The Aging Equine
We've all heard the statistics about aging America--as the population of baby boomers rounds the curve of 40, the average age of our society is inching its way up. In recent years, horses have experienced a similar population shift. While 100 years ago they were primarily beasts of burden, seldom kept around once their most productive years were over, now our horses are not only work and athletic partners, but are our companions. It has become increasingly important to us to preserve their good health through their "golden years" and to extend their lifespans as long as they can enjoy a "quality" life. Modern veterinary science has provided a vast array of knowledge and techniques that can assist us.
As horses get older, they experience a number of gradual, irreversible changes.
A large proportion of the equine population now remains active, to one degree or another, into their 20s and 30s, and their contributions to us are invaluable. Not only are older horses generally calmer than their younger brethren, their experience, education, and wisdom make them ideal teachers and wonderful friends. Ask anyone who owns an older horse; he or she likely will tell you he's worth his weight in gold.
Horses do age differently than humans. By comparison with us, they have quite abbreviated childhoods, lengthy and active adulthoods, then decline rapidly to succumb to some brief terminal illness. The average lifespan of a horse is said to be about 24 years; but as with humans, a horse's chronological age isn't always a good indicator of how old he really is. Some horses still are active at the age of 35, while others suffer significant signs of aging at 15.
Genetics plays a role--most of us know that ponies, on the whole, are longer-lived than horses, and that certain breeds (such as the Arabian) have a reputation for longevity. But good management is important in determining a horse's lifespan. Those horses which have had the benefit of good preventative health care--including vaccinations, dental care, good nutrition, and in particular, a rigorous parasite control program--throughout their lives, reap the benefits in old age. (Many researchers suspect that modern deworming drugs have probably contributed more to the horse's increased lifespan than any other single factor.)
That's not to say that all older horses remain problem-free where their health is concerned. Researchers have found that more than 70% of horses over the age of 20 have some sort of age-associated condition that requires special care. And indeed, it's quite reasonable to assume that your older horse is likely to suffer from some of the same bothersome symptoms that we aging humans experience. But we're really just beginning to understand all of the physiological changes involved in the equine aging process.
Until quite recently, research into geriatric equines was virtually non-existent, but a team led by Karyn Malinowski, PhD, at Rutgers University in New Jersey, has now spent several years investigating older equines with the help of a herd of 20-something ex-broodmares (mostly Standardbreds) maintained at the university.
"The focus is to improve the well-being of the geriatric horse," she says.
Malinowski notes that her research has important implications not only for helping horses lead longer and healthier lives, but also for providing information about the aging process in humans. Horses, as it turns out, provide a good model for the changes Homo sapiens experience, because their aging process is only about three times as fast as our own, and because like us, they tend to exercise voluntarily, and they sweat. Lab animals such as rats, on which aging research traditionally has been based, are poorer models because, says Malinowski, "they hate to exercise, and they age about 30 times faster than we do." The link between aging humans and aging horses might mean that we can look forward to a great deal more information to evolve from her research in the near future.
When Horses Age
As horses get older, they experience a number of gradual, irreversible changes. On the outside, the weakening of collagen, the body's protein "scaffolding," contributes to sagging skin and that characteristic loose lower lip. The downward migration of the roots of the teeth, combined with the atrophy of the postorbital fat around the eye sockets, gives an older horse's face a hollowed appearance and those typical depressions above the eyes. As the collagen continues to degrade, and muscle degeneration begins, he'll develop a sagging topline (sometimes resulting in a sway back). Many older horses also display gray (actually white) hairs around the eyes, temples, and nostrils.
It's relatively common for older horses to develop cataracts, but these rarely cause complete blindness. They do create a loss of transparency in the eye's lens. Because vision is a less important sense to horses than smell or hearing, older horses with some impairment of vision usually cope very well.
On the inside, more changes are occurring. Although the timing differs from horse to horse, aging inevitably results in reduced cardiopulmonary function, a decreased capacity for exercise, and impaired nutrient utilization (more on that in our companion article, Feeding the Geriatric Horse, on page 75). Some researchers feel aging takes the hardest toll on the digestive tract and on the skeletal system. The immune system also takes a hit, becoming less able to defend the horse against viral and bacterial challenges, so older equines might become more prone to respiratory diseases, allergies, infected wounds, and even surface conditions like thrush.
