15 Nutritional considerations for aged horses
Aging has been defined as “an irreversible, progressive and time-dependent decline of overall body functions, resulting from the interaction of genetic and stochastic factors” (Figuieredo et al 2008). One theory regarding the cause of aging and eventual death of multicellular organisms is related to macromolecular damage caused by reactive oxygen species produced by mitochondria, the so-called “mitochondrial theory of aging” (Terman et al 2010). The decay of metabolically relevant structures that occurs with age appears to be linked with total energy intake, in that dietary energy restriction is accepted as an interspecies factor for longevity (as discussed below). Regardless of the cause, aging in mammals is usually associated with changes in body composition, physical strength and physiological function. These are a result of a combination of intrinsic factors associated with the aging process itself, as well as factors indirectly associated with age, in particular degenerative diseases secondary to environmental or physical insults that progress with time and physical inactivity. However, it has been noted in the human literature that “a healthy lifestyle, incorporating a well-balanced diet and physical activity, cannot stop the years advancing, but may play a useful part in a healthful, active and independent old age” (Phillips 2003). We should have a similar goal of optimal nutrition and physical activity for the horses in our care, based on what we know about the aging process and the individual’s circumstances. More importantly perhaps, different dietary approaches may be required, depending on whether the horse is just old in years or is afflicted with one or more age related problems. This chapter will, start by outlining some key aspects of aging in humans and other species, followed by conditions associated with aging in the horse. It will then go on to discuss nutritional and management recommendations for the horse.
Body composition changes as age increases in most species studied to date. A principal change is a decrease in skeletal muscle mass, often referred to as “sarcopenia”, which is often associated with loss of physical strength (Evans 1995, Thompson 2009) and reportedly occurs in all humans, even if they remain active (Phillips 2003). Sarcopenia of aging, in healthy individuals, may be the result of an inability to maintain muscle protein synthesis after feeding (anabolic resistance), which is exacerbated by physical inactivity (Rennie et al 2010). A recent review of age-related changes in body mass concluded that the reduction in resting metabolic rate which favors weight gain due to fat accumulation with concomitant loss of fat free mass may be due in part to reduction in specific organ mass and metabolic rate (i.e., liver) (St-Onge & Gallagher 2010), although in the very elderly (over 75 years) body fat also tends to decrease, probably due to altered dietary habits (Phillips 2003). In humans, aerobic capacity is estimated to decline by about 1% per year and anaerobic capacity, as well as muscle strength, by about 0.5% per year between 30 and 70 years of age (Sutton & Brook 1986). Appropriate exercise training (Sutton & Brook 1986, Raj et al 2010) can reduce the rate of decline and have positive anabolic effects, even in frail (Aagaard et al 2010) and malnourished elderly patients (Hebuterne et al 2001). Resistance, as well as aerobic training, is generally considered to be safe for elderly people to improve strength and increase muscle mass (Graves et al 1994, Phillips 2003), but it is also usually recognized that a sensible exercise regimen throughout life is most advantageous in the reduction or inhibition of age related sarcopenia. Recent work has shown that 80 yr old humans who have partaken in lifelong aerobic or strength training regimens have maximal aerobic capacities and muscle strength that are comparable to that of sedentary individuals around 50 yrs of age (Booth & Zwetsloot 2010).
In elderly people, loss of weight is common and a low body mass index linked with unintentional weight loss is associated with an increased mortality risk (Phillips 2003, Dorner et al 2010), as is obesity (Dorner et al 2010) though for different reasons. The decline in food intake and loss of the motivation to eat, which is seen in many elderly people, may be related to a reduction in the central feeding drive, possibly secondary to increased effectiveness of cholecystokinin (Donini et al 2003), as well as physical problems such as poor dentition or social factors such as isolation. In addition, alterations in taste and olfactory perception have been suggested to reduce feed intake in older people and perhaps lead to inappropriate diet selection leading to inadequate protein and/or calorie intake (Schiffman 1997, Pelchat 2000). Protein-energy malnutrition in turn leads to impaired muscle function, poor wound healing, delayed recovery from surgery and ultimately increased morbidity and mortality (Donini et al 2003).
