Aging process: Why pets age and how we can influence the process (Proceedings)

In the last AVMA pet owner survey, more than 39 % of the owned pet population were 7 years of age or older and these percentages continue to grow. This change in pet population demographics is in part due to advances in the control of infectious disease, improvements in nutrition, newer surgical techniques, newer medications for controlling chronic disease, plus more comprehensive understanding of the aging process. Changing owner attitudes toward elderly pets (human animal bond / humanization of pets), has also contributed to potential increases in life expectancy.

An animal's life can be divided into four stages; pediatric, adult, senior, and geriatric. The senior years represents the transition period between the adult and the traditional geriatric period. Although the exact time of each stage could be argued, everyone would agree that smaller breeds live longer than that the giant breeds and each life stage would have a corresponding chronological difference. Human / Pet Age analogy charts reinforce this concept and are excellent client education and marketing tools. The chart also emphasizes the idea of comparable "time compression" in pets as it relates to disease progression, wellness testing intervals, and chronic drug monitoring. (See the age analogy chart).

Human/Pet Age Analogy

Aging is a complex subject influenced by numerous interrelated causative factors. The current causes / theories of 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, oxygen derived free-radical-mediated damage, and/or environmental pollutants; 3. Immune mediated or immune compromised processes; and 4. Genetically preprogrammed cell death initiated by a portion of the gene responsible for the cell's lifespan. Every cell is genetically preprogrammed for a specified number of divisions and will die at a predetermined point in time dictated by a specific cellular gene.

"Old age" is not a disease but represents the effects of time upon the physical, mental, and internal organs, but bears no absolute relationship to actual chronological age. Unfortunately each internal organ system will age at a different rate. While it is appropriate to use age as a benchmark of organ decline, any assessment of a patient health should be based on a complete health screening of mental and organ function because the organs degenerate at different rates.

The generalized changes associated with aging changes include dryness of all tissues, progressive degeneration of organ function, tissue hypoxia, cellular membrane alterations, decreased enzyme systems, decreased immune competence, and definite personality alterations. These progressive changes represent the complex interactions aging has on bodily functions however considerable individual variation exists even with litter mates. Three reference books one should consider owning when dealing with the older patients include Geriatrics and Gerontology of the Dog and Cat by Dr. Hoskins, the Veterinary Clinics of North America, 2005 Geriatrics edition, and Veterinary Drug Handbook by Plumb.

Gradual changes in the overall body condition are not easily detected by the owner. Regular weight monitoring combined with body condition scoring allows the veterinarian to better assess minor or earlier changes in overall weight status especially in multi-doctor practices. Based on ideal weight, older patients can be placed in three classifications; pets within the normal weight range, animals that are too thin, and patients that are overweight or obese. Decreases in activity and basal metabolic rates without a corresponding decrease in caloric intake produces the overweight pet.

Several studies have shown that geriatric pets are often too thin. The issue is whether this low body condition score is the result of some underlying disease state or just "normal' aging seen in dogs, cats and humans. Common age-related causes of insidious weight loss include metabolic diseases, cardiac failure, cancer, and maldigestion. Decreases in muscle mass and partial age-related inappetance can be "normal". Despite the category, a decreased or picky appetite needs to be investigated. Pathologic etiologies associated with a decreased appetite include dental disease, metabolic dysfunctions, a gastrointestinal disorder or cancer. In addition to the normal loss of olfactory neurons, normal loss of taste buds, masticatory muscle atrophy, a lack of sufficient saliva to swallowing dry food, or "senility" can also contribute to deceased appetite in the "healthy" older pet. Feline food aversion is often associated with environmental changes, palatability issues, nausea from IBD, chronic pain, stress, and/or medication administration with food.

Following a thorough physical examination and medical evaluation attempting to rule out cancer cachexia or various metabolic disease, this author advocates one or more of the following non-medical options for encouraging the pet to eat; more frequent meals; hand feeding the pet; adding water to the dry food; feeding canned food; warming the food; adding commercial flavor enhancers; and mild exercise prior to mealtime. Bowls used for feeding elderly cats should be wide and shallow so that the sides do not touch the cat's whiskers. Dietary fat helps make foods more palatable, an important consideration in older animals that may have a diminished sense of smell or taste. Benzodiazepine derivatives are commonly used to stimulate appetite and are effective in up to 50% of patients. Unfortunately, there uses have been associated with drug related hepatopathies especially in cats. Diazepam may be used in dogs or cats and is most effective when administered intravenously. Fresh, palatable food should be offered immediately following administration (0.2-0.5 mg/kg administered intravenously, a maximum dose of 5 mg per patient). Feeding usually starts within one minute and may continue for up to 20 minutes. Longer lasting oral benzodiazepam derivatives such as Oxazepam produce results within 20 minutes following oral dosing (2.5 mg per cat administered orally). Cyproheptadine (Periactin) 2-6 mg/dog PO BID and 1-2 mg/cat PO BID has been used as an effective appetite stimulant but may take several days before the desired effect. Alpha-2a Interferon (Plumb) has also been used effectively for long term appetite management as have anabolic steroids. In cats and humans weekly B complex injections has also been shown to be effective in increasing appetite in some cases.

