INTRODUCTION
Aging is a poorly understood and complex biologic process that is progressive and irreversible. However, age-associated pathologic changes and their effect on physiologic processes are manageable and possibly even preventable. The challenge to the practitioner is to differentiate normal aging from age-related disease and to minimize the impact of pathologic changes in our patients. With advancing knowledge of age-related diseases, combined with newer diagnostic testing and additional therapeutics options, veterinarians have numerous opportunities to better meet the challenges presented during the aging process and minimize or delay the impact of aging on our patients. The goal of the health care we provide our elderly patients must be to improve the quality of life, not just the length. With increasing veterinary competition and decreases in vaccine-associated income, addressing the needs of the aging patient can also be a significant profit center.
THE PROCESS OF AGING
Definition of Aging
Numerous definitions of aging exist. Dr. R. T. Goldston defines aging as “a complex biologic process resulting in progressive reduction of an individual’s ability to maintain homeostasis under internal physiologic and external environment stresses, thereby decreasing the individual’s viability and increasing its vulnerability to disease, and eventually causing its death.”4
Aging actually begins at birth. But aging is not a disease, rather the cumulative effects of time upon the physical, mental, and various organ systems. A patient’s age simply defines a point in time in that animal’s life. But that life stage dictates any client education issues, the preventative health care program, and disease risk assessment in that patient.
Signs of Aging
The common age-related changes in dogs and cats include dryness of mucosal tissues, progressive degeneration of organ function (lack of reserve), tissue hypoxia, cellular membrane alterations, decreased enzyme systems, decreased immune surveillance, and personality/behavioral changes (Table 1). These progressive changes are normal, expected, and represent the complex interactions aging has on bodily functions. Aging changes proceed at varying rates in different organ systems of the animal and may vary in degrees of clinical significance between individuals of the same age. In general, aging changes seen in older animals are fairly predictable. For example, the greying of the muzzle and thinning of the hair coat are consistent and early indicators of aging. Identifying various internal organ dysfunctions before they are blatantly obvious is more challenging.
LIFE STAGES
An animal’s life can easily be divided into four stages as follows:
Pediatric
Maintenance
Senior
Geriatric
However, disagreements arise when assigning specific age parameters to each life stage for cats and especially dogs. The various breeds have different rates of aging, with variable risk factors that results in significantly different life expectancies.
The pediatric life stage begins at birth and ends around one year of age. The next life stage is the maintenance period ending at around seven years of age. The senior period represents the life stage of transition between the adult and geriatric. The senior life stage signals the onset of detectable, progressive declines in physical condition, organ function, sensory function, mental function, and immunity.
In establishing age-related criterion for determining the onset of the geriatric period, size was the predominant factor according to a survey of ACVIM (American College of Veterinary Internal Medicine) and ABVP (American Board of Veterinary Practitioners) diplomates by Dr. Goldston. This group defined the beginning age of the geriatric patient3,4 as follows (also see Table 2):
Small breed dogs — 11.5 years
Medium breed dogs —10 years
Large breed dogs — 8.9 years
Giant breed dogs — 7.5 years
Cats — 12 years
The comparative age analogy chart (Table 3) provides a basis for client education of aging and age-related risks that allows the owner to better connect with the pet’s relative age.
The oldest dog on record was a mixed breed that lived to be 29 years old. The oldest cat was 39 years old.5 Although the maximum life span for most dogs is around 27 years, few ever reach age 20. In fact, the average life span of all dog breeds is reported to be only 13 years with large and giant breeds having a shorter life expectancy than small breeds.2 Cats housed indoors have an average lifespan of 14 years versus 3.5 years for cats that are kept outdoors1 Clearly there are a number of factors, most notably genetics, environmental, preventative health care, nutrition, and the human animal bond that can significantly influence aging and ultimate longevity.
CAUSES OF AGING
The underlying causes of why animals age are complex and have not been definitively determined. Recent advances in cellular and molecular biology have contributed significantly to our understanding of the fundamental mechanisms of aging in the domestic dog and cat and have given rise to four prominent subcelluar theories for the aging process (Table 4).