Arthritis, an inflammation or degeneration of the tissues associated with the joints, eventually catches up with almost every horse. A chronic degenerative condition of the cartilage associated with the ends of bones, arthritis compromises the ability of the joints to flex or to bear weight. Arthritis can be crippling, although if caught in its early stages, it can successfully be managed with injectible therapies such as polysulfated glucosaminoglycans (PSGAGs--brand name Adequan) or sodium hyaluronate. For some older horses, the symptoms remain mild--perhaps a little stiffness on cold mornings or after a night of inactivity in a stall, which they will warm out of as they begin to move. For others, arthritis can express itself in lameness-causing conditions such as ringbone and spavin.
Anti-inflammatory drugs, certain changes in foot care, and extra-deep bedding (and soft footing if he continues to work) can do much to make an old and arthritic horse more comfortable. Providing a maximum amount of turnout also is important for arthritic horses, since moving about helps them lubricate their joints. It's crucial that you not let an arthritic animal become obese, as the extra weight only increases the stress on the legs.
In addition to arthritis in the joints, the tendons and ligaments of older horses often lose elasticity, while cartilage replacement in the joints slows. All these conditions can interfere with an older horse's mobility, enthusiasm for exercise, and stability. Lean muscle mass also tends to decline, leaving the horse with a "saggy" outline.
Loss of condition is a major concern for older horses. Poor dental health, poor parasite control, and debilitating disease all can contribute. The digestive system's gradual decline in nutrient utilization (caused by chronic parasite damage earlier in life, or by subtle alterations in the digestive process) doesn't help matters. On top of this, many older horses lose their enthusiasm for feed, a condition called anorexia. Once an older horse starts to become ribby, it can be very difficult to persuade his system to put the weight back on.
Strict attention to the older horse's dental health is crucial. A horse's teeth are designed to continue growing throughout his life, but often the rate of wear outstrips any new growth. As a result, he might find himself with such worn-down molars that he has nothing with which to grind his feed. Those being fed dry hay and grain will need more frequent attention than those on lush pasture, but any older horse should have his teeth checked, and the sharp points floated, at least once every six months. You also should watch for other dental problems, such as broken or missing teeth and mouth abscesses. Because dental problems will compromise how well horses chew their food, owners should be vigilant about colic and choke (when food becomes lodged in the esophagus). Very aged horses might have to be kept off forages altogether; fortunately, it's frequently possible to maintain even the most toothless quite well on hay cubes, beet pulp, and grain pellets, all of which can be soaked in water to make a gruel or a mush.
Next on the list of common problems are tumors of the pituitary and thyroid glands. A 1989 study by Sarah Ralston, VMD, PhD, ACVN, and her colleagues indicated that more than 70% of horses over the age of 20 had at least subclinical signs of one or the other--that is, there were no obvious symptoms, but blood analysis revealed telltale alterations in the blood glucose and cortisol levels. Thyroid tumors appear to be more common in geldings, while mares are more likely to suffer pituitary tumors. Both are usually benign and slow-growing, but because they alter the secretion of certain hormones, they can have profound effects on the older horse's health. They both can cause glucose intolerance and decreased sensitivity to insulin, the hormone that regulates sugar levels in the bloodstream. As a result, blood levels of both glucose and insulin become abnormally high, and the horse usually experiences increased thirst and urination.
A horse with a thyroid tumor is at increased risk of obesity and founder, and might develop a swelling in the throat area (this can be surgically removed if it becomes very large, but this is rare). The implications of a pituitary tumor can include weight loss, a compromised immune system, increased thirst and urination, and a characteristically heavy (and often curly) hair coat that fails to shed in the summer--in other words, Cushing's disease.
Although both types of tumors eventually can progress to the point where they are life-threatening (and pituitary tumors, at least, are considered inoperable), the good news is that many horses do well for many years after symptoms develop. Appropriate care--which might include clipping the heavy coat of a Cushing's horse--and oral medications can help treat the symptoms and restore the older horse to an active and normal lifestyle for some time. The horse with a thyroid tumor will test positive for hypothyroidism and usually will respond well to powdered thyroxin, while horses with pituitary trouble might rally with the help of cyproheptadine or pergolide.
