Geriatric medicine has become increasingly important in equine sports medicine, although research regarding management, preventive health care and disease conditions in aging athletes is relatively limited. Between 10 and 26% of aged horses are used for competition,1,2 participating in a wide range of non-racing disciplines, including dressage, eventing and showjumping. In certain disciplines, it takes many years of training to reach elite levels of competition, so management of the horse in its older years can become very important. The term ‘geriatric’, first used in human medicine in the early 1900s, describes the life stage characterized by progressive decline in physical condition, organ function, sensory function, mental function and immunity.3 The age at which a horse is classified as geriatric or aged remains a subject for debate and can be defined in different ways, including chronological age, demographic age and physiological or functional age.4 Physiological age considers the point at which peak physiological functions begin to decline and, while it varies between horses depending on their use, genetic and environmental factors, this is often considered to be reached at around 15–20 years of age. There is limited available equine demographic information and few data providing population estimates and age distribution of horses. Anecdotally the life expectancy of horses is increasing and the proportion of aged animals in the equine population appears to be growing.5 While equine demographic characteristics vary considerably between different countries, studies suggest that between 10 and 33% of the equine population could be classed as aged.2,6–9 A common characteristic of aging body systems is progressive and irreversible change.3 Age-associated physiologic deterioration occurs subsequent to damage caused by intrinsic living processes, environmental factors, and age-associated disease.10 Factors influencing the rate of development of both normal aging and chronic diseases are often identical.11 When assessing health and disease in the older patient it is important to recognize and distinguish between benign signs of aging, physiological changes with age which can predispose to disease and clinical signs of disease associated with aging. However, as many of these signs are non-specific, accurately making this distinction in aging horses can be difficult and changes frequently considered benign signs of senescence, such as musculoskeletal stiffness or reduced joint flexibility and exercise intolerance/fatigue,5,12 are likely to represent clinical signs of disease. As in other species, there appears to be a decline in cardiorespiratory capacity with increasing age in the horse. There is an age-related decline in maximal heart rate, maximal stroke volume and maximal oxygen uptake (VO2max), which are reached at lower velocities in horses aged 20 years or greater (average 27 years) compared with young (average 7 years) and middle-aged (average 15 years) horses.13,14 During incremental exercise tests, maximal velocity attained by old horses (>20 years) was lower than that of younger horses (8.7 ± 0.5 compared to 10.8 ± 0.5 m/s, respectively).14 The velocity at which the onset of blood lactate accumulation is observed (blood lactate concentration of 4 mmol/L, VLa4) was also lower in old mares (7.5 ± 0.4 m/s) compared with young mares (10.2 ± 0.7 m/s).14 Older horses without clinical respiratory disease have decreased pulmonary surfactant phospholipid content;15 however, the importance of this age-related change in respiratory function in the development of disease has not yet been investigated. Healthy aged horses (average 27 years) have lower partial pressure of oxygen (PaO2) and increased arterial blood pH compared to young horses (Table 56.1).16 Increased alveolar to arterial PaO2 gradient suggests impaired arterial blood oxygenation in aged horses; however, there were no significant alterations in acid–base balance compared to young horses.16 Table 56.1 Arterial blood gas values, hematologic and serum chemistry values (mean ± standard error or standard deviation) of young and geriatric horses *Significant difference between young and geriatric horses (P < 0.05). It is interesting to note that many of the age-associated reductions in cardiorespiratory capacity were not found until horses were greater than 20 years of age. Further, despite these reductions, older horses still showed improvements in maximal stroke volume and VO2max after a training program.13 This shows that training can partially reverse some of the decline in cardiovascular function older horses. Aged horses demonstrate decreased ability to thermoregulate during exercise. During submaximal exercise tests, aged mares reached a core temperature of 40°C significantly more rapidly compared to young mares, and the average heart rate of aged mares on reaching this core temperature was significantly greater than that of the young mares.17 This reduction in thermoregulatory capacity is likely to be associated with the lower plasma volume, resting circulating blood volume and resting red cell volume in older compared to younger mares.17 Loss of muscle mass is common in geriatric humans; in elderly females, quadriceps cross-sectional