Pituitary pars intermedia dysfunction (PPID), commonly known as Cushing's disease, is one of the most common endocrine disorders in horses and ponies. It most commonly affects older horses (with the average horse age being approximately 20 years of age), however it has been reported in horses as young as seven years old. Overall, the majority (over 85%) of horses diagnosed with PPID are older than 15 years of age. PPID is caused by an enlargement of a portion of the pituitary gland (an important gland that is located at the base of the horse's brain).

Clinical signs

Horses with PPID typically are older horses that present with pasture-associated or seasonal laminitis, hair coat abnormalities (such as delayed, incomplete, curling of, excessive growth, or lack of hair coat shedding upon the warmer months of the year), and generalized muscle wasting/weight loss. Affected horses often have a history of impaired or delayed healing of wounds, which is a result of immune suppression associated with the disease. They are prone to recurring abscesses, tooth infections, sinusitis, and higher intestinal parasite loads than normal horses. Upon progression of the disease, horses may also have muscle atrophy along their back and develop a 'potbellied' appearance.

Complications

• Chronic Laminitis: Horses with PPID have an increased risk of developing laminitis, especially during the spring when turned out to pasture with lush grass.
• Insulin resistance: Horses with PPID are often prone to developing insulin resistance, meaning that glucose is not absorbed from the bloodstream into the body's cells as it should, resulting in high levels of glucose in the blood.
• High parasite load: Horses with PPID are shown to have higher intestinal parasite loads than normal horses.
• Neurological signs: As the pituitary gland continues to grow, it can often exert pressure to other portions of the horse's brain, which can on rare occasion cause the onset of neurological signs.

PPID Vs Equine Metabolic Syndrome (EMS)
PPID and EMS are both very common endocrine disorders, affecting up to 30% of certain breeds of horses and ponies. However, they are very distinct diseases which require different approaches with respect to treatment. PPID usually affects older horses with excessive hair coats in contrast to EMS which most often affects younger-to-middle-aged animals with normal hair coats.

EMS Vs PPID

	PPID	EMS
Age of onset:	Middle aged to older; 85% are older than 15 years of age	Can occur at any age
Cause:	Increased activity of the anterior pituitary gland which results in overproduction of adrenocorticotropic hormone (ACTH) and other related hormones.	An adipose-derived endocrinopathy, in which abnormal or excessive adipose tissue produces a hormonal and/or inflammatory environment that alters insulin responses to feeding and the activity of insulin in peripheral tissues.
Hair coat:	Long, curly haircoat	Normal haircoat
Shedding pattern:	Delayed, incomplete or lack of haircoat shedding	Normal shedding pattern
Weight status:	Muscle wasting and weight loss	Obese easykeeper, with obvious fat deposits in the crest of the neck and tail-head.
History:	Episodes of pasture-associated or seasonal (spring/autumn) laminitis	Episodes of pasture-associated or seasonal (spring/autumn) laminitis. May also be insidious, awith abnormal growth rings on hooves or chronic laminitis changes observed in radiographs without any obvious clinical signs.
Predisposed breeds:	Ponies, Morgans	Ponies, Morgans, Arabians, Saddlebreds, Quarter horses, Tennessee Walking Horses, Paso Finos
Fat distribution:	Abnormal, 'regional adiposity'; in which fat deposits collect in the crest of the neck, prepuce, supraorbital fossae, tail head, and focal deposits in flanks.	Abnormal, 'regional adiposity'; in which fat deposits collect in the crest of the neck, prepuce, supraorbital fossae, tail head, and focal deposits in flanks.
Immunity changes:	Decreased w/ delayed healing of wounds. Prone to recurring sole abscesses, corneal ulceration, intestinal parasites, rain rot episodes, dermatophilosis, tooth root abscesses, etc.	No apparent changes in immunity or healing
Water intake/urination:	Excessive water drinking and urination	No changes in water consumption or urination

PPID and EMS share several similarities. Both are endocrine disorders. Both are associated with horses that have a history of acute or chronic laminitis, especially seasonal or pasture-associated. The onset of laminitis in both disorders has been related to underlying insulin dysregulation. On rare occasions, EMS and PPID can occur concurrently in horses. Hormonal testing is required to properly diagnosis both EMS and PPID.