Generally speaking, it's safe to assume that some sort of tumor is present in the majority of horses over the age of 20. Other common types include abdominal fat lipomas, which are not malignant, but can cause problems because they tend to grow on slender stalks that can wrap around portions of the intestine and obstruct the flow of blood and abdominal contents. (These sometimes can be removed laparoscopically.)
The abdomen also might be the site of widespread cancers such as lymphosarcoma. Skin melanomas, particularly in gray (and occasionally pinto) horses, are a common problem. It's estimated that nearly 80% of gray horses over the age of 15 are affected by melanomas; fortunately, they usually are benign, but they do turn malignant and invasive on occasion. If a melanoma spreads to the abdominal region (its usual location), a horse could begin to lose weight and condition, become debilitated, and suffer from fluid collection in the abdomen. Large growths might impinge on the blood supply, the nerves, or the intestinal tract.
Liver and Kidney Problems
Next on the list of age-related complications are liver and kidney problems. While these are not as common in horses as they are in aging dogs and cats, they do occur, and once begun, they are progressive and irreversible. Their progress can be slowed, and their symptoms managed to some degree with dietary changes.
Liver failure is characterized by weight loss, lethargy, jaundice (a yellowing of the mucous membranes and in the whites of the eyes), loss of appetite, and an intolerance for fat and protein in the diet. It if becomes severe, it can trigger behavioral changes. Affected horses become irritable (as might we all), and can sometimes be witnessed aimlessly circling or pressing their heads against objects.
Since the liver is a site of synthesis of vitamins B and C, it's also common for affected horses to have deficiencies of these vitamins (both of which can be supplemented orally). A horse with liver failure needs increased levels of sugar in his diet to help maintain healthy blood glucose levels, so he should be fed a diet high in carbohydrates and low in protein and fat (more on this in our nutrition article, page 75).
Weight loss and anorexia can result from reduced kidney function, and these problems can be fatal. Horses are unique among animals in that they get rid of excess dietary calcium in the urine rather than in the manure. But when the kidneys don't function as they should, calcium (in the form of calcium oxalate) can build up in the kidney tissue, the urethra, or the bladder, as "stones" rather than being excreted. These stones can be extremely painful, and of course they often interfere with normal urinary function.
Those in the bladder or urethra sometimes can be surgically removed, but those in the kidney are inoperable.
A potentially lethal buildup of calcium in the bloodstream is possible when horses suffer kidney failure.
Reproductive function is one thing that tends to decline slowly in horses. Many equines remain fertile well into their late 20s, but eventually, a reduction in endocrine (hormone) efficiency compromises the reproductive capacity and leads to low sperm counts in stallions and an inability to "catch" or become pregnant in mares. On a related note, older geldings sometimes develop infections and swelling in the sheath, the result of their being less likely to "drop" to urinate. Regular sheath cleaning with surgical soap can help keep bacterial populations down and edema to a minimum.
Hoof growth is another area that isn't changed much by the aging process. Hooves generally keep growing at their usual rate even well into old age. In fact, if an older horse is working less (or not at all), he might actually wear the hoof horn down less vigorously and need more frequent trimming than when he was younger. If your older horse is not likely to be encountering hard surfaces or carrying much weight, it's often best to pull his shoes and let him run barefoot.
Other effects of aging are relatively subtle. It's been found, for example, that older horses tend to thermoregulate (maintain their internal body temperatures) less well than their younger counterparts. In extremes of heat or cold, they might become severely stressed. Therefore, it's essential to shelter an older horse from heat or humidity with adequate shade and shelter (and perhaps a fan in the barn aisle), and to blanket him warmly in winter and shelter him from cold snaps and high winds.
Geriatric horses also might need more calories to fuel their internal temperatures in bone-chilling weather. They can be encouraged not to become dehydrated by being offered warm water several times a day.
Aging and Athletic Capacity
One need look no further than the Olympic Games to get a reminder that a horse's peak athletic years might not be when he is five, or eight, or even 10. Horses in their teens abound in the highest levels of competition. It's not unheard of for some, like Lisa Jacquin's amazingly athletic show jumping partner For the Moment, to still be going strong in international competition at the age of 20. A remarkable number of older horses remain active and useful well into their 20s or even 30s, although for most the level of intensity gradually declines.
