PPID – It’s not “Cushing’s”!
What are Pituitary Pars Intermedia Dysfunction (PPID) and Cushing’s?
Pituitary pars intermedia dysfunction (PPID) was initially, and wrongly, called Cushing’s due to similarities with canine and human adrenal diseases. The pituitary gland is part of a hormone production pathway that relies on signals from the hypothalamus. Although PPID and Cushing’s may lead to an increase in ACTH, a hormone that regulates the adrenal gland, that is where the similarities between the two conditions end. Unlike Cushing’s, there is no consistent finding of increased blood cortisol or a dysfunction in the adrenal glands. In the horse, the disease primarily affects the neurons that produce dopamine, damaging the regulatory control of the middle lobe of the pituitary gland (pars intermedia). The pars intermedia of horses have more “melanotrope” cells, producing α-MSH. (Whereas humans and dogs have more “corticotrope” cells, producing ACTH.) The uncontrolled release of α-MSH from this region affects processes throughout the body, including metabolic rate, hair growth, and the immune system. In some instances, the pituitary gland enlarges, compressing other regions of the pituitary or the adjacent hypothalamus, leading to more uncommon signs.
What are the signs of PPID?
Most horses affected are over 15 years of age, but horses as young as 7 have been diagnosed with PPID. Because the pituitary area affected in horses produces more MSH, clinical signs are often first noted in hair. The most common signs of PPID are a long or curly hair coat, delayed shedding in the spring, loss of topline muscle, and lethargy. In more advanced cases, a decreased immune system may be observed. Manifestations include dental disease, recurrent foot abscesses, decreased wound healing or excessive parasite load. Once the condition progresses, ACTH levels increase and more signs present. With adenomas or very high levels of ACTH , abnormal thirst, excess urination or abnormal sweating may be observed.
Laminitis has also been associated with horses diagnosed with PPID, but most have concurrent insulin dysregulation or abnormal body fat associated with Equine Metabolic Syndrome. Insulin regulation is often investigated when PPID is suspected, especially when assessing laminitis risk and management. One does not cause the other, but they can coexist.
How is it diagnosed?
Diagnosis is achieved by having clinical signs suggestive of PPID as well as diagnostic testing. Most horses with obvious clinical symptoms will have an increase in ACTH, therefore a baseline ACTH measurement is performed. Seasonal variation in ACTH has been observed but new seasonal reference ranges have allowed accurate diagnosis year-round. Stress and illness can artificially increase the ACTH levels. Avoid sampling if your horse is ill or suffering from acute laminitic episodes.
In early PPID or cases where the condition is suspected but obvious signs have not yet occurred, a Thyrotropin Releasing hormone (TRH) stimulation test is recommended. After a baseline ACTH sample is collected, TRH is administered intravenously and another sample is collected after 10min.
Treatment Options for PPID
There is no cure for PPID, therefore management and treatment are life-long to reduce the clinical signs of the disease. Only one medication has been approved: pergolide mesylate (Prascend®). Clinical signs typically improve within 1-3 months. The ACTH levels are monitored regularly to ensure that the condition is being controlled, generally twice annually. Testing each fall, at minimum, is best as this is when levels are naturally the highest.
Because of immune suppression and the possibility of concurrent EMS or laminitis, regular veterinary examination are recommended. Regular vaccination and deworming, as well as periodic blood work if possible are all important. Dental examination and diet management must be included too.
Is PPID genetic or could it be prevented?
So far, the only known risk factor for PPID is aging – no gender or breed predispositions have been found.
For more information:
Equine Endocrinology Group, Recommendations for the Diagnosis and Treatment of Pituitary Pars Intermedia Dysfunction (PPID)
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