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What Exactly is Cushing’s Disease in Horses?

Disclaimer: I AM NOT A VETERINARIAN.  I am just a Cushing’s horse owner doing research and writing a blog with the information I’ve read. Do not take any information in this blog post as veterinary advice. The content in this post is not intended to diagnose or treat any animal. Please consult with a trusted vet about your horse’s health. Your vet is the only one who can diagnose your horse with Cushing’s Disease.

What causes Cushing’s?

Equine Cushing’s disease is also known as Pituitary Pars Intermedia Dysfunction (PPID). Horses with PPID have an overproduction of hormones by the pars intermedia, an anatomic region of the pituitary gland. In normal horses, the cells in this portion have very little activity because they are inhibited by dopamine. As the horse ages, the level of dopamine decreases. This means the pars intermedia is no longer inhibited, and the cells there start secreting high levels of hormones. 

Unregulated, these hormones cause excess production of glucocorticoids, which are involved in carbohydrate, protein, and fat metabolism as well as are anti-inflammatory and immunosuppressive. 

 One of the hormones that overproduce in Cushing’s horses is important for stimulating the skin cells to be the proper color, but in excess, it can cause darkened skin and cause the hair to grow at inappropriate times. Another hormone stimulates the production of cortisol, which is important for proper metabolism, resisting stress, and fighting off minor illnesses. Too much cortisol results in a decreased immune response that leaves the horse prone to pneumonia and sinus, tooth, and hoof infections. 

Excess hormones also cause muscle weakness, new fat tissue on the neck, head, rump, and abdomen, and increased fat pads – the combination of which can lead to saggy abdominal muscles and a pot-belly appearance. Increased thirst, resulting in increased urine output, is another common side effect. Laminitis is also common as well. 

Cushing’s primarily is found in horses over the age of 10, though the average age of diagnosis is 19. A study at an equine retirement center found that 14% of the horses there had PPID. Ponies are more likely to be affected than horses, but mares and geldings are equally susceptible. The Morgan horse breed also seems to more commonly get Cushing’s. Some studies theorize that as many as half the population of horses aged 14 and older have Cushing’s.

A long, shaggy winter coat is the most recognizable sign of Cushings in horses.

 

What are the signs and symptoms of Cushing’s?

Signs of Cushing’s include increased coat length (which was the first sign that my horse, Glory, had), failure to shed out the winter coat in summer, weight loss, increased drinking and urination, lethargy, increased sweating, and laminitis. Horses with PPID are also more susceptible to infections such as sinusitis, skin infections, and parasites. All Cushing’s horses do not show all of these symptoms. Glory, for example, barely drinks or urinates, to the point where we’ve started feeding electrolytes to encourage her to drink more!

How is Cushing’s treated?

Cushing’s has no cure, but medication and management can relieve the clinical signs and side effects. The most commonly prescribed drug approach is a low-dose Pergolide treatment. Pergolide is given daily for the remainder of the PPID-affected animal’s life and is a dopamine replacement agent that is also used to treat Parkinson’s in humans. Studies of Pergolide show improvement in most areas for the majority of treated horses, but the long term efficacy is unknown.

Another drug recommended for PPID horses is cyproheptadine, which seems to be less effective than Pergolide. Cyproheptadine is an antihistamine used to inhibit the serotonin hormone that stimulates the pituitary gland. It is often used to supplement Pergolide. Studies show that only 25% of horses show a reduction of clinical signs, though cyproheptadine is more likely to show improvement in horses with laminitis than Pergolide.

A third drug is also being used to treat PPID but is currently only being used in Europe and Canada. Trilostane inhibits the release of cortisol from the adrenal gland, working further down the line after the hormones have already been released. This drug has been used in dogs with Cushing’s effectively and found to be fairly effective in horses. A study following horses on Trilostane for one to two years found that 81% of treated horses showed improvement in laminitis, and all horses had reductions in excess thirst, excess urination, and lethargy.

In addition to drug therapies, management changes also are important to keep the PPID horse healthy. Owners should maintain a regular vaccination and deworming program because of the reduced immune response. Sugar and starch can increase the risk of laminitis, so avoiding sugary treats is a must. Pasture grazing should be limited, especially early in the growing season when the pastures have high sugar content, and on fall mornings after an overnight freeze. 

A grazing muzzle can limit your horses ingestion of grass. Glory is modeling a Greenguard Grazing Muzzle in this photo.

 

Whether the PPID horse can or cannot have alfalfa hay is disputed. At one time it was thought that Cushing’s horses should absolutely not have alfalfa. I had a discussion with my own veterinarian about whether we could feed the horses alfalfa for the extra protein, and she said that it would be fine because it wasn’t that much higher in sugar than what we were feeding. You should, of course, discuss all dietary concerns with your own vet for your horse’s specific situation! 

Some veterinarians also recommend chromium supplements to improve insulin effectiveness. Vitamin E, C, and zinc can also supplement immune function. These supplements have not been proven to be effective in helping PPID, but they are probably not harmful. 

If left untreated, Cushing’s horses experience chronic bouts of disease, a decline in health and comfort, and reduced quality of life. Correctly managed horses can improve and generally can live a happy, healthy life and continue with many of their normal activities. 

When should I talk to my veterinarian?

The best thing is to catch the signs of Cushing’s early so treatment can start. Make sure to monitor your horse for signs of lethargy, increased drinking, a saggy midsection and sunken back, and changes to skin and coat. Many vets say that by the time most horses develop the long, shaggy coat that is most commonly associated with Cushing’s, the disease is fairly advanced. So look for the other signs and get your vet to check your horse as soon as you suspect a problem. 

Have you ever had a horse with Cushing’s? Tell me about them and what kind of treatment they had in the comments!

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