Cushings disease is most common in aged horses and ponies, especially those 15 or older, but it has been diagnosed in horses as young as seven years of age. Bonnie S. Barr, VMD, DACVIM of Rood and Riddle Equine Hospital near Lexington, Kentucky explains what Cushing’s disease is, common clinical signs, diagnosis and treatment.
Equine Cushing’s disease is the most common disease of the hormonal (endocrine) system. A more appropriate term is pituitary pars intermediate dysfunction (PPID) because it reflects the location within the brain that is abnormal.
The pituitary gland is made up of three lobes and is located at the base of the brain. It is responsible for production of various hormones that regulate body functions. In horses with PPID, the middle lobe becomes enlarged resulting in an over production of hormones. This enlarged middle lobe of the pituitary can also affect structures adjacent to it, resulting in additional problems.
PPID is common in aged horses and ponies most often over the age of 15 years, but it has been diagnosed in horses as young as seven years of age. Both male and females are affected. All breeds of horses can develop PPID; however, ponies and Morgan horses have a higher incidence.
Clinical signs are variable and the disease progresses slowly. The most common clinical signs include abnormal hair coat (hair that does not shed out or long patches of hair), laminitis, increased water consumption and urination and changes in body conformation (pot belly, fat patches around eyes, neck or tail head). Other clinical signs include lethargy, change in attitude, abnormal sweating, increased appetite, muscle wasting, recurrence of infections and infertility.
The diagnosis of PPID can be challenging. Horses with subtle signs may need to be evaluated and tested every four to six months. The best indication of PPID is the clinical sign of an abnormal hair coat; specifically hair that does not shed out or long patches of hair.
The most commonly used tests are the dexamethasone suppression test and the measurement of resting plasma adrenocorticotropin hormone (ACTH). The dexamethasone suppression test involves the administration of dexamethasone and blood samples to determine the effect on the blood cortisol level. In a normal horse the cortisol value will decrease (or be suppressed by the dexamethasone’s affect on the pituitary gland); in a horse with PPID the value will not decrease. Because dexamethasone can cause laminitis, this test would not be used in laminitic horses.
Another test, which is safe for horses with laminitis, is the concentration of adrenocorticotropin hormone (ACTH) in the plasma. Horses with PPID have higher levels of ACTH than normal horses. Unfortunately, neither of these tests is 100% accurate and variability in result s occurs in certain times of the year.
Oftentimes blood insulin and glucose levels after fasting will be measured because horses with PPID can also have high blood insulin levels, suggesting insulin resistance. Insulin resistance can occur in horses with PPID, but it also occurs in horse with equine metabolic syndrome (EMS). EMS and PPID share some clinical characteristics, but the underlying causes are different.
The aim of treatment is to help decrease the signs of disease, not to cure the underlying cause. The drug of choice is pergolide, which reduces some of the excess hormone production in the pituitary gland. Prascend, which is pergolide mesylate, is approved by the FDA for treatment of PPID. Cyproheptadine is another drug that has been used to treat PPID. It also works at the level of the pituitary gland, although studies have suggested that it is not as effective as pergolide.
Appropriate management is important for horses with PPID. During the warm months, body clipping may be necessary. A special low starch diet can be fed to those horses with insulin resistance. Preventative measures such as regular deworming, dental care and appropriate farrier work are warranted.
PPID is a disease of older horses that can be managed. Recognizing the symptoms and early diagnosis are important steps in addressing the disease and managing its effect on the horse. A balanced approach combining proper medical treatment and management practices will provide relief from symptoms and extend the life of the PPID horse.
By Bonnie S. Barr, VMD, DACVIM