It's estimated that as the lean body mass (muscle) decreases, and tendons and ligaments lose elasticity, speed is the first to go, followed by agility, strength, and finally, endurance. If so, then it's a good idea to tailor your demands to your horse's changing athletic abilities. Your jumper might not, for example, be able to generate the speed needed for timed jump-offs anymore, but he could find a happy home in the hunter ring. Later still, he could continue to be productive as a school horse, teaching young riders the ropes.
Provided your horse still is basically sound, it's valuable to keep him working (at an appropriate level). He will maintain his condition better, keep his joints lubricated--and most horses seem to be happier when they have a job to do. As Eleanor Kellon, VMD, writes in her book The Older Horse, "Rule number one is that if you want the horse to continue to be active, it is best (and sometimes imperative) never to 'let him down' completely. This means avoidance of long periods of inactivity."
While stall rest is not advised for the typical healthy geriatric, she notes in her book, even turnout can lead to a significant loss of condition, depending on the horse's personality and the conditions of the turnout.
An older horse which loses significant condition also is at increased risk of injury.
In a study published in the December 1997 issue of the American Journal of Veterinary Medicine, Malinowski and colleague Kenneth Harrington McKeever, PhD, measured the exercise capacity of young vs. old Thoroughbred and Standardbred mares. They did this by using an incremental exercise test on a treadmill set at a 6% grade. Measurements of maximal oxygen uptake, blood lactate, and packed cell volume in the blood were examined and compared for the young mares (average age about five years) and the old ones (average age about 22 years--the human equivalent would be approximately 60-78 years). The conclusion was that older mares have a substantially (24%) lower maximal aerobic capacity. Some portion of the difference might be due to older horses' "being laid-back, so they don't tend to work as hard," according to Malinowski. The findings were consistent with expectations--especially since an older horse's maximal heart rate also seems to decline gradually, an observation similar to the decrease in cardiovascular function seen in humans.
"Compared with other species, however, older horses still have a tremendous innate aerobic capacity," she says.
In the discussion portion of the paper, authors Malinowski and McKeever remark that several questions about the older horse's athletic ability still remain to be answered. For example: at what age does aerobic capacity first begin to decrease in horses? (In humans, noticeable changes are first seen between 40 and 50 years, but no one has yet established the parameters for horses.) Another burning question: what amount of exercise will most enhance an older horse's cardiovascular health and aerobic capacity, without over-stressing him or putting him at risk of injury?
"The ultimate goal of all these studies," the authors note, "would be to adjust the exercise amount to meet the needs of the growing population of athletically active older equine athletes. These data are needed as many horse owners continue to train their older animals by use of exercise training protocols that, although appropriate for a younger animal, may not be appropriate for older equine athletes."
When is it time to retire your older horse from working? "There's no set age for retirement," says Ralston, who is also an equine nutrition expert and partner in the research studies on aging horses at Rutgers.
"Basically, if it ain't broke, don't fix it!" she says. "The worst thing you can do is just retire (an older horse) because he's reached a certain age."
Provided your horse basically is sound, keeping him going with some level of gentle exercise will be good for both his body and his mind. If, however, lameness, respiratory problems such as heaves, or other medical conditions compromise his ability to exercise cheerfully, it's probably best to retire him to a turnout situation. Keep in mind, however, that just because a horse is retired, doesn't mean he should be forgotten. He'll still need diligent care in order to retain quality of life.
Routine Health Care
Although their laid-back temperaments make up for a lot, older horses are somewhat higher-maintenance than their younger brethren. Because their immune systems aren't as efficient as they used to be, geriatric horses are vulnerable to disease and infection, and should be monitored closely for any sign of infectious illness. Vaccinations should be diligently administered, and it might be wise to include regular vaccinations for influenza, strangles (consider the new intra-nasal vaccine), and equine herpesvirus, even if you didn't give these when he was younger. Just as older humans often line up for "flu" shots because a dose of the disease will hit them harder than younger adults, so, too, aged horses might need some help to fend off disease foes.
