Cushing's disease is one of the few disease found in horses that is very rarely seen in horses under twenty years of age. Any breed of horse can develop Cushing's disease however it is most prone in Morgan horses and crosses with Morgans. Mares, geldings and stallions are equally at risk for developing the condition and both working horses as well as pleasure horses seem have about the same odds for Cushing's disease.
The first signs of Cushing's disease are usually noted in the coat. The horse will retain or develop a thick, winter like coat that does not shed, even in the warm summer months. Typically the long, thick hair will be curly or wavy in appearance and will be dull and much longer than the winter hair the horse would have normally grown.
If the condition is left without treatment the symptoms will become more pronounced and will expand to include excessive drinking and urination, a significant sway or dip in the back, loss of muscle mass and development along the back and rump as well as cuts and lesions that don't heal and laminitis and infections in the hoof that reoccur. Often the amount of water consumption will almost triple to up to 80 liters of water a day, while an average horse will drink about 30 liters per day.
Cushing's disease is caused by an imbalance in the hormones produced by the pituitary gland. The imbalance itself is due to a benign tumor in the pituitary that prevents the functioning of the gland and the production of the endocrine hormones needed in overall body functioning. As the endocrine problem becomes more pronounced and continues over time, more severe symptoms are noted. These symptoms are due to an overproduction of cortisol by the adrenal glands in response to the malfunctioning of the pituitary gland. Since the whole metabolism of the horse is based on a series of checks and balances, once one system is affected it impacts all the other hormonal and metabolic reactions. Horses with Cushing's disease will have less immunity and will be more likely to develop respiratory infections and other health problems as they are unable to fight off even minor infections.
Treatment for Cushing's disease can be effective and provides the best results when started early in the disease's progression rather than later. In older horses that are started on medications such as serotonin blockers and vasoconstrictors results will be good provided they are not suffering from chronic laminitis and hoof infections, which may not be helped by the medications. These medications will help with the physiological issues but will not actually address the tumor, it will continue to grow as there is currently no way to remove pituitary tumors in horses. Horses that are diagnosed and treated early in the disease may live years longer while horses that are not diagnosed until late in the disease may not benefit from treatment at all.
Some vets are now recommending an herbal treatment that includes adding crushed chaste berry to the feed to help regulate hormones in the body. There are both positive and neutral research findings on this supplement, but it is worth discussing with your vet if the horse is in the very early stages of Cushing's disease.