Equine Endocrine
Diseases
"Cushing's" and "metabolic disease" are terms that veterinarians and horse owners use commonly. But not everyone is familiar with the actual conditions to which these terms refer. Both are endocrine diseases that occur commonly in middle aged to older horses. Endocrine tissues of the body are those that secrete hormones or other chemicals that affect the metabolic processes of the body. Tissues classified as endocrine include the pituitary gland, pancreas, adrenal glands, thyroid gland, and parathyroid glands, among others. In very simplified terms, an endocrine disease is a condition that affects the ability of an endocrine organ to secrete the necessary hormone.
Cushing's Disease/PPID
"Cushing's" is a term used synonymously with Pituitary Pars Intermedia Dysfunction (PPID), which is more correct terminology. Originally, this disease was termed because of its similarities to Cushing's disease in humans, however equine PPID affects a different part of the brain than human Cushing's. The normal pituitary gland secretes hormones and other chemicals that regulate the body's everyday processes. Some of the chemicals released include oxytocin, dopamine, and melatonin. Dopamine plays a critical role in equine PPID. Normally, dopamine prevents the release of certain chemicals called POMC's* from the horse's pituitary gland, which is a process called negative feedback. In a horse with PPID, there is excessive growth of a specific portion of the pituitary gland causing a tumor. Dopamine release is decreased and the negative feedback does not occur, which allows the POMC's to increase leading to increased levels of the stress hormone cortisol. (*POMC's are proopiomelanocortins which are released by cells in the pituitary gland called melanotrophs. POMC's are broken down to different substances that are used by the body, including ACTH, β-endorphins and others.)
Equine Metabolic Syndrome (EMS)
Equine Metabolic Syndrome or EMS is a condition in horses that is characterized by obesity or regional fat deposition, insulin resistance, and laminitis. The definition of obesity is an increase in body weight because of excessive fat accumulation within the body. In a body condition scoring system, obesity would be defined as a 7, 8, or 9 on a scale of 1 to 9. An ideal body condition score is 5 out of 9. It is important for horse owners to know the body condition scoring system and be able to score their horses to help maintain a healthy body weight. Regional fat deposition occurs most commonly in the crest of the neck and tailhead, but is also seen in the sheath, above the eyes, and as large lumps along the horse's sides.
Laminitis can be either clinical, where the horse shows obvious lameness and coffin bone rotation, or subclinical, where the horse has only subtle signs such as abnormal growth rings ("founder lines") on the hoof wall.
Insulin resistance and obesity predispose horses to laminitis, and it must be controlled with weight loss and strict dietary restrictions. The insulin resistance that is associated with EMS is similar to type 2 diabetes in people. However, the high blood glucose that is found in diabetic people does not typically occur in horses and diabetes is extremely rare in horses.
The classic EMS horse is described as being an "easy keeper" that does not require much food and remains overweight. Onset of EMS is usually in adult horses over 5 years old. Breeds commonly diagnosed with EMS are those that are predisposed to obesity and insulin resistance, including all pony breeds, Morgans, Paso Finos and Arabians. EMS has been diagnosed in other "easy keeper" type breeds, but is rare in breeds that tend to be leaner, including Thoroughbreds and Standardbreds.
Diagnosis
The easiest test to perform is resting insulin concentration in the blood. Samples should be taken after horses have been off pasture and grain for 12 hours, as these feeds can cause an elevation in insulin. High resting insulin is sufficient evidence to diagnose EMS. High blood glucose concentration after fasting is also indicative of EMS, but it is not as commonly seen as increased insulin concentration. ACTH concentrations will not be elevated as they are in PPID horses.
Glucose and insulin tolerance tests can also be performed on suspected EMS horses. For a glucose tolerance, blood glucose is measured, then glucose is administered intravenously. The horse's blood glucose is continually measured at regular intervals over about 1-2 hours. A normal horse will have a spike in blood sugar soon after the glucose was administered, but it will decrease gradually. A horse with EMS will have a blood glucose that does not decrease as rapidly, which indicates insulin resistance. The insulin challenge is run in a similar manner. Blood glucose is measured and then a dose of insulin is given to the horse. The blood glucose is monitored at regular intervals and in a normal horse it will decrease rapidly after insulin is given and then increase gradually back to normal. Horse with EMS will show some level of insulin resistance and the decrease in blood glucose will not be as rapid. The insulin and glucose tests can also be run simultaneously.
How are EMS and PPID different when they sound so similar?
Both diseases have similar characteristics, but as described earlier, they have different causes. Certain traits are not consistent between the two diseases. For instance, insulin resistance is a primary component of EMS, but is not always present in PPID horses. ACTH is elevated in horses with PPID, but not in EMS. PPID and EMS affected horses can have a similar appearance, but the status of the endocrine organs in each horse can be different. They also require similar management practices, but PPID has a specific drug treatment, whereas there is not a specific drug used to treat EMS.