It's also vitally important to maintain a regular deworming schedule, and to examine the older horse's teeth at least once every six months for signs of decay or excessive wear. In addition, if you notice your older horse going off his feed or looking unusually subdued, it's worth running a blood test to detect early signs of liver or kidney dysfunction, or pituitary or thyroid problems.
Providing appropriate nutrition in an easy-to-chew format also is a pivotal part of maintaining an older horse. Because they grind and digest their feed less efficiently than younger horses, aging equines are at increased risk of both choke (when food gets lodged in the esophagus) and colic.
Chronic diarrhea, possibly resulting from a more fragile population of gut microflora and a decreased amount of digestive enzymes, also can occur and can be difficult to manage. Stress, feed changes, or intestinal infections might trigger diarrhea, and some horses might need fluid and electrolyte therapy to help them recover. Probiotics can be of great benefit to older or stressed horses. Or, to help older horses replace the gut microorganisms, you can allow them to graze in a field heavily used by other horses. The manure of these other horses will contain large populations of the bacteria, and your horse might pick them up in the same way a foal populates his gut, through coprophagy (eating manure). If worst comes to worst, your veterinarian might decide to stomach-tube your horse with fluid strained from normal manure in an attempt to repopulate his intestinal tract with the beneficial organisms.
Sensible management also includes protecting your older horse from undue stress. For example, he might not be able to hold his own with rambunctious youngsters in the pasture anymore, and would be happier in a field with his peers, who won't run him ragged. Because an older horse usually is less aggressive, you might find that in a group feeding situation, he gets chased away from his meal by bullying younger horses. It's best to provide him with a stall or separate paddock in which to eat, so he doesn't have to compete for his calories.
A Fountain Of Youth
A new hormone therapy designed for older horses, recently licensed for use in Australia, might in the near future change the way we manage our aging equines. EquiGen, an injectible form of the hormone somatotropin, promises to improve the nitrogen balance (and thus protein absorption) of older horses. Recent studies by Malinowski and her colleagues indicate that it might, in fact, do much more. It might improve the utilization of a number of other nutrients, making it easier for an older horse to keep good condition. It also seems to have a remarkable capacity to improve muscle tone and appearance.
Somatotropin, sometimes called growth hormone (although it has been demonstrated to have many roles in addition to its involvement in growth) normally is secreted by the pituitary gland. But as with most hormones, its level in the bloodstream undergoes a gradual, natural decline as a horse ages. Because somatotropin (the equine version of which is abbreviated eST) affects virtually every part of the body, its absence seems to allow the decline of many other systems.
"Once you take away somatotropin," says Malinowski, "there is a dramatic decrease in the condition of the horse."
In clinical trials conducted by BresaGen, the company that manufactures the recombinant version of the hormone, horses treated with EquiGen did show endocrine changes that are associated with improvements in nitrogen balance and therefore protein metabolism. The amount of nitrogen in their urine also was reduced, an indication that the treated horses were able to absorb more protein from their diets. It is for its role in improved nutrient utilization that EquiGen has been licensed for use in older horses...but it likely will have a number of other beneficial effects as well.
Somatotropin therapy has a precedent in other species, including dogs and humans. In dogs, ST administration partially restored immune function, and in geriatric humans, regular doses of ST improved body condition, musculature, and some elements of immune function, as well as decreasing adipose (fat) tissue mass and providing a sense of well-being. In one study on aged human males, somatotropin also improved strength and the ability of the subjects to perform a number of weight-lifting exercises. All of these effects are leading researchers to have high hopes for the hormone's use in horses.
Malinowski's team tested EquiGen injectible eST on a group of 16 Standardbred and Thoroughbred mares, which were 20 years old or older, giving daily injections for a period of six weeks. The aim of the initial study was to determine whether aged mares responded to the hormone with changes in feed intake, body weight, body condition score (based on fat cover), or immunocompetence (strength of the immune response).
The results of the study, says Malinowski, were dramatic. The mares given EquiGen developed greater muscle definition than the control mares. Although body condition scores and body weights did not alter, "eST helped the skinny mares retain their condition better, and heavier mares became trimmer."
