Pituitary Pars Intermedia Dysfunction - 'equine Cushing’s disease'

Description

Pituitary pars intermedia dysfunction (PPID), commonly known as Equine Cushing’s Disease, is a progressive disorder disrupting the horse’s endocrine (hormonal) system, typically affecting middle-aged to older horses. In horses Cushing’s disease is almost exclusively caused by enlargement - hypertrophy (increase in cell size) and hyperplasia (increase in cell number) - of the pars intermedia (intermediate lobe) of the pituitary gland, thought to be due to a decrease of the neurotransmitter dopamine from oxidative damage to neurons originating in the hypothalamus.

Simplified, a sequence of endocrinologic events occurs resulting in overproduction of various hormones from the pituitary gland including adrenocorticotropic hormone (ACTH), which stimulates cortisol release at the adrenal glands. There is also loss of the circadian pattern of cortisol secretion.

The characteristic clinical signs occur as a consequence of the excess circulation of hormones, the metabolic changes associated and the physical damage of the pituitary, hypothalamus or optic nerve.

Clinical Signs

Clinical signs can vary between individuals, even within an individual upon different phases of the disease. They include:

Hirsutism (long, thick or curly hair)
Polyuria (excessive urinating)
Polydipsia (excessive drinking)
Laminitis
Lethargy
Hyperhydrosis (excessive sweating) or anhydrosis (reduced sweating)
Insulin resistance
Increased appetite
Changes in body shape; cresty mane, muscle atrophy and pot bellied appearance
Abnormal fat distribution, including pockets above the eye
Immunosuppression
Poor resistance to parasites
Allergies & hypersensitivities
Infertility
Blindness due to compression of the optic chiasm
Cortisol induced insulin resistance

Diagnosis

Clinical signs will usually provide the information to make an initial diagnosis, with serum chemistry and endocrinology analysis confirming diagnosis.

A dexamethasone suppression test (DST) and thyroid stimulating hormone release (TRH) test are further diagnostics used.

Treatment

Horses with PPID require special care to support their clinical signs and suppressed immune system. Most require diligent attention to good husbandry in addition to pharmacologic therapy. It is important to note that there is no cure to PPID and that medical treatment is supportive and not curative.

Pergolide is a dopaminergic agonist and is currently the only product proven to decrease endogenous ACTH concentrations in horses with PPID. The starting dose should be gradually introduced over 4-6 weeks until a maintenance dose is established.

Reported side effects of Pergolide therapy include depression and anorexia. Often, these signs are temporary and will resolve over time. If they do not, the dose should be decreased temporarily or split and administered twice a day.

Although its use has not been documented to result in long-term improvement in clinical signs when given alone, Cyproheptadine may exert synergistic effects when combined with Pergolide, and the combination may result in outcomes better than those achieved with Pergolide alone.

Management

PPID horses should be managed fastidiously due to their suppressed immune system and clinical conditions.

They require regular:

Vaccination
Parasite treatment; worming and faecal egg counts
Dentistry
Hoof care, particularly if showing signs of laminitis
Diet management
Prompt response to wounds or infection
Thermoregulation management; clipping of excessive hair will help the comfort of affected horses in warmer months and shelter and rugging during colder months.