Understanding Equine Cushing's Disease

 

 

Cushing's disease affects about 15-30% of aged horses and is associated with severe conditions such as laminitis, which can drastically worsen a horse's quality of life. Equine Cushing's disease has been renamed pituitary pars intermedia dysfunction (PPID).  This dysfunction is caused by a decreased secretion of the neurotransmitter dopamine, which is responsible for inhibiting the secretion of ACTH.  ACTH is a hormone produced by a region of the pituitary gland called the pars intermedia. PPID is an insidious condition that may start developing in horses in their early teens and then progress to more advanced stages with aging.

 

Clinical Signs

"Phoenix", a 23 year old Arabian  with long guard hairs.

The most recognized clinical sign of a horse with advanced PPID is a long, shaggy hair coat. More subtle signs such as changes in behavior (the horse becomes more docile and dull), muscle loss (especially over the topline), weight loss, and long guard hairs on the back of the legs and under the jaw and neck are indicative of early stage PPID.  Other symptoms may include increased water consumption and urine production, increased susceptibility to infections (skin, sinusitis), excessive sweating, and laminitis.

 

Testing for Cushing's Disease

The most commonly used laboratory test to diagnose PPID is measurement of the hormone ACTH.  ACTH is produced in larger amounts by horses with more advanced PPID. This test will identify 60-70% of horses with PPID, missing about 20-30% of horses in the early stage of disease.

 

Another hormone test, the TRH stimulation test, is now being recommended to identify horses with early stages of PPID. Your veterinarian performs this test by injecting the hormone TRH into the horse's vein.   The horse's blood is collected 10 minutes later to test for ACTH levels. Identifying this disease at an early stage is beneficial as it is easier to control PPID during its earlier stages.

"Buzz" a 25 year old Welsh Pony with very poor shedding.

 

About 30% of PPID horses also have insulin resistance, so an oral sugar test should be performed in all horses with PPID in order to decide if they would benefit from restriction of sugar and carbohydrates in their diet.

 

Treatment

Pergolide is a drug which mimics the action of dopamine and inhibits excessive production of ACTH by the pituitary gland of affected horses. This is the drug of choice for treatment of PPID in horses. The only FDA approved pergolide available on the market is sold under the brand name Prascend. The initial dose of pergolide for an average sized horse is 1 mg (1 tablet) once a day. Tablets are small and most horses will eat them in their feed. However, pickier horses may need to have the pergolide hidden in a treat.  It can also be dissolved in a small amount of water and given by mouth using a syringe. As there is no cure for PPID, treatment with pergolide is life-long. No drug tolerance seems to develop. There are horses whose symptoms have been kept under control on the same dose for years.  Some patients may require an increase in their dose due to progression of the disease. The main side effect of this medication is decreased appetite, which is seen in less than 1/3 of the horses that are treated. This is addressed by decreasing the initial dose and then gradually increasing it until a therapeutic dose is achieved.

 

Management

Growth rings in the hooves of a horse suffering from repeated episodes of mild laminitis and PPID.

It is very important to have your veterinarian help you monitor the health of your horse once the diagnosis of PPID is made so that management changes and adjustments in the level of medication can be recommended.  A health examination and re-check of ACTH levels should be performed 4-6 weeks after starting Prascend, once a year on well-controlled patients, and every time there is exacerbation of clinical signs such as foot soreness, weight loss, recurrent infections, or growth of a longer hair coat.

 

In addition to medication, management strategies for horses with PPID include:

• Horses with a long hair coat can be body clipped in order to improve comfort level and facilitate heat exchange, especially in warmer months.
• Horses with PPID are predisposed to dental infection and sinus disease.  A thorough oral examination and dental care should be performed at least once a year.
• Hooves should be trimmed regularly. Corrective shoeing may be necessary for horses with chronic laminitis. 
• Some horses with PPID show weight loss, so a special diet may be necessary. Knowing if the PPID horse also is insulin resistant (by having an oral sugar test performed) will be crucial to decide if carbs and sugars should be restricted. 
• There is increasing evidence that obesity and insulin resistance in younger horses may predispose them to develop PPID later in life. This is one more reason to address obesity in younger horses.
• Supplements, such as those containing chromium and magnesium, as well as natural remedies containing chasteberry have not proven effective in either improving insulin sensitivity or reducing PPID symptoms.  If supplements are chosen they should be used only as adjunct therapy and not as a substitute for appropriate medication.  

We want to know what you think!

Research performed at three veterinary teaching hospitals showed that 97% of clients that owned PPPID horses were satisfied with the improvements obtained with pergolide treatment. GVEC currently has many horses that have been diagnosed with PPID and we want to know what you think. Please take a few minutes to fill out the questionnaire by using the link below.

https://www.surveymonkey.com/s/VYNFNJB