Chronic dietary restriction, however, appears to be the only nutritional intervention that consistently extends the life span of the animals evaluated (e.g., dogs, Kealy et al 2002; rats, Wohlgemuth et al 2010). Labrador Retrievers fed 25% less, than that consumed by those given ad libitum access, from weaning to 3.25 years of age, after which both were fed for maintenance, had increased median life span and delayed onset of signs of chronic disease (Kealy et al 2002, Lawler et al 2008). Dogs with a body condition score (BCS) of 5 or less (on a 9-point scale) at middle age (6–8 years of age) were more likely to live beyond 12 years of age compared to middle aged dogs with a higher BCS. Looking at the group as a whole, high fat mass and declining lean body mass were strongly correlated with death within 1 year (Lawler et al 2008).
The immune system becomes less competent with increasing age, which is often referred to as “immuno-senescence”. This change in immune function is thought to increase susceptibility to opportunistic infections, cancer and autoimmune conditions (Campbell et al 2004, Henson & Akbar 2009) and may reflect in particular a decline in T cell immune function (Henson & Akbar 2009). Aging is also associated with increased markers of chronic inflammation (Bruunsgaard & Pedersen 2003, Grimble 2003, Salminen et al 2007).
It has been recognized that stress is associated with proteolysis and a negative nitrogen balance that erodes cell mass (Hebuterne et al 2001), and older people may have a reduced capacity to respond in situations that require the mobilization of amino acids for protein synthesis in vital organs and the immune system. Studies also suggest a lack of adaptation to severe malnutrition in the elderly, with a preferential use of their fat-free mass and body cell mass as a fuel (Hebuterne et al 2001). A reduction in maximal mitochondrial ATP production rate (MAPR) and mitochondrial DNA abundance occurs with age in association with muscle weakness and reduced endurance in elderly people (Tatpati et al 2010). In a recent study an 8 hr infusion of branched chain amino acids resulted in an increased MAPR in young adults compared with the elderly suggesting that the response to amino acid supplementation may not be the same across all ages (Tatpati et al 2010). Dardevet et al (2002) showed a defect in the postprandial stimulation of protein synthesis in geriatric rats. Supplementation of leucine overcame this defect, possibly by enhancing the efficiency of translation during the protein synthesis process.
Osteoarthritis (OA) is the most commonly occurring musculoskeletal disease in people, and there is a strong relationship between age and the development of OA (Verzijl et al 2003). From about the age of 30 years, the matrix of minerals and collagen are removed from bone more rapidly than new bone tissue is put down, resulting in weakened structural support and increased risk of fractures, although the rate of age-related bone loss is influenced by many factors including level of physical activity (Phillips 2003).
Age-related differences were found in the microflora composition within the cecum, colon and rectum of beagles, but not in the stomach or small intestine (Hayek et al 1997). Whether such differences occur in the horse and what clinical relevance this might have is currently unknown, but probably warrants further evaluation. Similarly age-related changes in vagal afferents innervating the gastrointestinal tract have been reported in other species (Phillips et al 2010).
The term “geriatrics” was derived from the Greek words meaning “old men” and “medical treatment” and implies that there are health or physical problems, whether age-related or not, that need special attention as one ages. There is, however, considerable variation in the aging process (e.g., humans: Phillips 2003) and there is no set age at which an individual is automatically considered to be “geriatric” as opposed to “aged but healthy”. Some horses remain physically active and healthy well into their twenties and yet others become “geriatric” by mid teens. These individual differences need to be taken into consideration when determining optimal management and feeding practices. For the purposes of this chapter therefore we will refer in general to the aged horse rather than use the perhaps more specific term “geriatric”.
