By Jill Beech, DVM, DACVIM and Lydia Gray, DVM
Fortunately for our nation's horse population, interest in equine geriatrics among veterinarians and researchers has been increasing. As a result, it has been found that not only do senior horses have different preventive care needs, but also that certain disease conditions become more common as horses age. Many of the conditions we associate with "old age" in the horse--like lameness, weight loss, or poor shedding--actually might be preventable or treatable.
Normal "wear and tear" plus any of these practices--long years of an irregular exercise program, poor riding, poor footing, and poor foot care--can set a horse up for lameness. One common cause of lameness in the senior horse, degenerative joint disease (DJD) or arthritis, may cause pain which not only affects the horse's gait, but also makes proper foot care difficult. If a horse is unable to stand comfortably or bend its legs for hoof trimming or shoeing, inadequate foot care can result, making the original lameness worse. Some senior horses require pain-killers (analgesics) before their farrier's appointment to make the procedure easier. Also, owners must remember that just because a horse is not being ridden, his or her feet cannot be ignored; they still require regular attention and trimming.
Horses experiencing pain from DJD may require analgesics, but treatment should be under the advice of a veterinarian. Various different non-steroidal medications may be used (most commonly phenylbutazone), but these may have serious side effects when given at high doses and/or over a long period of time. Stomach and colon ulcers as well as kidney damage are potential side effects that can be fatal. Various other substances have been professed to benefit horses with arthritis, but there is little scientific proof of their effectiveness. Acupuncture is also used with anecdotal reports of success. Older horses with endocrine disorders and altered immune function appear prone to laminitis and foot abscesses. These conditions especially require the combined expertise and skills of a veterinarian and a farrier.
Owners of older horses might find that it is becoming harder and harder for their horse to maintain their weight. Although this condition can have many causes, inadequate nutrition and poor teeth are probably the most common. In general, as horses become aged, they require foodstuffs that are calorie-rich, easily chewed and digested, and contain additional vitamins. Their mineral intake should be balanced. As horses' teeth grow out with age, they can become loose or diseased, making effective chewing not only impossible, but painful. Tooth root abscess, which can lead to sinus infections, can produce a foul odor from the horse's mouth or nostrils. If this occurs, a veterinarian should be contacted. After about the age of 15 years, most horses should have their teeth checked by a veterinarian at least every six months to help prevent such conditions. Also, just because a horse is advanced in years doesn't mean he or she shouldn't still be part of a well-rounded deworming program.
Although aged horses tend to develop adenomas (benign tumors) of the thyroid gland, these are nearly always nonfunctional, requiring no attention unless their size causes problems, such as with swallowing.
Probably the most common endocrine disorder affecting the senior horse is pituitary pars intermedia dysfunction or PPID due to hyperplasia (increased cell growth) of part of the pituitary gland. This is sometimes referred to as a pituitary tumor or the clinical condition is called "Cushing's Disease." Despite the fact that not every horse with this condition will exhibit the "classic" signs of the disease, certain changes are considered to be hallmarks:
· long hair, which can sometimes be quite shaggy or wavy, has commonly been referred to as hirsute, although in human medicine, this term has a more specific definition, and would be accurate if simply describing hair change
· muscle wasting
· a "pot belly"
· cresty neck with or without other areas of abnormal fat deposition
· puffiness above the eyes
· laminitis or founder
· sometimes bouts of patchy sweating
· increased susceptibility to infections
The "classic" case probably does not require testing to confirm pituitary dysfunction, but the less obvious cases do. Various blood tests may be performed but routine hematology and serum biochemistry panels are not diagnostic of the condition. Because of variability in serum or plasma concentrations of ACTH and alpha-MSH when they are above reference range, single baseline measurements may not be very useful in the diagnosis of this disease unless a value is extremely elevated ; it is advisable to obtain more than one blood sample for testing. Plasma concentrations of ACTH or alpha-MSH that are consistently increased above reference range during months other than mid-August through October are strong evidence that a horse has PPID. Season influences hormone concentrations (ACTH and alpha-MSH), and normal horses and ponies sometimes have high concentrations during mid Aug through October, making it preferable to test during other times of the year when possible. However, normal basal levels of these hormones do not rule out PPID, and in these cases, it is helpful to have your veterinarian perform stimulation tests to determine whether there is an abnormal rise in ACTH or alpha- MSH. ACTH is currently the most frequently measured hormone as there is no commercial laboratory assaying equine alpha-MSH. One should always enquire whether the laboratory assaying your horse’s hormones has good quality control and tests which are validated for horses. .
For many years the dexamethasone suppression test--where the horse's cortisol level is measured before and after giving this man-made steroid—has been considered the gold standard for making the diagnosis, but the test is not consistently abnormal in horses with PPID, and season can also influence results. Occasional normal horses can have abnormal responses during the fall months.
Measuring the horse’s baseline cortisol levels is NOThelpful as horses with PPID often have low or normal, not increased, concentrations. Also, obtaining a blood sample in the morning and one in the afternoon or evening to evaluate whether there is daily variation of the cortisol concentration is not reliable as many factors can influence the levels. Failure to detect a difference between morning and afternoon or evening levels does not mean the horse has PPID.
Although there are no controlled trials, pergolide appears to be the best treatment, although not all animals respond to the same dosage. Excellent husbandry, including frequent teeth care is very important. These horses also may require more aggressive parasite prevention programmes.
As horses age, other diseases and conditions become more common. Neoplasia or various types of cancer can affect older horses. Clinical signs, diagnosis and treatment depend upon the organ affected. Unless there is one, single, accessible tumor that can be removed, there might not be an effective treatment.
Senior horses also are more prone to certain types of colic than young horses. Obviously, poorly chewed food can be a problem. Lipomas tend to occur in middle-aged and older horses and can strangulate intestine, requiring surgery.
As horses age, they often develop cataracts and/or floating material in the fluid of the eye. Although these can result in impaired vision, horses seem to function quite well even with major changes in their eyes.
Although there are other conditions that affect the older horse, this overview covers the more common ones. Good, consistent care and an appropriate preventive health care program can do a great deal toward maintaining the well-being of our aged equine companions.
Lydia Gray, DVM, in conjunction with Jill Beech, DVM, Dipl. ACVIM, New Bolton Center, University of Pennsylvania College of Veterinary Medicine and AAEP member.