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 winter.
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."
One of the most critical signs is chronic, insidious laminitis. Though the relationship of Cushing's and laminitis is not well understood, according to Philip Johnson, BVSc, MS, MRCVS, Dipl. ACVIM, Dipl. ECEIM, University of Missouri at Columbia veterinary school, excess cortisol may increase the risk of laminitis through several mechanisms: reducing blood supply to the lamellar tissue; weakening hoof lamellar attachments; impairing ongoing hoof lamellar restitution; and reducing glucose delivery to the hoof cells. "These are some plausible mechanisms that might contribute to the risk of laminitis associated with glucocorticoids, but the scientific evidence for which — if any — is truly significant, is lacking," Johnson says.
Excess ACTH leads to excess cortisol production by the equine adrenals. Pituitary damage and excessive cortisol is the common thread to Cushing's in each species, except for dogs with primary adrenal gland disease.
Cortisol normally helps maintain blood pressure and cardiovascular function, regulates the immune system's response to infection and inflammation and balances the effects of insulin in regard to glucose and also in the regulation of fat and protein metabolism.
Cortisol regulates nerve tissue function, muscle tone and connective tissue repair. Its primary function is to help the animal respond to stress. In one way or another, almost all cells throughout the body are responsive to cortisol.
While Cushing's in dogs and people is really attributable directly to excess cortisol, the condition in horses is distinctly different, in that excess cortisol is not always very significant, but the production and release of excessive quantities of pars intermedia-derived POMC peptides is inescapably significant and represents an equine-specific difference.
Cortisol exerts its effect on the functioning of the cell by entering the cell and interacting with a receptor in the cell's nucleus. The cell's function is altered depending on the extent to which cortisol is or is not acting to control the transcription of specific proteins at the genetic level.
It is not possible to define optimal in this context. The active concentration of cortisol within the cell is defined by both the circulating plasma concentration and the extent to which the intracellular (cytoplasmic) concentration of cortisol is changed by a cell-specific enzyme.
This "steroid-converting enzyme," 11-beta hydroxysteroid dehydrogenase (HSD), is capable of either increasing or reducing the cytoplasmic concentration of active cortisol in different cells.
According to Johnson, this enzyme regulates the extent to which glucocorticoid receptors are activated by cortisol. The enzyme, a product of the cell, either will "allow" cortisol to activate the nuclear transcription process or not. Depending on its isoenzyme type, it can destroy cortisol before it gets close to its receptor, or it can produce more cortisol (from its inactive cousin, cortisone), increasing the local effectiveness of glucocorticoids.
"The old-fashioned idea that the activity of cortisol in a given tissue is simply a function of its circulating concentration is thus no longer applicable," says Johnson.
"Under normal circumstances," he says, "the concentration of cortisol within the cell is adjusted by HSD within the cell itself. This ensures that the requirements of the cell at any given time are met. It has been suggested that Cushing's syndrome may sometimes be attributed to abnormal HSD activity within the cells."
Specifically, within the abdomen in people and in laboratory rodents, excess fat tissue contains increased HSD that contributes to the medical problems attributable to abdominal obesity. Human researchers have likened this phenomenon to a tissue-specific Cushing's disorder, known as "Omental Cushing's."
According to the theory, "increased HSD activity in the cells leads to increased cortisol within the tissues," Johnson explains. Increased HSD in a given tissue implies increased cortisol effect in that tissue. Exactly what cortisol is doing in that tissue is not always clear.
As a part of studies to better understand why laminitis sometimes occurs in the context of excess glucocorticoids (hypercortisolism), Johnson and his team, Drs. Seshu Ganjam and Nat Messer, developed a test for HSD in the tissues of horses. They then compared the level of HSD in the tissues of the skin and hoof of normal, healthy adult horses with the levels found in horses with laminitis. They were able to show that HSD could be identified both in the skin and hoof tissues and that the level of HSD was substantially elevated in the tissues of the foot and skin of horses with laminitis. They also hypothesized that skin/hoof HSD activity may be increased as a result of insulin resistance in some laminitic horses.
"Under some circumstances," says Johnson, "it appears that HSD is increased in the hoof tissues. The implication is that the action of cortisol is increased locally — cortisol does many different things to the cells. If HSD is increased locally, it could be either a result of laminitis or a contributing risk factor for laminitis. We do not know."