The cumulative cellular damage theory suggests that ionizing irradiation of genetic material (DNA, RNA), plus environmental pollutants and oxygen-derived free radical damage to subcelluar organelles underlies the aging process. This would support the use of antioxidants and/or free radical scavengers in an attempt to slow the aging process. Accumulations of intracellular substances such as lipofusion, glycosylated compounds, and beta amyloid in the central and peripheral nervous system affects the aging process in the waste product theory. The immunologic theory suggests various immunologic events such as antigen/antibody reactions and/or deceases in immune surveillance of infectious agents or neoplasia may also hasten the aging process. The genetic theory says that aging change for each cell is due to an intrinsic pre-programmed genetic code determined at conception. This specific gene acts as a "biologic clock" measuring either time or some repetitive cellular process such as cell division. At the predetermined time an alteration in the cells biochemical pathways results in the cellular dysfunction and eventual death.3
CHANGES DURING AGING
Common to all aging systems is progressive and irreversible change. However, the specific rate of aging change is less predictable and determined by numerous factors including genetics, nutrition, environment, immunity, stress, and disease.2 Knowledge of the common age-related changes and age-related pathologic changes allows the veterinarian to plan and manage more effective health care programs for elderly pets.
In the older dog, the integumentary system undergoes normal aging changes associated with cellular atrophy of the epidermis, dermis, and adnexal structures. Follicular atrophy results in fewer hair shafts; in addition, atrophy of the pigment-producing melanocytes results in greying of the remaining hairs. The hair coat becomes dry, scaly, and dull from decreased quantity and quality (more waxy) of sebum production. The skin thickens and becomes less pliable with age. The nose and footpads become hyperkeratotic and the nails are longer and more brittle.7,8
Dental disease is the most common age-related change in the alimentary system. Dental calculus and tartar accompanied by varying degrees of periodontal disease and tooth loss also increases with age. Fatty infiltration of the salivary glands results in decreased saliva. Appetite may be suppressed from taste bud atrophy and decreased sensation of smell. Esophageal function is decreased by neuronalloss. The esophageal muscle loses tone and may fail to adequately relax at the cardia. The ability to secrete hydrochloric acid by the parietal cells is decreased as is gastric epithelial turnover. Hepatic function may decrease as intracellular fat droplets accumulate in the hepatocytes and the number of hepatocytes decrease. Colonic motility may decrease with age and,combined with drier colonic mucosa, predis-poses the older pet to constipation.7,8
A gradual decline in renal function is a normal part of aging. Because the kidneys have such a reserve capacity,any decline in functional nephrons may be insignificant. However, renal failure is the third leading cause of death in dogs and number two in older cats. Glomerular filtration rate may decrease as a result of decreased renal perfusion, decreased systemic pressure, and altered permeability of the glomerular membrane.7,8 Evaluation of renal function including a BUN and urine specific gravity is an important component of any Senior Care™ program.
Muscle mass loss is a normal finding in the geriatric patient. This change is associated with a combination of inactivity, progressive atrophy of existing muscle cells, a decrease in muscle cell numbers due to fibrosis, reduced oxygen transport to the muscles, and a decreased sensitivity to ATP. Some additional muscle atrophy can also be attributed to decreased dietary protein.
The quality of articular cartilage is also influenced by aging. The cartilage matrix is composed of proteoglycan macromolecules (hyaluronic acid and glycosaminoglycan [GAG]), type I collagen, and 80% water. In the older patient, the production of GAGs, chondroitin sulfate, and type I collagen is diminished by the decreased numbers of chrondrocytes. Car-tilage homeostasis shifts toward catabolism and the aging cartilage has limited ability to regenerate and main-tain itself in the face of the cumulative effects of ongoing trauma. The tensile strength and shock absorption qualities are diminished resulting in cartilage splitting, cartilage fragmentation, inflammation, synovitis, and microfractures in the subchondral bone.4 Primary idiopathic osteoarthritis is the eventual outcome.
Mild hypothermia is often problematic in the older dog and cat and is usually attributed to less subcutaneous fat and a decreased basal metabolic rate, combined with lack of peripheral vaso constriction. Hypothyroidism can also be a common cause. The old dog and cat become heat seekers. The resulting behaviors may manifest as reclusiveness, periodic trembling, reluctance to go outside for eliminations, and/or sleep cycle disturbances.
Personality and behavioral changes can also be associated with age-related functional changes in the central nervous system. Elderly patients desire more attention, are more jealous, are more irritable, are less mentally alert, dislike change, and have altered sleep cycles. Some have clinical signs related to significant impaired cognitive mental function commonly referred to as senility, dementia, or cognitive dysfunction syndrome (CDS).
