Equine Pituitary Pars Intermedia Dysfunction (PPID) – NAVLE Study Guide

Overview and Clinical Importance

Pituitary pars intermedia dysfunction (PPID), formerly known as Equine Cushing's Disease, is the most common endocrine disorder of aged horses. PPID affects approximately 20-25% of horses over 15 years of age. Unlike human and canine Cushing's disease (which affects the pars distalis), equine PPID specifically affects the pars intermedia of the pituitary gland. The disease results from oxidative neurodegeneration of dopaminergic neurons in the hypothalamus, leading to loss of inhibitory control over melanotropes in the pars intermedia.

Understanding PPID is critical for NAVLE success as it appears frequently in questions involving geriatric horse care, laminitis workup, and endocrine testing. Early recognition and treatment significantly improves quality of life and survival.

Pathophysiology

Normal Pituitary Function

The equine pituitary gland consists of three main regions: the pars distalis (anterior lobe), pars intermedia (intermediate lobe), and pars nervosa (posterior lobe). The pars intermedia contains melanotrope cells that are directly innervated by dopaminergic neurons from the hypothalamus. Dopamine acts on D2 receptors to inhibit melanotrope proliferation and proopiomelanocortin (POMC) transcription.