Nat Messer, DVM, Dipl. AVBP, University of Missouri at Columbia veterinary school, displayed results of experiments performed by Fabiana Farias and Seshu Ganjam investigating 11-beta HSD activity in adipose tissues from nonobese and obese/IR horses, and showed no differences between groups. Moreover, unlike the situation in humans and some other laboratory animal species, 11-beta HSD activity did not differ based on whether it was measured in subcutaneous fat compared with visceral fat.
"I think that the development of chronic insulin resistance (IR) is the major problem in horses," says Nicholas Frank, DVM, PhD, Dipl. ACVIM from the University of Tennessee.
"There is agreement that the development of insulin resistance is regarded as important in terms of risk of laminitis in obese horses,"Johnson says.
What the Missouri team has shown is that, "unlike the situation in humans that develop obesity-associated insulin resistance, heightened activity of the 11-beta HSD enzyme in visceral adipose tissue does not appear to be an important mediating link between accretion of obesity and development of insulin resistance. The role of heightened 11-beta HSD activity in skin and hoof lamellar interface tissues in the laminitis context points to a role for cortisol, but continues to lack a satisfactory explanation: specifically, is the increased 11-beta HSD a consequence of laminitis or was it there in such a manner as to predispose to laminitits?" Johnson asks.
Getting an early handle on PPID
"The most challenging question regarding PPID is the issue of mild/early disease," Frank says. It is relatively easy to diagnose advanced PPID because horses with hirsutism are easily recognized, he adds. However, if we want to match the situation with Parkinson's disease in human medicine, "we should be more focused upon slowing the development of disease or even preventing it," he says.
Dianne McFarlane, DVM, PhD, Dipl. ACVIM, Oklahoma State University veterinary school, has shown that PPID is a disorder that develops as dopaminergic neurons undergo oxidative damage. It is a degenerative process that occurs as horses age, but may also be accelerated under some circumstances.
Factors that determine the rate of degeneration might include genetics, diet, environmental exposure to oxidants or pre-existing conditions, such as chronic inflammatory or metabolic disease. "My work with obesity and insulin resistance leads me to believe that horses that are chronically affected by these conditions are predisposed to PPID," Frank says. For such horses, especially those placed in confinement (off good grass pasture) on poor-quality grass, hay supplemental vitamin E is advisable.
"Unfortunately, currently available diagnostic tests (with the possible exception of a new oral domperidone test under development) do not seem to detect early/mild PPID," Frank says. This leaves practitioners trying to decide whether to be aggressive and try to prevent the disease or wait until it gets severe enough to diagnose.
"I advocate the early use of pergolide (dopamine agonist) in horses that we suspect will develop the condition at a younger age (i.e., the horse that has been chronically obese and insulin-resistant) and the provision of adequate antioxidants in the diet — vitamin E primarily," Frank says.
What causes the dopaminergic neurons to degenerate? According to McFarlane, "Oxidative stress results in modification of cellular components including proteins, DNA and cell-membrane lipids, because of excessive exposure to exogenous or endogenous free-radicals." This leads to neurodegeneration. McFarlane looked at the pituitary and the hypothalamus of horses with PPID, using immunohistochemistry to see if there was degeneration of the dopaminergic neurons and also to look for markers of oxidative stress, as a potential mechanism of why those neurons might be damaged.
McFarlane found a marked decrease in dopaminergic neurons at the level of the pituitary and hypothalamus both at their nerve terminals and cell bodies. Oxidative stress markers were increased, and, similar to what is observed in humans with Parkinson's disease, there was an accumulation of the protein alpha-synuclein in horses with PPID.
A neurodegenerative disease
"In summary," McFarlane says, "PPID likely is a neurodegenerative disease, and oxidative stress and abnormal protein accumulation is associated with the neurodegeneration of those dopaminergic neurons." This theory fits with what is known, both in how horses with PPID respond to treatment with pergolide, a drug that replaces dopamine, and also from the older work from the 1980s showing decreased pituitary dopamine concentration," McFarlane says.
Regardless of the progress in determining the probable cause of the disease, equine practitioners don't have an early enough diagnostic marker yet of PPID. What might be effective would be to recognize and to treat affected horses during the early stages of the disease, before the neurons are irreparably damaged.
Researchers now are looking for improved markers of the disease that will allow veterinarians to recognize affected horses earlier, and to see whether they can give them neuroprotective therapies, such as antioxidants, to slow or stop PPID. "(Lack of early recognition/diagnosis) keeps us from being able to say that antioxidant supplementation will help this condition," McFarlane says.