Cushing's disease is a fairly common disease in older horses. It is characterized by a long hair coat that does not shed in the summer, laminitis or founder of the feet, and (usually) a crusty neck and large fat deposits around the tail head, as well as other symptoms. This disease is related to a growth on or increased size of the pituitary gland, which causes an increased production of various hormones.
The major concern with these horses is that founder can occur, as that can be deadly. However, a recent paper in the Journal of Equine Veterinary Science indicates this disease affects many other organs as well. One interesting finding is the increase in lipofuscin. Lipofuscin is an aging pigment that is an indicator of free radical injury; it accumulates in the nervous tissue of horses with Cushing's disease and even in some normal horses as they age. However, the amount is greater in horses with Cushing's disease, which can lead to degeneration of nervous tissue. The liver in these horses was also affected, so liver function could be decreased. An important finding was a lung inflammation called bronchiolitis in horses with Cushing's disease. This inflammation makes the horses more susceptible to lung infections, and I have seen this in several horses in our practice that have recurrent infections. The heart of these horses was also affected with lipofuscin, indicating free radical damage that can lead to fibrosis or scarring. Along with treating these horses with pergolide, it is possible we should be treating with some type of antioxidative drug to prevent damage to other organs.