Healthy Horses: Endocrine System Function and Flaws
Horses suffer from few endocrine disorders compared to humans and companion animals; however, they're not totally off the hook, explained Emily Graves, VMD, MS, Dipl. ACVIM, of Michigan State University, to horse owners in attendance at the Healthy Horses Workshop held in Ft. Collins, Colo., on July 28.
The endocrine system is a very complex system that consists of the hypothalamus, pituitary, thyroid, parathyroid, adrenal glands, pineal body, pancreas, and reproductive organs.
"The most common endocrine disorders dealt with today by equine practitioners and owners are pituitary pars intermedia dysfunction (PPID or Cushing's), equine metabolic syndrome (EMS), insulin resistance (IR), and suspected hypothyroidism," Graves said. "While other diseases are known to occur in equid species, such as diabetes insipidus and hyperthyroidism, they are extremely rare."
Pituitary Pars Intermedia Dysfunction
Although it is often referred to as Cushing's syndrome (a term borrowed from human medicine), PPID is the preferred label because the disease effects a different portion of the pituitary gland than its human counterpart.
"The pituitary gland dysfunction in equids stems from overgrowth, meaning hyperplasia or tumor, of certain cell types within the gland's par intermedia," Graves explained. "These cells are hyperactive or present in high numbers, and thus lead to production of abnormally high levels of many pituitary hormones (such as andrenocorticotropic hormone or ACTH)."
Common clinical signs are varied, but the most common by far is the failure to shed a haircoat fully. Graves said other signs include:
- Long, sometimes curly haircoat;
- Chronic infections;
- Repeated laminitis episodes, sometime associated with hoof abscesses;
- Excess or inappropriate sweating;
- Increased water consumption and urination;
- Loss of muscle mass (if a horse is left untreated) typically of the back and hindquarters;
- "Pot-bellied" appearance;
- Infertility and lack of estrous cycles; and
- Abnormal mammary gland development.
Lab results for PPID horses usually show low lymphocytes, an increased neutrophil count, and intermittent high blood sugar.
"The veterinary community has not yet determined the cause of, and effect for, every PPID sign," Graves said. "One understood consequence is the effect of long-term high blood cortisol concentrations."
Cortisol is known as a "stress" hormone that can cause immune system suppression over time, which can lead to chronic infections.
There is no cure for PPID, but most of the problems associated with clinical signs can be managed by frequently changing bedding or constant turnout (to avoid wetness in the stall from excessive urination), body clipping long-haired animals, bathing frequently (especially in hot weather conditions), constantly monitoring for signs of infection, and treating any infection appropriately.
However, the most damaging of PPID-associated problems is laminitis. Unlike acute laminitis, PPID-related laminitis occurs gradually over a period of time, making it hard to recognize in the early stages.
"This disease is thought to be related to cortisol and other hormonal effects on glucose action in the tissues of the hoof wall," Graves explained.
If a horse is diagnosed with PPID-related laminitis, a collaborative effort between the veterinarian and farrier is needed to correct or minimize the damage to the foot structure.
PPID Treatment Options
While there is no cure for PPID, several medications are commercially marketed to treat the disease. These include:
Pergolide This is the most commonly used drug. Studies have shown that this once-daily treatment improved hormone test levels and owner-perceived improvement in the horse. The horse community held its breath earlier this year when the Federal Drug Administration discontinued the use of pergolide for human use. Without the demand from the human market, it was unclear if the drug would be available for equine use. Fortunately, the combined efforts of veterinary groups, horse owners, and the FDA allowed the drug to be imported in bulk for compounding by pharmacies.
Cyproheptadine This is another widely used drug that has similar properties to pergolide. It has sometimes been used in conjunction with pergolide when horses have shown minimal response to pergolide alone. "Although the combination of the two drugs is more expensive, this combination can be a very useful treatment for some equine patients," Graves explained.
Bromocriptine Bromocriptine is another drug that mimics pergolide; however, it is rarely used as it has been shown to be poorly absorbed into the system and has been associated with severe side effects.
Trilostane Although not readily available in the United States, Trilostane has been used overseas for many years, with some success in reversing clinical signs in PPID-affected horses.
Chasteberry "In the herbal remedy field, some veterinarians and owners have reported positive results following use of chasteberry supplements," Graves said. "Anecdotal reports exist that chasteberry is helpful to PPID horse. However, a comparison study concluded that an herbal product containing chasteberry had no beneficial effect on horses with PPID compared to a group treated with pergolide."
When it comes to PPID, veterinarians aim to reduce the clinical signs and bring the horse back to normal cortisol production. Early recognition is important to offer the best quality of life.
EMS and Insulin Resistance
Equine metabolic syndrome (EMS) shares many characteristics with PPID, according to Graves. "Both of these disorders alter cortisol metabolism," she explained. "It is thought that EMS results from excess production of active cortisol, primarily in fat cells or adipose tissue. The pituitary gland functions normally in patients with this disorder."
The underlying causes of the disorders might be different, but they can present in similar fashion (abnormal fat deposits along the crest, over the tail head, and geldings' sheaths, as well as laminitis).
A diagnostic criterion for EMS includes:
- Insulin resistance (IR);
- History of or active laminitis; and
- Excessive fat deposits in typical regions.
Insulin resistance describes a condition in which the body fails to appropriately respond to insulin. "In a classic scenario, the individual has both abnormally high blood sugar and blood insulin concentrations," Graves said. "Once the pathologic state of IR develops, poor utilization of glucose from the diet and intermittent high blood sugar occurs (similar to Type 2 diabetes mellitus in humans)."
High-carbohydrate diets can exacerbate this state by increasing insulin production when eaten.
Management of EMS horses focuses mostly on reducing the incidence and effects of laminitis as well as adjusting the horse's diet to include low-carbohydrate grains and hays.
"An important recommendation is to feed grass hay or other feed sources that are low in water-soluble carbohydrates or non-structural carbohydrates (NSC)," suggested graves. "NSC content below 12% is suggested for IR horses and ponies, in both EMS and PPID patients."
Vegetable oil, rice bran, or another high-fat source can be added to the diet instead of concentrated grains or feeds containing high-levels of carbohydrates. Adding oral antioxidants, such as vitamin E, has also been recommended. Graves urged owners to discuss treatment options with their veterinarians before implementing any treatment program.
Horses might not have endocrine disorders frequently, but that doesn't mean they don't have them at all. Researchers are working to decipher what causes these disorders and the best way to treat them.
About the Author
Chad Mendell is the former Managing Editor for TheHorse.com .