The Secrets of (Horse) Longevity

The changes are subtle–mild stiffness, deepening hollows above the eyes in the evening light, a spine that dips a bit more. “How old is he?” friends ask. “Oh, about 15,” you answer year after year until you run across his papers and discover your beloved companion is nearly 25. At some point, your vision of your horse shifts. You no longer picture him sailing over fences. Instead, you see him nibbling the green grass beneath the fence in the summer evenings. You begin to worry about the end of your years together and seek ways to prolong those years. Welcome to ownership of an old horse.

In practical terms horses are considered “old” around age 20. However, an increasing number of horses are living much longer. At the Tufts University School of Veterinary Medicine1, for instance, the percentage of horses over age 20 presented to the teaching hospital rose from 2.2% in 1989 to 12.5% in 1999. Anecdotally, it is not uncommon for horses to live well into their 30s and even early 40s. Ponies tend to be overrepresented in the population of aged horses. In a National Animal Health Monitoring Systems study that surveyed owners of 10,000 U.S. equines, 15.2% of ponies were older than 20.

So what can you do to keep your horse happily in that pasture for years to come? Aging is inevitable, its progression determined by a combination of genetics, care, and luck. Ideally, we would breed for traits that enhance longevity: strong feet and legs, good dental occlusion, and a decreased susceptibility to life-shortening diseases. In that perfect world horses would have access to free exercise, a natural diet of mixed grasses with small amounts of concentrate, and consistent hoof, medical, and dental care throughout their lives. These early factors help a horse reach his 20s in good health.

While owners can’t control genetics and luck once a horse is born, they can impact longevity through lifelong thoughtful care–attention to housing, nutrition, hoof care, exercise, and preventive medical care such as parasite control and vaccination.

You Are What (and How) You Eat

In the aforementioned Tufts study the gastrointestinal (GI) tract was the most commonly affected body system in the older horses presented, with problems manifesting as conditions such as colic, esophageal choke, and dental disease. As horses age, dental malocclusions become more common, with hooks, severe points, ramps, and “wave mouth” observed frequently. Additionally, these horses will often develop diastemata, or spaces between the teeth into which food becomes packed, leading to periodontal (around the tooth) and tooth root disease. Aged teeth can also wear until they have a slick chewing surface. All of these factors can negatively impact chewing, possibly decreasing feed digestibility and contributing to conditions such as choke and large colon impaction. Additionally, large intestine efficiency can diminish with age. Laurie Beard, DVM, MS, Dipl. ACVIM, associate clinical professor at Kansas State University’s College of Veterinary Medicine, notes2 that in her experience poor dentition is the most common cause of weight loss and malnutrition in the geriatric horse. While a thorough yearly dental examination can catch many of these issues, Beard notes that “dental abnormalities in geriatric horses may be so severe that they may not be completely corrected with dentistry.” These horses might need dietary adjustments such as eliminating hay or providing roughage in a chopped or pelleted form.

However, not all older horses require a so-called geriatric diet. For the healthy mature horse with good dentition, Nat Messer, DVM, Dipl. ABVP, professor in the Equine Clinic at the University of Missouri College of Veterinary Medicine, advocates avoiding feeds rich in carbohydrates and instead focusing nutrition on a diet of hay and/or pasture, water, and a minimum of grain-based feeds. While Messer lauds the recent development of low nonstructural carbohydrate (NSC) commercial feeds, he notes there is “nothing that says horses need to eat something out of a brown paper bag.”

The final component of the geriatric horse’s diet–fresh water–is often overlooked. Water hydrates the animal and softens feed fibers, making them more digestible. You can increase your horse’s water intake by adding salt to his feed, as adding loose salt to any pelleted feed tends to work better for maintaining water intake than simply providing a salt block. Use water heaters in the winter if your horse is reluctant to drink very cold water.

Common Age-Related Conditions

According to Messer, diet also is an important factor in minimizing the risks of two age-related conditions in horses: equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID), also known as equine Cushing’s disease.

While clinical signs of EMS and PPID can be similar in appearance and in effect, the diseases have different causes, treatments, and underlying changes within the horse. The PPID or “Cushingoid” horse often appears as the classic sway-backed, pot-bellied, shaggy-haired geriatric. In these horses the pars intermedia portion of the brain’s pituitary gland becomes hyperplastic, or enlarged, producing excess amounts of various hormones, one of which is the adrenocorticotropic hormone (ACTH). Clinical signs of PPID include excessively long hair coat (hirsutism); excessive drinking, urination, and sweating; immune suppression; abnormal fat deposition; and laminitis.

These last two signs often cause owners to confuse PPID with EMS. Horses with the latter disorder can appear overweight and cresty-necked and are often afflicted with laminitis, but they typically have a normal coat and can be younger than PPID patients. Equine metabolic syndrome is characterized by insulin resistance, producing intermittent blood sugar spikes similar to what happens in a person with Type II diabetes. These horses might also be subfertile and display exercise intolerance. While EMS treatment relies primarily on dietary management with strict attention to providing a low NSC (less than 10-12%) feed along with exercise, PPID horses can benefit from medical treatment. First, however, Messer stresses the importance of diagnosing PPID accurately through blood testing. The two most commonly used tests are the dexamethasone suppression test and the endogenous ACTH test.

