This condition also leads to reduced ability to fight infections, so older horses with ECD are prone to infectious diseases such as EPM, tooth root infections, sinusitis and common respiratory infections. There are special tests to support the diagnosis and specific treatments, so it is well worth having a workup for this problem. If untreated, horses with ECD get progressively thinner, dehydrate, founder or suffer chronic infections that don't respond well to antibiotics.
In general, it is the complexity of secondary problems that may become untreatable. For example, severe laminitis, pneumonia or severe sinusitis, which cause pain and interfere with breathing and eating, are serious and possibly irreversible. Weight loss may also be too rapid to reverse. In some horses, the pituitary gets so large it actually puts pressure on the hypothalamus, that part of the brain that regulates "flight or fight" responses, emotions, thermoregulation and other hormonal functions. The horse cannot maintain his body temperature or he has seizures.
Failure of The Immune System
One reason old horses do not respond to conventional treatments is failure of the immune system brought about by old age. Treatment of infections in older horses must be aggressive and often requires a combination of antibiotics, which can be expensive. While it's always worth treating an infection, a lack of response is a cardinal sign that the "back-stop" function of the immune system is failing.
Like people, older horses are prone cancer, which causes pain, loss of function and lack of response to conventional treatments. Old gray horses invariably develop melanoma of the skin. Other common cancers include lymphosarcoma, squamous cell carcinoma ("cancer eye"), and fibrosarcomas. At the present time, there are not many options for treating cancers in horses, since radiation and chemotherapy are extremely expensive or not feasible because of the size of the animals.
If the cancer is localized to one area, there are possibilities, although they may be expensive, of local treatment with cryotherapy, surgery, and/or local injection of chemotherapeutic agents. Most of the time, however, cancer is a life-threatening disease without many options. Tumors often cause extensive local damage making organs dysfunctional such as the loss of swallowing ability (mouth, tongue and esophageal tumors), interference with breathing (nasal, tracheal or lung tumors), digestion (intestinal tumors), urination (bladder, genital tumors), defecation (melanomas of the tail), or even walking (spinal tumors). If any of these areas are affected to the point where the horse is in constant pain, experiences serious weight loss or organ system failure, the condition may not be treatable.
Inefficient Intestinal System
Along with the relative decrease in the effectiveness of the aged horse's cheek teeth, the intestines of older horses appears to be less efficient at digesting their food. This is why some horses appear to be dropping weight slowly, despite feeding them more. Before assuming this is the problem, the intestinal absorption of the horse should be tested, and a complete blood chemistry panel and hormone analysis performed.
The problem of age-related poor intestinal digestion may not be hopeless, in that diets higher in proteins and calories can offset the eventual progression of weight loss. If there are other coexisting causes of weight loss (e.g. dental disease or kidney failure), this can be serious, because you just will not be able to get enough food into the horse to keep up. If despite your efforts to push the feed, the horse continues to lose weight, this could become very serious once the horse is ribby, especially for maintaining body temperature in the winter, and fighting off any stresses or infections that are encountered from time to time.
Arthritis and Navicular Disease
Pain and function (mobility) are the big issues that determine quality of life. Although certain joints sustain the greatest wear and tear throughout the life of a horse, it is not always these "obvious" sore joints that act up later in life.
Horses with arthritis are generally stiff, especially in the morning after they have lain motionless for a period of time. The problem gets even worse in cold or wet weather. The stiffness worsens over time, and eventually the horse is so painful that he doesn't want to move, and may even groan. Arthritis in the legs is more obvious, in that the horse's mobility has declined, and there is a progressive "crouching" stance or actual limping. However, neck arthritis can also be a big problem, although more subtle. In this case, the horse exhibits difficulty reaching the ground. Pain and loss of function (immobility) add up in the arthritic horse.