In a paper by Brosnahan and Paradis (2003b), in which the management and activity of 218 older horses was surveyed, the owners perceived their horses as being “old” at approximately 22 years of age. They also found that, in horses aged 16.5 years or more, age became a negative factor in a proposed purchase. The proportion of horses over 20 years of age has increased in recent years. One retrospective study reported that in 1989 only around 2% of the equine referral cases at a university veterinary hospital were over 20 years of age, but this had increased to 12.5% by 1999 (Brosnahan & Paradis 2003a) and approximately 20% by 2003 (Brosnahan & Paradis 2003b). Pony breeds were significantly over represented in the ≥30 years of age group. This progressive increase in old horses presented for veterinary care is probably due to both improved health care and nutrition as well as an increased willingness of owners to maintain older horses (NRC 2007).
In a survey of the horse population in the United States done in 2005, 7.6% of the total population was reported to be over 20 years old (National Animal Health Monitoring System 2006). Previous studies suggested that a quarter of those horses might have been 30 years or older (Paradis 2002). It has been estimated that around 30% of the UK equine population is aged 15 years or older (Hotchkiss et al 2007, Ireland et al 2011a), with 11% between 20–30 years and 2% over 30 years (Ireland et al 2011a). Horses over the age of 40 years are no longer uncommon in the United States (Ralston, personal observation). An Arabian-cross gelding, Elmer Bandit, competed successfully in competitive trail rides until his demise at 38 years (Hayes 2010).
A 2002 study on causes of mortality in older horses in the UK reported that cardiovascular deficiencies were the major cause of death (41%), followed by injuries of the locomotor system (18%, 61% of which were degenerative disorders) and colics (11%) (Leblond et al 2002). In Australia, a survey of horses and ponies over 15 years old (McGowan et al 2010), reported that mortality rate was 9.4 per 100 horse years with the majority being euthanized (89%) due to musculoskeletal disorders (21%), weight loss (13%) and colic (12%). In the general aged horse population, 43% had cardiac murmurs, 50% were lame, 69% had hoof abnormalities and 96% were found to have some form of dental abnormality. A recent study in the UK reported similar findings (Ireland et al 2009, Stevens et al 2009). Ireland et al 2009 found a slightly higher overall mortality rate (11.1 per 100 horse years at risk) and the most frequent reasons for death were musculoskeletal disorders (28.8%), colic (19.5%) and chronic diseases (19.5%). At a UK equine charity in 2009 the average age of the horse requiring euthanasia was 20 years with 66.3% due to osteoarthritis (average age 21.9 years), 11% colic and the remaining 22.7% due to respiratory or hepatic problems, laminitis, ocular lesions and neoplasia (Jarvis 2010).
In the Brosnahan and Paradis (2003a, b) papers, gastrointestinal, musculoskeletal and respiratory problems were the most frequently reported problems in horses ≥20 years of age. A survey in Australia found that owners considered ~43% of horses >20 years to be suffering from a health disorder compared with ~33% in the 6–10 year age group (Cole et al 2005). In a more recent survey (McGowan 2009), owners of elderly horses were most concerned about weight loss (maintaining the horse’s body condition), arthritis/lameness and dental care.
• Neoplasia: Pituitary pars intermedia dysfunction (PPID) and thyroid tumors are common in horses over 20 years of age (Beech 1987, Ralston et al 1988, Dybdal 1997, Brosnahan & Paradis 2003a, Mcfarlane et al 1998, McFarlane & Cribb 2005 ) see Box 15.1. Although prevalence in the general population has not been documented, in one study of 24 horses over the age of 20 years, only one mare did not have neoplastic changes in the pituitary and/or thyroid (Ralston et al 1988). Other tumors such as intestinal lipomas, melanomas, and squamous cell carcinomas also are more commonly found in the older horse (Howarth et al 1991, Brosnahan & Paradis 2003b, Springer et al 2010).
• Increased incidence of colics, especially impactions: In one survey, impactions comprised 88% of colic episodes in the older horse versus 29% of episodes in horses of all ages (Carson-Dunkerley & Hanson 1996), and an increased incidence of strangulating lipomas has also been recorded in aged horses (Edwards & Proudman 1994). In contrast, increasing age was not associated with an increased incidence of colic in a recent study in the UK (Ireland et al 2011b).
