PPID (Pituitary Pars Intermedia Dysfunction)

PPID (pituitary pars intermedia dysfunction), historically called equine Cushing's disease, is a progressive neurodegenerative condition of the hypothalamic-pituitary axis. Dopaminergic neurons that normally inhibit the pars intermedia degenerate, leading to unregulated production of ACTH, MSH, and other POMC-derived peptides.

Signalment: Aged horses and ponies (> 15 years); no breed predilection but ponies may be overrepresented.

Clinical signs:

• Hypertrichosis (hirsutism): Long, curly coat that fails to shed; pathognomonic but occurs in advanced disease
• PU/PD: Secondary to cortisol-induced diabetes insipidus-like state
• Weight loss & muscle wasting: Swayback, pot-bellied appearance
• Laminitis: Chronic or recurrent; major cause of morbidity/mortality
• Recurrent infections: Sinusitis, skin infections, hoof abscesses, parasitism due to immunosuppression
• Other: Lethargy, hyperhidrosis, infertility, delayed wound healing

Baseline ACTH:

• Gold standard screening test
• Seasonal reference ranges essential — ACTH normally rises in fall (August–October)
• Typical non-fall reference: < 50 pg/mL; fall reference may be up to ~100 pg/mL depending on lab
• ↑ ACTH = PPID; borderline = consider TRH stimulation test

TRH stimulation test (early disease):

• Administer TRH IV, measure ACTH at 0 and 30 minutes
• Exaggerated ACTH response = early PPID when baseline is normal/borderline

Insulin & glucose:

• Many PPID horses have concurrent insulin dysregulation/EMS
• Check fasting insulin and glucose; dynamic testing (OST) if indicated

Dexamenhasone suppression test:

• Not recommended in clinical practice due to risk of inducing laminitis

Resting cortisol:

• Unreliable due to diurnal variation and stress; not recommended

Pergolide mesylate:

• First-line treatment; dopamine agonist
• Dose: 1–2 mcg/kg PO q24h (typical starting dose ~1 mg/500 kg)
• Titrate based on clinical response and ACTH levels
• Side effects: transient inappetance/dullness in first 1–2 weeks (start low, go slow)

Supportive care:

• Diet: Low NSC (< 10%), maintain ideal body condition, avoid high-sugar treats and pasture during peak sugar times
• Deworming: Regular fecal egg counts and targeted deworming (immunocompromise increases parasite susceptibility)
• Dentistry: Ensure adequate mastication for weight maintenance
• Farriery: Aggressive laminitis prevention and monitoring

Monitoring:

• Recheck ACTH every 6 months (adjust for seasonal timing)
• Monitor for laminitis signs (digital pulses, hoof heat)
• Track weight, coat shedding, water consumption