While many veterinarians consider the dexamethasone suppression test to be the “gold standard” for PPID diagnosis, it requires a veterinarian to take two blood samples, just before and 20 hours after administering dexamethasone. Steroids such as dexamethasone have been linked to laminitis, but Messer says it is unlikely the dexamethasone suppression test will cause laminitis on a one-time basis. The test measures the response of cortisol (a hormone produced in response to ACTH) to dexamethasone administration. In a normal horse dexamethasone administration will suppress cortisol levels. In a PPID horse cortisol will resist suppression.

The endogenous ACTH test requires only one blood sample and eliminates the need for dexamethasone administration. Sample processing requires an exact approach, or the test results will be invalid.

Pain and stress can affect both tests’ results, as can season. According to Beard, owners should avoid testing horses in late summer/fall as the pituitary gland is more active this time of year (likely due to decreasing daylight), causing hormone levels to fluctuate and altering test results.

Properly diagnosed PPID can be treated several ways, but pergolide mesylate is the most commonly used (and generally most effective) medication. Pergolide is a prescription drug that owners administer to affected horses daily. According to Messer, the earlier a horse with PPID is treated, the less likely his pituitary will become severely hyperplastic. Therefore, once a veterinarian diagnoses a horse with PPID, treatment is critical to prevent the progression of the disease.

While EMS and PPID are distinct diseases, they can produce similar outcomes. In both conditions, affected horses are often insulin resistant and laminitis-prone. According to Messer, diet can minimize the risks of developing either condition. While EMS is clearly linked to obesity, Messer states that “there is some evidence to suggest obesity causes a greater risk of PPID,” as well. Adipocytes (fat cells) produce inflammatory cytokines (mediators of inflammation), which create oxidative damage within the body that can lead to both laminitis and damage to parts of the brain that contribute to PPID development.

Despite the array of nutraceutical manufacturers claiming product efficacy against PPID and EMS, Messer states he does not think these products contain any “magic bullets.” As he puts it, most herbal remedies are either unproven or have been disproven in scientific testing.

An Ounce of Prevention

Other changes documented in older horses occur within the immune system. Geriatric horses have shown lower antibody titer response to influenza vaccination and have lower numbers of total lymphocytes (a type of white blood cell that plays a central role in directing and coordinating immune responses). Horses with PPID might also experience suppressed immunity. With these factors in mind, part of older horse management includes controlling exposure to disease-causing agents and/or maximizing the immune response. Work with your veterinarian to develop a vaccination protocol that considers age, exposure risk, and underlying medical conditions.

Parasite control is critical for older horses as well, as a horse with parasite infection will be less likely to absorb nutrients properly from feed. Messer believes that “an important (step an owner can take to provide a long and high-quality life) is targeted deworming.” Targeted deworming is the practice of identifying and deworming most frequently the horses that are the highest parasite larvae/egg shedders. In a 2010 article3 Kaplan and Nielsen discussed the increase of drug-resistant parasites and owners’ continued reliance on an outdated deworming system. Rather than methodically deworm every horse every eight weeks, Kaplan and Nielsen suggest identifying the horses that are responsible for the greatest percentage of a farm’s parasite load through monitored fecal egg counts and by testing each dewormer’s efficacy through fecal egg count reduction tests. The latter tests the egg count from a horse’s manure at the time of deworming and then again two weeks later.

Proper hoof care can also enhance a horse’s quality of life. With laminitic horses, for instance, hoof care can mean the difference between life and death. Even for horses with mild-to-moderate arthritis–another common older horse concern–keeping hooves trimmed and balanced reduces strain on the joints.

Fit as a Fiddle

The musculoskeletal system is the second most commonly age-affected system in the horse behind the GI tract. As previously discussed, laminitis is a common cause of lameness in older horses, largely due to its association with PPID and EMS. As for arthritis, researchers believe that older horse cartilage is more brittle than that of a younger horse.4 Thus, as a rule, older horses benefit from free exercise such as pasture housing rather than excessive stall confinement. Additionally, “Training can overcome the age-related reduction in muscle oxidative capacity in horses,” states Beard. “Training can also attenuate the severity of exercise-induced muscle fiber damage.”

Messer agrees that if older horses “are capable, exercise is good for them.”

Quality vs. Quantity

The true secret of longevity in the horse is not to focus on longevity. Though the thought of owning a record-breaking old horse might be wonderful, the true focus must be on the animal’s overall well-being. Ultimately, no medication, surgery, or preventive care can halt the aging process. Eventually, the long-lived horse might outlast the quality of his life. So, when does the responsible owner consider the painful choice of euthanasia? As markers of a declining quality of life, Messer lists “unexplained weight loss; musculoskeletal infirmity that is not responsive to reasonable amounts of medication; inflammatory airway disease that does not respond to reasonable treatment; dental disease where there is no viable solution; and finally, the lack of ability to maintain a healthy body condition score and to move around and do things horses are meant to do–to be a normal horse.”

Whether he walks the trail, packs the grandchildren, baby-sits weanlings, carries therapeutic riding program participants, or lives out his years in leisure, the senior horse deserves this ultimate indicator of a quality life: to be a normal horse.