• Dental abnormalities: Loss of teeth and abnormal wear patterns are extremely common with advancing age because the teeth of the horse continue to erupt throughout life until approximately 20 years of age, after which time they begin to loosen and are shed (Graham 2002). With age, the enamel ridges of the cheek teeth, considered by many to be essential to efficient mastication of forage, become worn, and the resultant marble-like occlusal surface is known as a “smooth mouth”. This can lead to “quidding” (inadequate mastication with wads of partially chewed feed dropped from the mouth) of long fiber forages, weight loss (Knottenbelt 2003) and potentially an increased risk of choke (Ralston & Breuer 1996).
• Reduced maximal contractile function of the aortic valve: This abnormality is associated with an increase in collagen content of the valve (Bowen et al 2006) and a high incidence of valvular regurgitation (Stevens et al 2009).
• Alterations in respiratory function: Changes include lower partial pressures of arterial oxygen and carbon dioxide plus, potentially, impaired transfer of oxygen across the alveoli (Aguilera-Tejero et al 1998). The incidence of recurrent small airway obstruction is higher in the older horse (Deaton et al 2004, Hotchkiss et al 2007) .
• Increased risk for and severity of bacterial and viral infections, potentially due to decreased immunocompetence. For example, more severe clinical signs of equine viral arteritis infection were seen in aged (over 20 years old) vs. young animals kept in similar conditions (Traub-Dargatz et al 1985). The ability of horses to produce antibodies in response to viral antigens in a vaccine gradually decreases with age (Goto et al 1993, Horohov et al 1999). It has been suggested that low plasma ascorbic acid concentrations associated with pituitary dysfunction may adversely impact immunocompetence (Ralston et al 1988).
• Increased risk of tendon and ligament damage: There is reportedly a reduction in the crimp pattern of collagen fibrils and a decrease in overall fibril diameter associated with advanced age (Patterson-Kane et al 1997, 1998). In the horse, unlike man, the biochemical characteristics of the collagen component of equine cartilage are apparently not influenced by age alone (Brama et al 1999); however osteoarthritis is a very common cause of lameness and poor performance in horses of all ages (Kidd et al 2001) and is commonly observed in the joints of older horses (Brama et al 1999). Degenerative joint disease was reported to be a frequent reason for euthanasia in old horses (Leblond et al 2002). In a report of horses housed at a welfare facility, 55.4% of horses ≥ 15 years of age were treated with a nonsteroidal anti-inflammatory drug (NSAID) because of osteoarthritis, while 62.2% of animals ≥ 20 years and 65.3% older than 25 years were treated with an NSAID (Jarvis 2010).
• Increased risk of death when undergoing general anesthesia for surgery: It was reported that horses over 12 years of age had a higher risk of death during general anesthesia than younger horses (Johnston et al 1995). The reasons were thought to be multifactorial and may reflect the types of surgery older horses are presented for (predominantly emergency colics as opposed to elective orthopedic or cosmetic procedures) as well as overall effects of aging.
• Decline in mare fertility: The reproductive cycles of mares change as they age, probably associated with pituitary dysfunction leading to altered ovarian function and loss of primordial follicles (Carnevale et al 1993). Eventually many old mares will enter a period of reproductive senescence with reduced oocyte viability (Madill 2002).
• Parenchymal and vascular lesions in the brain: A variety of lesions have been reported by Capucchio et al (2010) and another study suggested that there are a number of age related changes in the brain of horses, including lipofuscin deposition (Jahns et al 2006).
Pituitary Pars Intermedia Dysfunction (PPID)
• Of unknown etiology although oxidative stress and/or abnormal accumulations of misfolded proteins such as α-synuclein may contribute to the neuronal damage and cell death. Research does not suggest that systemic oxidative stress or antioxidant failure contributes to the development of PPID but mitochondrial antioxidant dysfunction and increased mitochondrial reactive oxygen species production may play a role.
