Equine Cushing's Disease

Equine Cushing’s Disease, or Pituitary Pars Intermedia Dysfunction (PPID), is the most common endocrine disorder in older horses. It results from hyperplasia or adenoma formation in the pars intermedia of the pituitary gland, leading to excess production of ACTH and cortisol dysregulation. Clinical signs include hirsutism, laminitis, muscle wasting, PU/PD, and increased susceptibility to infections. Understanding PPID’s pathophysiology, diagnosis, and management is essential for NAVLE success and equine practice.

Equine Pituitary Pars Intermedia Dysfunction (PPID)

Also Known As (AKA):

Equine Cushing's Disease

Etiology:

PPID is primarily a neurodegenerative disorder characterized by the loss of dopaminergic inhibition of the pituitary pars intermedia, leading to hyperplasia or adenoma formation.

Pathophysiology:

Degeneration of hypothalamic dopaminergic neurons results in reduced inhibition of melanotrophs in the pars intermedia.
This disinhibition leads to excessive production of pro-opiomelanocortin (POMC) and its derivatives, including adrenocorticotropic hormone (ACTH).
Elevated ACTH levels stimulate cortisol secretion from the adrenal glands, contributing to the clinical manifestations of PPID.

Clinical Signs:

Hypertrichosis (hirsutism): long, curly hair coat that fails to shed.
Muscle wasting, particularly along the topline.
Lethargy and decreased performance.
Laminitis: a common and potentially severe complication.
Polyuria and polydipsia.
Hyperhidrosis or anhidrosis.
Abnormal fat distribution, such as supraorbital fat pads.
Increased susceptibility to infections due to immunosuppression.

Laboratory Findings:

Complete Blood Count (CBC):
- Mild anemia.
- Neutrophilia with possible hypersegmentation.
- Lymphopenia.
Serum Biochemistry:
- Hyperglycemia.
- Elevated liver enzyme activities.
- Hypercholesterolemia and hypertriglyceridemia.

Diagnostic Imaging:

Not typically utilized for PPID diagnosis; however, advanced imaging like MRI can identify pituitary enlargement in research settings.

Confirmatory Diagnostic Tests:

Basal Plasma ACTH Concentration:
- Elevated levels suggest PPID; seasonal reference ranges should be considered due to physiological variations, especially in the autumn.
Thyrotropin-Releasing Hormone (TRH) Stimulation Test:
- Measures ACTH response to TRH administration; an exaggerated response is indicative of PPID.

Treatment:

Pharmacological Therapy:
- Pergolide mesylate, a dopamine agonist, is the treatment of choice.
Management Practices:
- Regular body clipping to manage hypertrichosis.
- Dietary modifications to address insulin dysregulation and reduce laminitis risk.
- Routine farriery care to manage or prevent laminitis.

Prognosis:

With appropriate management, many horses experience improved quality of life.
Lifelong treatment is typically necessary.
Early diagnosis and intervention are crucial to mitigate complications such as laminitis.

Additional Considerations:

Regular monitoring of clinical signs and endocrine parameters is essential to adjust treatment protocols effectively.
Awareness of seasonal variations in hormone levels is important for accurate diagnosis and management.

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