Equine Cushing's (Pituitary Pars Intermedia Dysfunction, or PPID), similar to the human and canine disease, is characterized
by pituitary damage, excess production of ACTH and subsequently cortisol.
Battling Cushing's disease: Excessive, curly hair coat growth, either over the entire body or over the neck and shoulders
is a classical clinical sign of Cushing's disease in horses. (Photo: Dr. Nicholas Frank, DVM, University of Tennessee)
Hypercortisolism is less significant in many affected equids, compared with humans and dogs. Unlike the syndrome and disease
in those species, the disease in horses originates from hyperplasia or adenoma of the pituitary pars intermedia, rather than
adenoma or carcinoma of the adrenal in dogs, or adenoma of the pars distalis in both dogs and people.
Current thought is that equine Cushing's may be the result of primary hypothalamic and secondary pituitary disease, produced
by oxidant damage to the hypothalamic-derived dopaminergic neurons that innervate the pituitary pars intermedia.
Pituitaries of PPID-affected horses may enlarge up to five times their normal size. This enlargement compresses the adjacent
pituitary and hypothalamic tissues, often resulting in further functional loss. Loss of dopaminergic inhibition is critical
to the pathology of PPID. Pituitary pars intermedia tissue dopamine concentration may show an eight-fold decrease.
There has been a significant apparent increase in incidence recently in horses, most likely due to well-cared-for horses living
to older ages, since it is primarily seen in horses 18 years of age and older, and predominantly in the stouter breeds, e.g.
Morgans and ponies, although equids of all types and breeds are susceptible.
Now there is an actual treatment, emphasizing the need to characterize the medical status of the older horse. An increase
in research and literature has raised awareness of PPID. And, other "treatments" for old/geriatric horses are more readily
available — especially senior-horse diets.
In people, PPID affects mostly females ages 20 to 50, with typical clinical signs of exposure to elevated cortisol, including
excessive weight, especially in the upper torso and face; elevated body temperature; depressed immune function with potential
for increased incidence of infections; thin and visibly damaged skin/bruising; decreased bone density leading to fractures;
anxiety; irritability; depression; PU/PD; and hyperglycemia.
Women show excessive hair growth on the face, neck, chest, abdomen and thighs. (People do not have a pituitary pars intermedia.
In humans it's mainly hypercortisolism, but in horses hypercortisolism is only evident in about 20 percent of PPID cases.
The "other" PI — pituitary-derived hormones — are important in equine disease, not human).
Cushing's in dogs presents with signs of hair loss, pot-bellied appearance, increased appetite, PU/PD, fragile blood vessels
and thin skin that bruises easily.
In horses, as opposed to thin skin and hair loss, Cushinoid horses show excessive coat growth (hirsutism), either over the
entire body or predominantly over the neck and shoulders, which doesn't shed like the normal winter coat. Hirsutism is theoretically
caused by increased PI-derived POMC (proopiomelanocortin) peptides, which normally stimulate increased coat growth prior to
Other signs include elevated body temperature and sweating; depressed immune system, leading to an increased incidence of
infections (respiratory disease — sinusitis, alveolar periostitis, bronchopneumonia; skin infections; abscesses of the foot;
buccal ulcers, gingivitis, periodontal disease; delayed wound healing; hampered protein and fat metabolism, seen as decreased
muscle deposition (epaxial and rump) and increased fat deposition, especially along the crest of the neck and over the tail
head; lethargy; dental abnormalities; PU/PD; and hyperglycemia.
Dental abnormalities are not a part of PPID per se; they are common coincidental medical issues in their own right in older
horses. Harold Schott, DVM, PhD, Dipl. ACVIM, Michigan State University veterinary school, says, "Horses with PPID have also
been described as overly docile and more tolerant of pain than normal horses." These signs are attributed to increased plasma
and CSF concentrations of beta-endorphin, that are "60- and more than 100-fold greater, respectively, in horses with PPID
than in normal horses."