• Difficult to definitively diagnose in the early stages and especially in those animals without the classic signs of hypertrichosis (previously referred to as hirsutism – recent work has confirmed that there is persistence of hair follicles in anagen). There are profound effects of season on many of the diagnostic tests currently used.
• Hypertrichosis is a common clinical sign (with a 90% positive predictive value for identification of PPID using post-mortem examination as the gold standard). Owner surveys suggest 14–30% of aged horses have hair coat abnormalities.
• Other clinical features include: muscle atrophy, increased risk of laminitis, polyuria, polydipsia, hyperhidrosis, abnormal fat distribution, and insulin resistance (although whether this is specific to PPID horses or aged animals or the horse population as a whole is currently unknown), increased risk for opportunistic infections, behavioral abnormalities (including lethargy), other neurological conditions as well as raised basal ACTH concentrations. Not all are present in every case.
Maximal heart rate during exercise reportedly decreases with age (McKeever & Kearns 2001). It also has been suggested that unfit older horses may not be able to thermoregulate during exercise as well as can younger unfit horses due to changes in resting plasma volume and cardiac deficits (McKeever & Kearns 2001). More recent work has confirmed that aging may compromise the ability to handle the combined demands of exercise and thermoregulation perhaps due in part to a decreased absolute pre-exercise plasma volume (McKeever et al 2010). Aged horses may also have altered endocrine responses to exercise (McKeever & Malinowski 1999).
Mares over 20 years old had significantly lower maximal aerobic capacity and exercise capacity than young mares maintained under the same conditions and levels of training (McKeever & Malinowski 1997, see also Walker et al 2010). In a retrospective study of unfit Standardbred mares (McKeever et al 2010), a statistically significant reduction in maximal aerobic capacity was estimated to have occurred between 18 and 20 years of age, regardless of diet or exercise history. However, the old horses tested at 20 to 30 years old were still able to complete strenuous standardized treadmill tests and no special feeds or training regimens were provided to them. In the recent UK review ~61% of owners reported a decrease in intensity of exercise with increasing age but around 21% of horses were still competing with a median age of 18 years (Ireland et al 2011a) and the majority of horses were still in some ridden exercise. Similarly, Brosnahan and Paradis (2003b) reported that around 60% of the old horse population in the New England region of the USA were used regularly in some form of athletic activity, with 10% of old horses in active competition. This underscores the fact that many old horses can be maintained in good body condition and continue to be used for athletic endeavors well into their 20s and even 30s, despite reduced exercise tolerance.
Prior to the 1980s, it was commonly accepted that horses over 16 years of age (the American Quarter Horse Association definition for “aged”) were in some way geriatric, and the general public perception was that there was little value in applying interventions to reverse or prevent age related deterioration in health (Ralston, personal observations). It was rare to see a horse over 30 years of age and most of these animals were thin and frail. However, recently these perceptions have changed. This is likely to be the result of a number of factors including increased use of more effective, easily administered anthelmintics as well as improved overall health care since the late 1970s and the introduction of “senior” feed formulations in the 1990s. In addition, the increased prevalence of people owning horses purely for pleasure/leisure activities as opposed to commercial purposes such as draught or high level competition may have resulted in more owners being willing to maintain their “faithful companion” as a “pasture ornament” and continue to care for them into old age. Though there is some evidence that reduced frequency of some routine management practices such as farrier care and anthelmintic administration may occur in older horses (Mellor et al 2001), the highly popular use of high priced “senior” feeds is evidence that a significant proportion of owners are willing to spend extra to maintain an old horse’s health (Brosnahan & Paradis, 2003b).
