Equine Cushing's Disease (Pituitary Pars Intermedia Dysfunction)

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Key takeaways

Equine Cushing’s disease, or pituitary pars intermedia dysfunction (PPID), is a common hormone disorder affecting the control of ACTH levels within the body.

  • ACTH is the major hormone controlling cortisol, which is responsible for maintaining blood sugar levels, wound healing, and other critical bodily functions
  • Horses with PPID show symptoms such as a thick hair coat, muscle loss, a pot-bellied appearance, and increased drinking and urination
  • Some horses with PPID develop serious complications, such as laminitis
  • Diagnosis of PPID involves a physical examination and series of blood tests.
  • Lifelong treatment with pergolide, a medication which restores ACTH control, lessens symptoms and decreases the likelihood for laminitis
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A closer look: Equine Cushing's Disease (Pituitary Pars Intermedia Dysfunction)


The symptoms of PPID vary in severity and many horses are not diagnosed until late in the disease progression. One of the most significant complications of PPID is laminitis, where the coffin bone separates from the hoof wall surrounding it. In some cases, horses are not diagnosed with PPID until their first episode of laminitis. Laminitis causes severe lameness, and horses often position themselves in a “sawhorse” stance, where they lean backwards with their front feet extended forwards. Horses showing a sawhorse stance require emergency veterinary care. Many horses with PPID have repeated episodes of laminitis if not properly managed.

PPID also impacts inflammation and healing. Horses may experience chronic, repeated infections, such as skin or eye infections. Wounds may also take longer to heal than expected.

Risk factors


PPID is a common condition in horses, particularly horses over the age of 15. Estimates suggest that between 15-30% of horses over the age of 15 have PPID.

Horses showing symptoms of PPID such as a thick hair coat, muscle loss, a pot-bellied appearance, weight loss, and increased drinking and urination require prompt veterinary assessment. With appropriate medical management, horses with PPID have a good quality of life and minimal symptoms.

Possible causes


The most common cause of PPID is a benign mass in the pituitary gland. The mass dysregulates hormone secretion by the pituitary gland, disrupting ACTH levels. ACTH is the major hormone that controls cortisol levels in the body. Ultimately, the altered ACTH is what produces the symptoms associated with PPID.

Main symptoms


Testing and diagnosis


Diagnosis of PPID begins with a physical examination and blood work. If PPID is suspected, then specific testing for the condition is pursued. The main tests available for PPID are:

  • Plasma ACTH level: ACTH is the hormone that regulates cortisol levels. Horses with PPID have elevated ACTH levels.
  • TRH stimulation test: TRH is a thyroid hormone that stimulates ACTH production. This test measures baseline ACTH levels, then measures ACTH levels again after TRH is administered.
  • Glucose tolerance test: Many horses with PPID have insulin dysregulation, similar to diabetes. This test identifies issues with insulin control.

Test results are influenced by many factors, including the time of year when testing takes place. This variation makes diagnosis of PPID challenging in some cases, particularly in horses that have minimal symptoms. Repeated testing may be required in some cases if PPID is still suspected despite negative results.

Steps to Recovery


PPID is incurable, but can be managed with the medication pergolide. This medication restores ACTH control, minimizing the symptoms of PPID. After starting treatment, horses are retested for ACTH levels every 1-3 months until ACTH levels are stable and the horse has improved clinical signs. During the maintenance phase of treatment, horses continue to be tested every 6-12 months. Some horses experience reduced appetite when starting pergolide. Working with a veterinarian to adjust the dose is crucial to restoring appetite while maintaining treatment of PPID. Some severe cases benefit from adding cyproheptadine, an appetite stimulant, alongside pergolide as a treatment strategy. Note: pergolide is a prohibited substance under many show management organizations, including the FEI.

Additional treatment strategies for horses with PPID include:

  • Body clipping during the summer if they do not shed
  • Blanketing during the winter
  • Monitoring closely for signs of laminitis
  • Altering the diet to prevent weight loss
  • Routine deworming, vaccination, and dental care to prevent infections
  • Prompt treatment of any wounds or infections

PPID is a lifelong disease. Most horses’ symptoms worsen over time without treatment. With pergolide treatment, many horses have improved symptoms and have a good long-term prognosis. Severe cases that do not respond to pergolide or have other complications, such as laminitis, have a poorer prognosis. Repeated episodes of laminitis is a common cause of euthanasia for horses with PPID.

Prevention


There are no preventative measures available for PPID.

Typical Treatment


  • Pergolide
  • Cyproheptadine
  • Management changes

Is Equine Cushing's Disease (Pituitary Pars Intermedia Dysfunction) common?


PPID is common in horses over the age of 15, with between 15-30% of horses affected.

References


Clair Thunes, PhD - Writing for The Horse
Alexandra Beckstett, The Horse Managing Editor - Writing for The Horse
Janice E. Kritchevsky , VMD, DACVIM-LAIM - Writing for Merck Veterinary Manual
Amy Young - Writing for UC Davis Veterinary Medicine Center for Equine Health
- Writing for Veterinary Diagnostic Laboratory
Kelsey Hart ​DVM PhD (Physiology), et al. - Writing for The Equine Endocrinology Group

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