, please visit our website, mwvets.com and follow the Services menu bar to Equine.  Essentially, a benign adenoma (tumor) develops in the pituitary gland, which causes overproduction of some hormones, most importantly ACTH.

Diagnosis of Cushing’s Disease

Our veterinary clinic usually starts with a simple blood test, called Endogenous ACTH.  A blood sample is sent to the University of Guelph.  The cost of the test is approximately $100, plus shipping.  If the ACTH level is normal and does not support a diagnosis of Cushing’s disease, your veterinarian may consider other endocrine disorders, such as Equine Metabolic Syndrome, Hypothyroidism, Hyperadrenocorticism, or simply obesity and laminitis.

Treatment of Cushing’s Disease

A drug named Prascend (pergolide mesylate) has been registered in Canada and is the drug of choice.  It is a tablet, which is given orally to the horse daily, and is relatively affordable.  Pergolide is a dopamine receptor agonist which was originally approved in 1982 for the treatment of Parkinson’s disease in humans. 

In addition to treatment with Prascend, the following measures should be taken to ensure your horse remains healthy and the drug continues to be effective:

  1. A nutritional plan is of utmost importance.

  2. The most appropriate diet to feed a horse is grass or grass hay on a free choice basis, so long as this forage is not too rich in sugars. Test your hay.  The vet clinic or your nutritionist can test your hay for you. They should ask for the wet chemistry test, including sugars and minerals. That way you can custom mineral balance your hay ration to ensure your horse is properly fed.A slow feeder net should be used to slow down consumption. Grain is not necessary for most horses, unless in serious training/work, and many commercial feeds have high amounts of sugars that should be avoided. If your hay analysis is high in sugar, it may be necessary to soak your hay for 30 minutes prior to feeding. This will help leach out some of the excess sugar. A grazing muzzle may be necessary in order to restrict or slow down the amount of sugar-rich grass your horse may be exposed to. To maintain weight, a horse or pony should consume 2% of its body weight in hay per day. That’s 20 lbs of hay for a 1000 lb horse, or approximately ½ - 2/3 of a small square bale. To lose weight, you can safely reduce this to 1.5% of body weight.

  3. Body clipping, especially in the hot months to help manage uneven hair growth.

  4. Consistent monitoring for signs of infection, lameness, dental and eye issues.

  5. Avoid stressful situations, which can further increase cortisol levels.

  6. Monitor your horse’s body condition using the Henneke scale.  An ideal weight is a score of 5-6.

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