In a survey of 165 non-hospitalized horses over 20 years of age (mean age of 26.5 years, the oldest being 44 years old) initiated in 1998, only 4% were reported by their owners to be in poor body condition (scored on a scale of 0 to 5 with 0 = very poor, 5 = very fat; Brosnahan & Paradis 2003b). In this population of old horses in the Northeastern United States, 68% were considered to be in moderate to good body condition and 28% were reported as fat or very fat. In Australia, owners of horses and ponies aged ≥15 years (mean 20.5 years) reported that 30% were overweight (BCS 3/5) and only 2% were underweight (BCS<2/5; McGowan et al 2010). In a UK survey, ~8% of old horses were reported to be underweight and 10.5% overweight (Ireland et al 2011b). Weight loss or difficulty in maintaining adequate body condition is, however, not uncommon in elderly horses and in the survey study of Ireland et al (2011b) 17% of owners reported weight loss in their horse within the last 12 months. Weight loss may arise for many reasons such as dental abnormalities, renal and hepatic disease, and PPID. Risk of weight loss may be higher in the cold winter months. Horses over 20 years of age were reported to require higher feed intake during winter months, when maintained outdoors in 3 sided shelters in Colorado, than did middle aged mares housed under the same conditions (Ralston et al 1988). As noted above, aged mares also had difficulty with thermoregulation during exercise (McKeever & Kearns 2001) Therefore, ration and general management may be contributory factors in the failure of old horses to maintain acceptable body condition, especially in the winter months. Similarly, with the apparent increase in the prevalence of obesity in the general horse population (see Chapter 28) it may be expected that an increasing proportion of horses over 20 years of age may be overweight with the associated complications of lameness, laminitis and insulin resistance.
Chronic parasitism may impair digestive ability due to scarification of the intestinal mucosa in both the large and small intestines. It has been hypothesized (Ralston et al 2001b) that chronic parasitic scarring of the large intestine contributed to the apparent malabsorption and/or maldigestion reported in some earlier studies of digestion in aged horses (Ralston 1989, Ralston & Breuer 1996). This type of lesion could cause a reduction in the absorption of nutrients if a sufficient percentage of the absorptive surface area is affected, although this has not been documented in horses. Reduced apparent digestion of fiber, protein and phosphorus was reported in horses over 20 years of age, based on studies done in the 1980s and early 1990s (Ralston 1989, Ralston & Breuer 1996). The horses used in the original studies, however, had not had the lifelong benefit of modern intestinal parasite control, having been born in the 1960s and 1970s, before paste anthelminthics were commonly available. The digestive deficits reported were essentially the same as those reported for horses that had had their entire left and right colons surgically removed, leaving only the cecum and small colon for absorption (Ralston et al 1986, Bertone et al 1989). Similar deficits were not found in aged horses studied in the 1990s that had had good gastrointestinal parasite control all of their lives (Ralston et al 2001b). This was recently confirmed by Elzinga et al (2011) in a study that showed no age by diet differences in 8 adult (5–12 years) and 9 aged (19–28 years) mares of similar stock-type breeding, all of which had received regular anthelmintic treatment and had normal dentition. They were evaluated after 5 weeks on three different commonly fed formulated diets (high roughage, high fat and fiber, and high cereal).
The observed reduction in fiber digestion also might have been due in part to abnormal dentition, since dentition was not assessed in the first two studies (Ralston 1989, Ralston & Breuer 1996) and major abnormalities were not present in the third (Ralston et al, 2001b). However, common dental abnormalities such as points or hooks <3 mm in size did not adversely affect digestion of nutrients in middle aged (10 to 15 years old) horses (Ralston et al 2001a) and may not affect fecal particle size or apparent digestibility of nutrients (Ralston et al 2001b, Carmalt & Allen 2008). However, both groups of researchers did note that extremely poor dentition might adversely impact feed intake and may contribute to weight loss through reduced feed intake. Carmalt and Allen (2008) also speculated that there may be a minimum amount of tooth necessary for effective mastication, but that the morphological limit is currently unknown. In addition, there is evidence that age is associated with increasing severity of degenerative changes in the equine temporohyoid joint (Naylor et al 2010), which may predispose horses to temporohyoid osteoarthropathy, which in turn causes difficulty in chewing. More work is needed into the link between dentition, digestion and weight maintenance.