area was a third smaller and voluntary isometric strength of the quadriceps muscles was significantly lower compared to young adult females.18 There was a significant correlation between quadriceps size and strength, suggesting that weakness is predominantly due to reduced muscle mass.18 In humans, reduced muscle strength with increasing age is considered to be a result of both loss and atrophy of muscle fibers, with a reduction in the proportion of type I fibers.19 Similar changes in the profile of muscle fiber types have also been identified in aged horses, and can explain some of the lower exercise capacity seen in the older horse. The proportion of type I muscle fibers decreases and there are alterations in the activity of muscle enzymes, consistent with decreasing oxidative capacity of equine skeletal muscle, with increasing age.20,21 Post-mortem studies have identified age-related changes in articular calcified cartilage thickness, hyaline cartilage and subchondral bone thickness and bone density and micro-architecture.22–24 Age-related degenerative changes in the surface of the navicular bone and the deep flexor tendon were found in older horses without a history of lameness and navicular disease.25 Age-related changes have also been reported in components of articular cartilage. For example, non-enzymatic pentosidine cross-linking increases linearly with age, with a 10-fold increase between horses aged 5–30 years.26 Another study found that while glycosaminoglycan content remained constant in samples taken from the metacarpophalangeal joints of horses ranging from three months to 31 years, there was a decline in hydration and cellularity of cartilage with increasing age, in addition to an age-related decrease in proteoglycan size through the loss of glycosaminoglycan chains.27 These changes may predispose older horses to osteochondral disease due to the cartilage becoming stiffer, more brittle, and more vulnerable to damage by mechanical loading.26,27 Some age-related differences in hematological parameters have been described in the horse, although, again, these are not usually evident until the horse is over 20 years of age. Clinically healthy aged horses have larger red blood cells (RBCs), with a higher erythrocyte mean cell volume (MCV)28,29 and a higher mean cell hemoglobin concentration (MCH).28 Increased MCH could reflect larger RBC size rather than a true increase in MCH concentration.28 Anemia is not a feature of healthy aged horses until they are very old (≥30), and then reduced RBC count might be compensated for by an increased MCV or cell size.30,31 A proportion of aged horses do have decreased lymphocyte counts compared to younger adult horses and 17% horses aged 20 years and 13% of horses aged 30 years and older showed lymphocyte counts below the laboratory reference range.28,31 This result is likely to be associated with immunosenescence in the older horse, which affects some horses to a greater degree than others (see below). The effect of age on biochemical parameters in the horse is limited. Most studies have reported no significant differences in any of the biochemical parameters assessed between healthy young and aged horses.28,29,32 Some minor variations in several biochemical parameters have been reported between different age groups of Arab mares.33 They found a decrease in alkaline phosphatase (ALP) with increasing age, with the lowest mean values found in mares > 20 years of age. The same study also reported that phosphorus concentrations were below physiological levels in horses ≥14 years old and mean values for blood urea nitrogen (BUN) and creatinine increased with increasing age, although the highest mean creatinine found in the >20-year-old group was still within the laboratory reference range used.33 There are few endocrine changes in aged horses that can be clearly linked with performance. In a small study, thyroxine (T4) and insulin-like growth factor (IGF)-I levels were significantly lower in older mares than younger mares, and further, certain IGF-binding proteins had lowest circulating concentrations in aged mares.34 This led to research on the effects of exogenous administration of equine somatotrophin to aged horses on feed intake, body score, digestion, aerobic capacity, and immunocompetence.35–37
Veterinary aspects of the aged equine athlete
Introduction
Definition and demographics of the aged equine athlete
Physiological alterations associated with aging
Cardiorespiratory changes
YOUNG HORSES
GERIATRIC HORSES
REFERENCES
pH
7.404 ± 0.005
7.428 ± 0.007*
16
PaO2 (mmHg)
101.7 ± 1.6
90.2 ± 2.2*
16
PaCO2 (mmHg)
43 ± 0.7
41.5 ± 1.0*
16
P(A-a)O2 (mmHg)
7.2 ± 1.7
22.1 ± 2.5*
16
Mean cell volume (MCV) (fL)
47.8 ± 2.2
49.7 ± 2.2*
3
Mean cell volume (MCV) (fL)
43 ± 1
49 ± 2*
19
Mean cell hemoglobin (MHC) (pg)
16.3 ± 0.9
17.1 ± 0.8*
3
Lymphocytes (×109/L)
3.1 ± 1.1
2.2 ± 0.8*
3
CD4+/CD8+ cell ratio
4.07 ± 1.54
3.30 ± 1.31*
25
Ionized calcium (mEq/L)
3.00 ± 0.02
3.18 ± 0.05*
16
Ionized calcium pH corrected (mEq/L)
2.98 ± 0.03
3.14 ± 0.05*
16
Thermoregulatory changes
Changes in the musculoskeletal system
Hematological, biochemical and endocrine parameters
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Veterinary aspects of the aged equine athlete