The greater muscle definition observed in these mares by Malinowski's team appeared to come at the expense of fat deposits, without altering nutrient intake. The overall effect was that the mares looked toned and younger. (The team was quick to note, however, that increased muscle tone does not necessarily translate to improved strength.)
Another finding was that eST did, indeed, have a beneficial effect on certain portions of immune response. Mares given eST did respond by producing a greater number of total circulating leukocytes (a type of white blood cell), although lymphocyte numbers and efficiency were not altered.
"There was a dramatic increase in the number, but not the function, of certain white blood cells," says Malinowski, "specifically neutrophils. We think that might be significant; the animal might be a little better protected (against invading pathogens) because there are more (white blood cells) circulating."
Interestingly, two weeks after the treatment ceased, the eST mares did demonstrate a twofold improvement in the lymphocyte response to several deliberately introduced irritants (such as pokeweed pollen). This indicates that certain immune mechanisms--but not the immune system as a whole--might respond favorably to regular injections of eST.
These results were encouraging. For Malinowski's team, the next logical question was: will eST improve aerobic capacity or exercise tolerance in aging horses? Certainly if it did have that effect, it would be a major improvement in the quality of life for older equines.
In a paper recently published in The Veterinary Journal, that very question was examined by challenging 15 healthy, aged mares (with the definition of "aged" in this case being 20 or more) with a standardized exercise test (SET) on a high-speed treadmill. The mares were unfit at the start of the study, and received eST for a total of 89 days, with exercise tests performed before, during, and after the treatment period at 21 days, 43 days, 89 days, and 127 days. While they were exercising, measurements of oxygen uptake, blood lactate levels, and plasma protein, hematocrit (a measure of the volume of red blood cells compared to the total volume of blood), and the concentrations of certain enzymes that indicate muscle damage and possible "tying-up" were taken.
In this case, the results were disappointing. The mares given somatotropin demonstrated no improvement in either aerobic capacity or exercise tolerance when compared to the control mares. Whether eST will have an effect on the strength of older horses remains to be seen. But the result of this study should be important from a regulatory standpoint, as it is evident that the hormone is probably not an ergogenic (performance-enhancing) substance.
Malinowski hints that eST might still have some other, unexplored benefits.
"I'm predicting that eST will have a beneficial effect on uterine tone and fertility in older mares," she says.
In any event, the beneficial effects of eST appear to last only as long as the drug continues to be administered.
"They look great when they're on it," says Malinowski, "but within two weeks after they go off the drug, they look like old ladies again. And their blood levels (of somatotropin) return to pre-treatment levels."
Although the drug currently is available only as a daily injectible, Malinowski suspects that it might not need to be administered every day to be effective; that question is currently under investigation. However, it's relatively clear that once a horse is put on somatotropin, for best benefit he or she should remain on it for life.
Also being explored are other methods of delivering the drug. Should an oral format be developed, it might very well change the entire way we manage our older horses. Of course, EquiGen is not yet available in North America, and Malinowski believes that it will be a few years before the regulatory bodies in the United States and Canada give their thumbs-up.
Such Sweet Sorrow
Although our focus in this article has been the successful management of the aging process, there comes a time for every horse when the accumulated wear and tear become too much. A dwindling appetite, chronic pain, immobility (or a lack of desire to move), and self-imposed isolation from the herd and from humans--any or all of these symptoms can tell you that your horse's systems are shutting down. When that fateful day is reached, humane euthanasia is the most generous final gift you can give your old friend (see related article on page 41). As Ralston observes, "He's not writing his memoirs. He doesn't have any affairs to put in order. Though it may be wrenching personally to let go, euthanasia is the kindest thing for a horse who no longer enjoys any sort of quality of life."
But just so we don't finish on that sad note, remember that horses which have been gently started, well cared for throughout their lives, and exercised consistently (but not overworked) can enjoy longer and healthier lives than ever before. As the years go by, you might have to adapt your demands to your horse's capabilities, but any diminished performance ambitions will likely be more than compensated for by the enjoyment you get out of your relationship with an older horse. My own two are 15 and 27 years of age and are still both actively working, and I can honestly say they bring me more joy with each passing year.
About the Author
Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.
POLL: University Equine Hospitals