Cushing’s Disease in Horses

What Is Cushing's Disease in Horses?

Also known as pituitary pars intermedia dysfunction (PPID), Cushing’s disease is the most common endocrine disorder diagnosed in horses. Although Cushing’s disease in horses carries the same name as an similar disorder that affects people and dogs, the disease process is different because the equine version affects a different aspect of the pituitary gland. Endocrine disorders are those that affect the production of hormones.

All breeds of horses can be diagnosed with Cushing’s disease, but ponies and Morgan breeds may be predisposed. 

Symptoms of Cushing's Disease in Horses

Cushing’s disease is a progressive disorder. The symptoms may be slow to present but will ultimately progress with time.

The most common signs observed in horses with Cushing’s disease/PPID include: 

• Failure to shed winter coat fully, or patchy shedding

• Long and/or curly coat

• Chronic infections

• Laminitis or multiple laminitis episodes

• Hoof abscess or recurrent hoof abscess

• Excessive or inappropriate sweating

• Increased water intake and urination

• Lethargy

• Weight loss

• Accumulation of fat in abnormal areas

• Loss of muscle mass (especially over the back and hind quarters)

• “Pot-bellied” appearance 

• Infertility or lack of normal estrus cycles in a mare 

• Abnormal mammary gland development 

• Suspensory ligament degeneration 

• Recurrent corneal (eye) ulcers

Causes of Cushing's Disease in Horses

PPID in horses is a progressive, degenerative disease of the hypothalamus. The hypothalamus is a structure in the brain that controls the production of hormones through communication with the pituitary gland. In a healthy horse, the endocrine system is a balanced network of structures and organs that work together in a series of feedback loops to produce and regulate hormone levels throughout the body.

In horses with PPID, loss of function in the hypothalamus leads to problems with a region of the pituitary gland called the pars intermedia (PI). The pituitary gland is located at the base of the brain. Normally, the pituitary gland makes hormones that help regulate normal body functions. PPID occurs when there is an abnormal growth (benign tumor) of the PI portion of the pituitary gland.

As the tumor grows, it causes the PI portion of the gland to expand. In turn, this causes an increase in production of a hormone called adrenocorticotropic hormone (ACTH), which then stimulates the adrenal gland to produce more of the hormone cortisol.

Cortisol is known as the “stress hormone” and controls many important bodily functions such as blood sugar levels. Excessive and consistent elevated levels of cortisol can lead to problems in horses such as increased susceptibility to infection, insulin resistance, laminitis, and muscle wasting. 

How Veterinarians Diagnose Cushing's Disease in Horses

The veterinarian will diagnose PPID through a combination of your horse’s medical history, a physical exam, and diagnostic tests. There are two main diagnostics tests for PPID: a baseline plasma ACTH concentration, and a thyrotropin releasing hormone (TRH) stimulation test.

The baseline ACTH test is a simple blood sample that your veterinarian will send to a lab. The TRH stimulation test is also a blood sample but it requires two blood draws: a baseline sample and a second sample taken after a hormone called thyrotropin is given to your horse intravenously. The second blood sample is collected 10 minutes after the thryotropin injection. Both samples will then be sent to the lab to compare the ACTH levels in each.

For early signs of PPID, TRH stimulation testing is recommended as the first test because baseline ACTH levels may be falsely negative in the early stages of PPID. In the later stages of PPID, or in horses with numerous symptoms, baseline plasma ACTH level testing may be enough to make a diagnosis. Your veterinarian may require your horse to be fasted for 12 hours prior to the blood draw.  

Other tests that may be recommended include overnight dexamethasone suppression testing and magnetic resonance imaging (MRI) specific for pars intermedia enlargement. Oral domperidone challenge testing and a combined dexamethason suppression/TRH simulation with cortisol measurement are no longer recommended. 

Treatment of Cushing's Disease in Horses

The first-line treatment for Cushing’s disease is an FDA-approved tablet called Prascend® (pergolide). Some horses may need to be started on Prascend® gradually as it can cause a decrease appetite.

Your veterinarian will likely recommend retesting the horse's ACTH levels once the medication is given for 1-3 months, and then every 6-12 months thereafter. Routine blood work monitoring is important to make sure the Prascend® dose is effective and clinical disease is not worsening.

Recovery and Management of Cushing's Disease in Horses

After a horse is diagnosed with PPID and started on Prascend®, it is very important that it is given regularly. In addition to medical treatment with Prascend® for PPID in horses, routine wellness care is of great importance and should include:

• Maintaining ideal body weight

• Routine hoof care

• Feed of low sugar diet (grain and hay included)

• Restricting grass intake (especially in the spring and fall when the sugar in grass is at the highest level)

• Routine dental care 

• Routine parasite control

Because horses with PPID can have a weakened immune system, it can predispose them to a number of conditions:

• Laminitis 

• Hoof abscesses

• Corneal ulcers

• Chronic infections 

If you or your veterinarian notices that your horse is having recurrent medical issues, then it may be recommended to perform diagnostic testing for Cushing’s disease.