Pituitary pars intermedia dysfunction (PPID), or Equine Cushing Disease, in the horse is a slowly progressive disorder with characteristic clinical features. Also known as equine Cushings disease because of its similarity to Cushings disease in humans. Unlike humans, the condition in horses is caused by adenomas or adenomatous hypertrophy of the intermediate lobe of the pituitary gland located in the brain.
The clinical signs include hirsutism (excessively long hair), hyperhidrosis (increased sweating), increased water intake and urination, muscle wasting, increased susceptibility to bacterial and parasitic infections, lethargy, bulging eyes and a predisposition to laminitis. Laminitis is generally considered one of the most serious secondary diseases.
PPID is caused by functional hyperplasia of the intermediate lobe of the pituitary gland. This enlargement apparently occurs because of an age related decline in the neurotransmitter dopamine, which has an inhibitory effect on the intermediate lobe of the pituitary gland. The specific reason(s) for the high incidence of laminitis in horses with PPID is unknown. They may be partially related to a deficiency in circadian fluxuations of corticosteroid secretion. This has potential adverse effects on the vascular integrity in the hoof and structural stability of the hoof’s lamellar connective tissue.
The diagnosis of PPID is made by consideration of the clinical signs, age of the horse, and laboratory testing. There are numerous laboratory tests including measurement of adrenocorticotrophic hormone (ACTH) and/or the dexamethasone suppression test.
Currently the one treatment available which has shown clinical improvement for Cushings Disease is a drug called Pergolide. The drug replaces dopamine, so it is generally given for life. Sometimes this drug can be used in conjunction with another drug called Cyproheptadine. While these drugs can provide effective management of the disease, they do not appear to be a permanent cure.