The pathogenesis of CDS is a progressive age-related neuronal degenerative disease associated with lipofusion or beta amyloid plaque formation leading to neuronal dysfunction and eventual neuron death. In addition, other neuronal toxins, neuronal cellular infiltrates, cerebral hypoxia associated with arteriocapillary fibrosis, and endothelial proliferation, or the lack of specific neurotransmitters (ACH, GABA, dopamine) can all adversely affect neuronal function resulting in personality changes and abnormal behaviors in older dogs and geriatric cats.6
Failing eye sight and hearing loss are common findings in older dogs. The most common cause of hearing impairments in dogs is acquired neuronal loss associated with degeneration of the cochlea.
Gradual increases or decreases in the overall body condition score are often associated with aging. These changes are often insidious and not easily detected by the owner. Regular weight monitoring combined with body condition scoring allows the veterinarian to better assess minor changes in overall weight status.
According to a study funded by the Morris Animal Foundation for Animals, in the older dog, the four most common causes of disease-related death were 1) cancer, 2) cardiovascular disease, 3) renal failure, and 4) epilepsy and hepatic diseases (equal number of occurrences). In cats, the top four fatal diseases included 1) cancer, 2) renal failure, 3) cardiovascular disease, and 4) diabetes mellitus.9
CLINICAL MANAGEMENT OF SENIOR PETS
From a clinical perspective, how do we transpose this knowledge of aging to our older patients? Progressive practices are using a Senior Care™ program as the vehicle to provide high-quality health care to their older patients. The cornerstones of a Senior Care™ program include client education programs, programs for prevention and early detection of age-related problems, and timely and appropriate medical intervention (Table 5).
One major obstacle in delivering quality health care to the older pet is the lack of knowledge by the owners of the aging process and the early warning symptoms of disease. Many of the age-related diseases have such an insidious onset, most owners do not recognize the problem until it is well advanced. Sadly, at this point in the course of disease, many of the early management alternatives/options may no longer be viable choices.
Unfortunately once observed, some uninformed owners fail to seek veterinary assistance for a particular age-related problem, falsely assuming that their pet is “just getting old” and that nothing can be done. As a consequence, those owners and their pets endure chronic progressive problems that might have been preventable, curable, or at least effectively controlled in the early stages.
A major focus of any geriatric client education program must be emphasizing the awareness of the aging process and the future impact aging will have on the pet’s health care. Empowering our clients to be more active health care partners is a must. Implementing a Senior Care™ program in the clinic provides a way to educate clients on the early warning signs of disease and treatment options.
One of the most significant advances in human geriatric medicine has been the introduction of routine health screening coupled with empowering the patient to be more responsible for their health care. Routine screening improves the early identification of a problem. As a result, the patients are being treated earlier and are living longer. The veterinary profession does a very good job of managing older, chronically ill patients. Instead of reacting to age-related disease, a Senior Care™ program in the clinic is a proactive approach to health care advocating routine age-related physical examinations and a program of diagnostic screening of apparently “healthy” older pets. A program of early detection before disease symptoms are obvious to the owner, before sedation or general anesthesia is administered, before specific medications are prescribed, or before chronic medications are refilled. This early intervention strategy actively probes for problems before they become profound, expensive, or even uncontrollable.
Finally veterinarians must be able to provide timely and appropriate medical options to manage age-related diseases while constantly looking for ways to couple any medical advances and newer technologies to their patients. There is a continual challenge to stay abreast of current knowledge of aging and the pathophysiology of multiple organ dysfunction. Various drug interactions must be throughly investigated when treating animals requiring multiple drug therapy. Anesthesia of the older patient requires special considerations prior to, during, and following a procedure. Newer safer anesthetics and anesthetic monitoring equipment are proven investments in progressive small animal practices. Meeting the specific nutritional demands of aged patients requires continual updating as newer nutritional advances important in modifying aging and in specific disease-managing diets are made available.
SUMMARY
Age is not a disease but rather manifestations of the body’s decreased repair (lack of reserve). Detecting these changes and attempting to modify their impact on our patients is what geriatric medicine is all about.
William D. Fortney, DVM
Department of Clinical Sciences
Kansas State University, Manhattan, Kansas, USA
