Laurie A. Beard
Aged Horse Health and Welfare
Interest in geriatric equine medicine has been increasing recently, with more than 20 scientific studies published on related topics from 2008 to 2013. Seven studies in the past 3 years documented the management and health status in geriatric horses, whereas other recent studies described the effects of aging on immune function, colic, and other body systems. The definition of a geriatric horse varies, but 15 to 20 years of age or older has been suggested as a guideline. In this chapter, the term geriatric refers to horses 15 years of age or older. In one study, horse owners reported that signs of aging begin at 22 years of age, but horse age becomes a factor for potential horse owners when horses are 16.5 years of age. The normal life span of horses is reported to be from 20 to 40 years of age, yet many horses have finished their performance or breeding careers by half of this age. Today, geriatric horses are valued for their companionship rather than just their ability to work or perform. Geriatric horses can remain valuable after finishing a successful athletic career because they are often reliable mounts for younger, inexperienced riders. Age does affect the primary use of horses, with most owners of older horses reporting a decrease in exercise intensity in their horse. Most (60%) geriatric horses are still used for pleasure riding, with most being ridden approximately 3 times a week in one study. Most horses that are 30 years or older are retired, but about 30% are still used for pleasure riding.
It is difficult to accurately determine the number of geriatric horses in the U.S. horse population, but it appears to be increasing. According to data from the National Animal Health Monitoring System, the percentage of horses older than 20 years in the United States has increased from 5.6% (1998) to 7.6% (2005). About 30% of the horses in Queensland, Australia and in the United Kingdom are geriatric, with 11% of horses in the United Kingdom being 20 years or older. It is estimated that 0.2% of horses in the United States and 2% of horses in the United Kingdom are 30 years or older. It appears that breed and size of horse play a role in longevity. In several studies, pony breeds were overrepresented in horses 30 years or older.
It is difficult to distinguish between normal age-related changes and diseases associated with aging. Common signs of aging include graying of hair, muscle atrophy or loss (e.g., development of a swayed back), lack of joint flexibility, low or sloping pasterns, drooping of the lower lip, and weight loss. However, not all geriatric horses are underweight, and some geriatric horses are overweight. An association does exist between increasing age and reduced body condition. Twenty-six percent of all geriatric horses are overweight, and only 4.5% are underweight. In contrast, only 10% of geriatric horses 30 years or older are overweight. Most horses 30 years or older are in good body condition, but 16% are in poor body condition. Owners tend to underestimate their animal’s body condition score, compared with veterinarians. In other words, some horses that are regarded as having a normal body condition by their owners are scored as overweight upon veterinary examination. Diseases or problems reported by owners in geriatric horses are common and include lameness, colic, and ocular and dental problems. Specific diseases reported frequently in geriatric horses include pituitary pars intermedia dysfunction (PPID), equine metabolic syndrome, recurrent airway obstruction, and arthritis. These conditions are mentioned in this chapter but are covered in detail in other sections.
Horse owners frequently observe clinical signs of disease in their geriatric horses, but underrecognition of health problems is still a concern. Geriatric horses do not receive as many routine veterinary health care visits as younger, actively competing horses. Most geriatric horses are vaccinated against infectious disease; however, the percentage of geriatric horses receiving annual vaccination is significantly less than in adult horses. Dental disease or problems are also underrecognized by horse owners because most geriatric horses are found to have dental abnormalities when examined by a veterinarian. Owners of geriatric horses may misinterpret changes as normal aging signs rather than development of disease. Reduced frequency of routine veterinary health care may lead to delay in recognizing significant health problems in older horses.
Effect of Age on the Cardiovascular and Respiratory Systems
The capacity for exercise and the aerobic capacity both decrease in geriatric horses. Not surprisingly, geriatric horses are not able to run as fast or as long on a high-speed treadmill, compared with younger horses. Aging results in reduction of cardiac output in horses and humans because of a decrease in both stroke volume and heart rate. Conditioning improves stroke volume but does not increase maximal heart rate. During exercise, geriatric horses do not thermoregulate as well as younger horses, which may be a result of lower preexercise plasma volume. This could be clinically important information for horse owners competing older horses in athletic events, in particular endurance riding. Monitoring for signs of heat stress is therefore very important in geriatric horses being ridden in warmer climates.
Age also affects lung function. Geriatric horses have impaired transfer of oxygen from the alveoli to the capillaries, and older horses have a lower Pao2 and Paco2, and a higher P(A-a)o2 gradient and higher pH values, than younger horses. A lower Pao2 is also reported in older horses under anesthesia. The lower Paco2 and increased pH may be caused by hyperventilation in the attempt to maintain Pao2. In one recent study, abnormal respiratory sounds were heard upon auscultation in 22% of geriatric horses and in 32% of horses 30 years or older at rest. Rebreathing examination resulted in auscultation findings consistent with lower airway disease, including expiratory wheezes and coughing in some of these horses. Recurrent airway obstruction (RAO) is a common disease in older horses. It is difficult to determine whether the impairment in oxygen transfer is a result of subclinical disease such as RAO or is a result of other age-related mechanisms.
Cardiac murmurs caused by aortic regurgitation were considered to be the one common finding in older horses; however, both mitral and aortic valve regurgitation were recently reported as common findings in these animals. The prevalence of cardiac murmurs in geriatric horses is 20%, with 7.5% of the murmurs consistent with aortic valve regurgitation and 9% consistent with mitral valve regurgitation. In horses 30 years or older, cardiac murmurs are detected in 36.2% of horses. In one study, most of these murmurs were left-sided, with 17.4% being consistent with mitral valve regurgitation and 18.8% being consistent with aortic valve regurgitation. Most of these murmurs were not severe (only 28% of the murmurs were scored higher than III/VI in severity). Most of the horses had no other sign of cardiac disease, and the presence of a cardiac murmur was not associated with a higher mortality rate. An increased risk for fatal hemorrhage associated with phenylephrine administration was recently reported in geriatric horses treated for left-dorsal displacement of the large colon. The reason for the hemorrhage associated with phenylephrine in older horses is unclear.
