Demographics of aging in the horse (Proceedings)

Definitions and Theories on aging

Aging is not a disease. It occurs in all organisms if they live long enough! Some  definitions that are helpful when speaking about aging include life span, life expectancy, chronologic age, biologic age and demographic age.  Life span is the maximal attainable survival time for a given species. Life expectancy is the average observed years of expected life from birth.  Chronologic age denotes the exact number of years, months, days that someone is alive, while biologic age is when molecular, physiologic and biochemical functions are expressed as a % of maximal function.

Finally, demographic age is concerned with survivorship relative to a population. For instance if 100-75% of the population is still alive then the animal may be in the mature category.  If 75-25% of population is alive then you enter the old category.  You are considered very old if 25% or less of the population at that age is surviving.  This may relate to the 15-20 year old horse as being mature, the 20-29 year old horse as being old and the > 30 year old horse as being very old.

There are numerous theories on aging in humans. Molecular theories propose that aging is the result of changes in the DNA template which regulates the formation of cellular proteins.  The Cellular theory says that cellular protein changes occur with the passage of time under environmental stresses. Others adhere to the systemic theories that claim the aging of the entire organism is ascribed to decrements in the function of a key system such as the immune or neurologic system.

The oxidative stress or free radical theory suggests that aging is the result of accumulation of oxidative damage.  In truth all of the above probably contribute to the process of aging under the Unified theory - Organism's biologic process is genetically determined at conception, lasts for the duration of life. Environmental stressors accelerate or decelerate the process and the balance between the two determine our biologic age.

What is considered old in the Horse?

Many have asked the questions about aging in the horse and is our definition of old changing.  In the past 20 years it appeared that we were seeing more horses over the age of 20 in our hospital. A small study did indicate that from 1989 to 1999 we saw an 7 fold increase in horses presenting with problems who were 20 years of age or older.  The numbers rose from 2.2% of the hospital population to 12.5%. In a survey of equine veterinarians in New England, all practitioners responded yes when asked if they had horses > 20 years of age.  30% of the responding veterinarians stated that between 21-40% of their caseload was composed of horses >20 years of age.

So are horses living longer than before?  It is hard to separate all the different factors – economic vs increased longevity.  In the NAHMS study in 1998, 7.5% of resident equids were > 20 years of older. Ponies had the highest % over 20 years of age at 15.2% (only 5% of horse population). Miniature horses - 2.7 % > 20 years of age. Largest cause of mortality in horses over 30 days of age was “old Age” (29.5%). In New England the number was double the national average.

In a review of early old horse studies, researchers would consider horses old if they were 15 years or older.  In a 2000 survey of owners with older horses, owners considered a horse generally old at age 22. They first noticed aging changes in their horses around 23 years of age.  Age was considered a negative factor in the purchase of a horse if they were > 16.5 years old.

Are there specific breeds that live longer than others?  There does not appear to be any breed of horse that has a “longevity” gene with the exception of the pony.  In multiple studies ponies have a higher percentage of animals in the > 30 years of age category especially when compared with the % of ponies in the younger population.

Anecdotally, mules are also stated to have long lives. Why the difference?   Some say that ponies originally come from areas in the world that require a hardier constitution.  Others state that perhaps the smaller body size plays a role as seen in other species. Wilmink found a difference in 2nd-intention wound healing between horses and ponies. Ponies healed faster than horses in a standardized wound, so perhaps there are physiologic differences that contribute to long life span.

A retrospective study of horse owners showed that activity levels decrease or change as the horse ages.  Greater than 50% of the horses over 20 years of age were still used for pleasure riding and 37% of the horses were retired.  Interesting though was the 10% of the surveyed owners said that they still competed they horses.  One famous aged horse was Bandit, a competitive trail riding horse that finished his last ride when he was 37 years old.

Are the changes we see due to old age or disease?

It is difficult to clearly denote whether some of the signs we see in the aging animal are due to ageing changes or due to disease processes.  For instance, older animals are known to have decreased spontaneous activity.  In the older horse we may wonder if the decrease in spontaneous activity is not secondary to old age but a result of degenerative orthopedic disease.  The general signs of aging in the horse include loss of “top line” muscle mass - sway back, graying of haircoat and musculoskeletal stiffness.  But one could argue that the decreased muscle mass is secondary to decreased activity and the stiffness is secondary to arthritis.

One may think that the older horse would tend to be thin and frail. When owners were asked to body condition score their horses, 29% old versus 33% of the young horses were scored as fat or very fat!  Only 4% of the old horses were considered in poor or very poor body condition. Management practices of the geriatric horse were also similar to that of younger with perhaps a slight decrease in dental care and deworming.

Older horses have many of the same medical issues that young horses have but there are some conditions that appear to increase with age.  The disease most commonly associated with old age in the horse is pituitary pars intermedia dysfunction (PPID), or commonly known as equine Cushing's.  A more in depth presentation of this problem will be undertaken in the next hour.  Other diseases that appear to increase in horses with age include colic (particularly strangulating lipomas), dental disease, lameness other than laminitis and neoplasia (particularly melanomas). When owners were asked about the use of medications for chronic problems, 25% of the old horses versus 6% of the young horses were on regular medications.  The medications fell into 3 categories of treatments – pain relief, recurrent airway inflammation and PPID.

The current knowledge base is the older horse is increasing as people become more interested in aging research and in how to provide for the aging population of horses. The affects of aging on specific body systems have been investigated.  In the reproduction system, we know that mares > 19 years of age ovulate 2 weeks later in the spring than younger mares. (Vanderwall, Wood) Older mares generally ovulate smaller follicles and have more multiple ovulations than younger mares. (Carnevale et al) The fertilization rates for young fertile and aged, subfertile mare are similar at 80% but embryonic loss between fertilization and day 14 is 7-8 times higher in the aged subfertile mares. The high rate of early embryonic loss in aged subfertile mares may be due to inherent developmental defects in their embryos. (Brinsko et)

In human immunologic studies, immune function declines substantially with advancing age. Advanced age favors humeral response as cellular response declines faster. Aged horses have a decrease in peripheral lymphocytes and a shift in lymphocytes subsets (decreased CD4 and CD8) and a significantly lower proliferative response to mitogens than younger horses. (Horohov)

Antibody production after exposure to rabies vaccine showed  82% of old horses and 50% of young horses had titers < 0.5 IU after 8 weeks.  Old horses may have greater difficulty maintaining an adequate antibody titer after 1 rabies dose – current recommendations may need to be reconsidered. (Muirhead, McClure)

Researchers have also looked at cartilage degeneration in horses with age. Though GAG remain constant throughout life, there is an age-related decrease in proteoglycan size though the loss of GAG chains. (Platt et al)  Older horses have greater pentosidine crosslinks which may predispose older horses to osteochondral disease due to stiffer and more brittle cartilage. Exercise studies by McKeever demonstrated a lower (-24%) maximal aerobic capacity in older mares (22.0+0.4 years) than young mares.  This type of research may be important in how we condition the older athlete.

More research continues to be conducted in the aging horse with the hopes that we will continue to enjoy their company and good health for longer periods of time.