What is Cushing's Disease in Horses

Cushing’s disease in horses can occur when a tumor develops on the pituitary gland. This tumor, called an “adenoma,” causes the pituitary gland to send an excessive load of hormones throughout the body of the horse. That includes cortisol, which is a hormone that is directly associated with stress. As the tumor grows, higher levels of hormones are created and pushed throughout the body of the horse.

It begins when the hypothalamic neurons begin to degenerate. This causes them to produce lower levels of dopamine, which begins to eliminate the restraints that are placed on pituitary secretion. 

Both humans and canines can experience Cushing’s disease as well. It is not an infectious disease. To distinguish the equine version of Cushing’s from the canine and human versions, it is often diagnosed today as PPID, which stands for “pituitary pars intermedia dysfunction. How PPID works in horses is actually closer to Parkinson’s disease in humans than the disease to which each shares a name. 

In horses, Cushing’s disease is usually found in mature horses that are at least 7 years old, but it can affect any horse at any age. Although there are not specific breeds that have a higher risk of suffering from an adenoma, pony breeds do tend to have more cases of Cushing’s disease than other breeds.

What Are the Symptoms of Cushing’s Disease in Horses?

According to Equus Magazine, pituitary dysfunction may affect elderly horses at a high rate. In a survey of 218 elderly horse owners, 3 out of 10 horses displayed the symptoms of Cushing’s disease, but only 8% of the horses had received a formal diagnosis. 

A 2003 Tufts University study of nearly 500 horses above the age of 20 found that Cushing’s disease was the most common specific health diagnosis shared by the group. 

The symptoms of Cushing’s disease can be slow to develop. They can also be unique to the individual horse, depending on the excessive load of hormones being produced. Once an adenoma is present, however, the signs and symptoms of the disease are progressive.

These are the most common symptoms that horses experience when the adenoma begins to grow on the pituitary gland.

    Changes in Body Shape. Horses with Cushing’s disease tend to develop large deposits of fat along their mane. Many will develop a pot-belly shape over time. It is common for muscle wasting to occur as well, but this may happen at any point of the body instead of one specific spot on the horse.
•    Changes to the Coat. Horses that have an adenoma will experience coat changes. The hairs become longer and the coat becomes thicker. The horse may experience year-long shedding, especially in warmer climates. This typically makes the horse appear to be shaggy and some horses may begin to resemble Curly horses. 
•    Excessive Thirst. Horses with Cushing’s disease tend to experience thirst frequently. They will head to the water trough or their water access point numerous times throughout the day. Because they are drinking an excessive amount of water, horses with this health issue tend to experience excessive urination as well.

Additional symptoms of Cushing’s disease may include weight loss, inflammation of the hoof structures, and ulcers in the mouth. Horses with this disease are also more prone to infection and take longer to heal. Owners or handlers may see cuts, scrapes, or bruises on the horse that take longer to heal compared to other horses in the herd.

Sporting horses with Cushing’s disease may experience a decreased level of athletic performance. Some horses may experience increased sweating because of the adenoma. Changes to the reproductive cycle may occur in mares and stallions with Cushing’s have been known to become infertile. Certain neurological deficits may develop and, over time, blindness may occur.

How is Cushing’s Disease Diagnosed?

Although the symptoms of Cushing’s disease are indicative of the presence of an adenoma, there are other health issues that can cause similar symptoms in horses. If Cushing’s disease is suspected, a veterinarian must example the horse and complete a full blood profile to rule out any other potential cause. This includes a complete blood count and summary of serum chemistry. 

Once completed, another set of blood tests can help to formally diagnose the disease and begin the creation of a treatment plan for the horse. These tests may include a resting ACTH blood test. A TRH Stimulation Test may also be performed. 

How Is Cushing’s Disease Treated in Horses? 

No definitive treatment current exists for Cushing’s disease in horses, but it can be controlled effectively. One of the most common treatments is a medication called Pergolide, which is given to the horse orally in doses of up to 5mg. Bromocriptine is another medication commonly given to horses if Pergolide does not work. Cyproheptadine is a third prescription option and it was the most common treatment until Pergolide was developed.

Some horses may require two or all three of these medications to stabilize their hormone levels.

Additional management practices must be included with a Cushing’s treatment plan as well. Horses with this disease are very prone to laminitis, so there must be regular visits to a professional farrier to maintain hoof health. Horses with this condition should have limited access to a lush pasture as well to avoid dietary issues.

Cushing’s disease reduces the effectiveness of a horse’s immune system, even when being treated. Any wounds the horse suffers, including minor scrapes and scratches, must be properly cleaned immediately.

Some veterinarians may recommend certain supplements or specific ingredients to benefit the health of the horse too. Antioxidants are a common item that is recommended, especially plant adaptogens, as they support the immune system and can help the horse be able to fight oxidative stress.

These supplements can also be useful in the management of Cushing’s disease in horses.

•    Omega-3s. These healthy fats can help the horse’s internal systems provide a normal response to inflammation.
•    Chaste Berries. This supplement provides an assist to the endocrine functionality of the horse.
•    Amino Acids. Supplements like threonine, methionine, and lysine help to support the development of lean muscle mass for the horse.

Some horses may also benefit from selenium being added to their feed. They normally obtain the selenium they need from foraging. It prevents cellular damage, but if given in high doses, it can be toxic. Before adding this supplement, it is necessary to test soil and forage selenium levels and to have it measured in the bloodwork of the horse. 

Before giving any horse a supplement, it is important to discuss the overall treatment plan for Cushing’s disease with a veterinarian. Certain supplements may interfere with the functionality of prescription medication.

Dietary Needs of a Horse with Cushing’s Disease

Horses with Cushing’s disease, especially elderly horses, have many special needs that must be met. Routine activities must include vaccinations, dental health, daily hoof care, and deworming. Other preventative procedures may be necessary as well, depending upon the overall health and medical history of the horse.

Horses with Cushing’s disease tend to have specific dietary needs that must be met as part of the preventative routine. Horses with an adenoma may have metabolism and insulin issues that are similar to diabetes mellitus. For that reason, traditional grains and horse treats may be restricted from the diet of the horse. Even pasture may be restricted in some instances. A diet that is high in starch and sugar could negatively impact the health of the horse.

Alternative feed options may include beet pulp that is free from molasses, pellets, or a low-carb feed that is specifically made for senior horses. Avoid alfalfa and clover and stick with grass hays since they contain lower levels of carbohydrates. 

Horses with Cushing’s often need a specific high fiber, low sugar feed. Mineral and vitamin supplements are often added to this type of feed, so speak with a veterinarian about what will benefit the horse specifically. Some horses may need fats added to their recommended feed for weight support.

Providing a Good Life to a Horse with Cushing’s Disease

Simple things can make life better for horses with Cushing’s disease. One of the easiest things that owners or handlers can do is keep the coat of the horse trimmed. This will prevent the horse from overheating in the warmer months and can prevent cases of rain rot from happening in humid climates.

Parasite control is a must for the ongoing treatment plan as well. Horses with PPID are extremely prone to internal parasites. Periodic fecal egg counts and an individualized deworming program are usually part of the treatment process.

A full panel of routine bloodwork is usually part of the treatment plan too. That allows the medical history of the horse to have reference values so that abnormal findings can be recognized and treated immediately. 

Cushing’s disease in horses may not have a cure, but it can be effectively managed. With a little time and work, even horses with this diagnosis can still live fulfilling lives.