Why dogs and cats age and how we can influence the process (Proceedings)

In the last AVMA pet owner survey, more than 39 % of the owned dog population were 7 years of age or older and these percentages continue to grow. This change in pet population demographics is due to in part due to several inter-connected causes. Changing owner attitudes toward their pets, has significantly contributed to increases in their pet's life expectancy. More owners than ever consider their pets as "family members" and therefore are more willing to invest the time, energy and resources required to appropriately manage those common chronic infirmities associated with aging. In parallel with the changing human-animal-bonding, the veterinary profession along with industry has responded with significant developments in comprehensive health care options, both preventative medicine and disease management, which were not accessible or affordable a decade ago. With a better understanding of the aging processes and age-related diseases; we are better positioned to increase the longevity and the quality of life for our senior pets. Enhanced senior diets, superior diagnostic techniques, new drugs, safer anesthesia, newer surgery techniques, cancer chemotherapy, improved dental care, pain management strategies, and the use of multimodal strategies in managing chronic progressive age-related diseases, have changed the health care options s now available for senior pets. Once reserved for only referral centers, today, the primary care veterinarians are able to provide their older patients the high quality care that the average pet owner demands and expects. In addition, the implementation of "Senior Care" program in the more progressive hospitals has made significant increases in the "Standard of Care" those hospitals provide to their older patients.

An animal's life can be divided into four stages; pediatric, adult, senior (middle age), and geriatric (senior / super senior). The senior / middle age years represents the transition period between the usually uneventful "healthy" adult years and the traditional "geriatric" age period where serious age related diseases are much more prevalent. This transition period signals the patient's initial decline in physical condition, organ function, sensory function, mental function and immune responses. An appropriate medical response would be to take a better history, perform an age-related physical examination, recommend routine diagnostic screening tests, advocate a senior diet, and start training the clients to look for age-related diseases.

Although the exact time of each stage could be argued, everyone would agree that in general cats live longer than dogs, smaller breed dogs live longer than that the giant breeds and each life stage would have a corresponding chronological difference. Human / Pet Age analogy charts (table 1) reinforce this "time compression" concept and are excellent staff and client educational tools. These relative age charts also emphasizes the concept of comparable "time compression" differences in pets as it relates to wellness testing intervals, disease progression, and chronic drug monitoring.

Aging is the sum of the deleterious effects of time upon the cellular function, microanatomy and physiology of each body system with each organ system aging at a different rate. These biological aging changes manifest in progressive deteriorations in physical condition, organ function, mental function, and immune response, but not necessarily correlating with the patient's actual chronological age. How old an organism is in actual time is referred to as "chronological" aging and should be distinguished from "biological" aging," the relative functional age of each of an individual's diverse organ systems. Because there is little correlation between biological and chronological age, each animal must still be evaluated in depth and as an individual.

Aging is not a specific disease, but rather a complex process influenced by genetics, environment, stress and nutrition. These factors affect progressive irreversible degenerative changes of all body tissues. Aging in dogs and cats is associated with gradual and progressive deterioration in the delicate body systems that eventually results in decreased physiological functions. As the functional organ reserves are gradually lost, the long-term result is a physiological decline of the major organ systems leading to an altered response to stressors, infections and various drugs. At some stage in the progressive decline (these "tipping points"), all physiological reserves are exhausted resulting in overt changes in diagnostic screening tests, biochemical parameters, and/or the onset of clinical symptoms of age-related disease.

Aging is a complex subject influenced by numerous interrelated causative factors. The current causes / theories, aging can be broken down into four general categories; 1. Accumulations of toxic substances or compounds within cells; 2. Cumulative cell damage from ionizing irradiation, environmental pollutants, and more importantly, oxygen derived free-radical-mediated damage; 3. Immune mediated or immune compromised pathology; and 4. Genetic factors. Genetics is estimated to be responsible for about 30% – 35% of a dog's longevity. Every cell is genetically preprogrammed for a specified number of divisions or will die at a predetermined point in time dictated by a specific cellular gene.

Since there is little we can do about the genetics of aging, most of the current age-related research involves the limiting the production and preventing the cellular damage associated with oxygen derived free radicals. The "Free-Radical Theory of Aging" was first proposed by Denham Harmon in 1956. The now proven theory states that the excessive free radicals associated with aging or produced from chronic inflammation hasten have deleterious effects on cells, cellular DNA, cell organelles, and cell membranes.

As animals age, their basal metabolic rate (BMR) decreases. The consequences include weight gain if the there is not a corresponding decrease in caloric intake; less metabolic heat generated resulting in a "cold" patient; and decreased cell turnover in GI tract; and a decrease in metabolism of various drugs. The mitochondrial energy production in less efficient, resulting in excess oxygen derived free radical production. These free radicals damages cells, cellular organelles, cell.

Membranes, and DNA. In addition, there is also an age-related decrease in the body's "natural" free radical scavenger systems. Both factors combined results in an absolute excess in free radicals often referred to as "oxidative stress" . Age related diseases where free radicals play a significant role in the pathogenesis include; CDS, arthritis, cataracts, liver disease, skin changes and some cancers.

Antioxidants are substances that "protect" against the effects of free radicals and are often referred to as "anti-aging" compounds in the media and advertisements. In dogs, anti-oxidants either in the diet or as supplements, decrease cellular damage from free radicals plus support the patient's immune function. However, since they do not have the same mechanism of action, an anti-oxidant "package" is a better strategy than a "pick and choose" plan. Vitamins E, C & Zinc neutralize the damage from free radicals; Fatty acids DHA, EPA and SAMe protect the cell membranes form free radical damage; Lipoic Acid and L-carnitine limits the production of free radicals; and Carotenoids and Flavonoids inactivate the free radical acting as scavengers. To aid in supporting the patient' s immune function, Vitamin E helps with T cell activation, Lutein helps with B cell activation including vaccine recognition and beta carotene increases circulating antibodies and optimizes vaccine antigen recognition. In summary a "package" of anti-oxidants makes more therapeutic sense than the "pick and choose" selection process.

Unfortunately supplements are not regulated; therefore quality assurance issue is always an issue. In addition to quality and balanced age specific ingredients, plus increases in beneficial dietary fiber, most premium senior diets contain some a combination of anti-oxidants, referred to as an "anti-oxidant package". In my opinion, recommending a senior diet for all your "healthy" senior patients is an appropriate practice. Anti-oxidants containing "prescription" diets are also used as part of a multi-modal strategy in managing specific disease processes ie; CDS and arthritis in older dogs.

Human/Pet Age Analogy

One cannot talk about slowing the aging process without a discussion of the role of exercise in the overall well being of the older patient. In people the benefits of regular exercise are well documented. A regular scheduled exercise program should be our professional recommendation for every senior patient in our practice. In dogs, exercise will help minimize stiffness, maintain important muscle mass, will keep the joints limber, and is also beneficial in weight control. Older dogs with difficulty sleeping can benefit from a brisk 10 minute walk before bed time. In addition, several studies have shown the two 10 minute daily walks can increase a patient's mental attitude by increasing the oxygen flow to the brain. The only obvious contraindication to an exercise program would be an unstable joint or cardio-pulmonary compromise.