Effective thermoregulation is deceased in the aging dog. Their decreased ability to pant and decreased cardiac output make them more prone to heat stroke. Age-related cold intolerance is often attributed to less Sub Q fat, decreased basal metabolic rate, decreased cardiac output and decreases in peripheral vaso-constriction. Cold intolerance can also be a sign of hypothyroidism. The resulting response to cool ambient temperatures may manifest as behavioral problems including reclusiveness, trembling, and reluctance to go outside for eliminations, and/or sleep cycle disturbances. In addition to suggested environmental changes and thyroid evaluation, warm bedding and outdoor garments may help alleviate some of the abnormal behavior.

The integument changes are the most obvious to the owner. The skin and hair undergoes some degree of pigment loss (graying), hyperkeratosis, follicular atrophy, and decreased sebum production. The nails become longer and more brittle. The dryness of the skin and coat can be helped with increased grooming, less bathing, post-bathing conditioners, topical emollient (oils) or humectants (moisture) sprays, and essential fatty acid nutritional supplements.

Progressive loss of muscle mass is a normal finding in the older pet especially the geriatric patient. This change is related to a combination of inactivity combined with a decrease in muscle cell numbers due to fibrosis, atrophies of existing muscle cells, and decrease sensitivity to ATP. Additional muscle atrophy can also be attributed to decreased dietary protein when using a "low protein" prescription diets. Unfortunately this muscle wasting may be responsible for or exaggerate hind limb weakness. The resulting difficulty climbing stairs or jumping can be confused with the symptoms of arthritis and disuse atrophy. In dogs systemic or oral anabolic steroids combined with increased dietary protein and a sensible exercise program has been advocated in management of the generalized muscle weakness and muscle wasting conditions. The use of creatine / phosphocreatine supplements is also indicated.

Decreases in hearing and vision are common age-related changes that may have associated changes in behavior patterns. Unfortunately these sensory dysfunctions may not be recognized as such and are often mis-diagnosed or mis-interpreted as senility or Cognitive Dysfunction Syndrome as the symptoms are often similar. Hearing loss associated with decreased sound wave conduction from the external ear to the cochlea (conduction deafness) can be helped with amplification. However specific loss of nerve function (neurogenic deafness) is the most common cause of deafness in older dogs. The use of a high frequency dog whistle is a temporary solution until the higher frequency recognition is finally lost. The normal aging changes in the lens called nucleus or lenticular sclerosis should be always differentiated from opacities of the lens/capsule called cataracts. All cataracts should be staged (immature, mature or hyper mature) to better assess the cataracts impact on vision and the appropriateness of lens extraction. Neither nuclear sclerosis nor an immature cataract should affect a pet's vision.

Progressive functional renal nephron loss may impact drug and anesthetic selections. Advancing progressive renal nephron loss can be easily determined by urine specific gravity, BUN/ Creatinine, and perhaps persistent micro-albuminuira. Once more than 66% of renal function is lost, the patient losses the ability to concentrate their urine. The bun/creatinine start to rise after more than 75% real function is lost. With the remarkable adaptation of nephron in slowly progressive renal disease, the patient can survive and a very small percentage of kidney tissue. . This baseline testing is also helpful in trending, the concept of repeated monitoring / charting of a biological parameter that is helpful in "predicting" the patient's future health status.

Since significant renal proteinuria (persistent microabuminuira or overt proteinuria) can be the first laboratory finding with chronic progressive real disease. A complete urinalysis with specific gravity, dip Stix, and sediment examination is an essential part of a senior care program. Microabuminuira using ELISA based technology is a very sensitive test but not very specific. Conversely a urine protein/creatinine ratio while not as sensitive a test, is much more specific. Both Heska and IDEXX have proteinuria alogrhythms charts available that include step by step workup and management options.

Liver function maybe altered by fatty infiltration and/or hepatocyte damage as indicated by elevated alkaline phosphates. Mild elevations (2-3X high normal) in ALP could be considered normal in the older dog. The implications are at what point should the "healthy" patient with an elevated ALP be worked up or just periodically monitored (trended)?

Decreases in both cellular and humeral immunity are associated with the aging process and explain the increase prevalence of tumor growth and infection rates in the senior and geriatric patients. That said, what should the vaccination schedule be for the older pets.... none, more, less or the same?

The four most common causes of death in older dogs, based on a 1993 study funded by the Morris Animal Foundation for Animals, re cancer, cardiovascular disease, renal failure, with epilepsy & hepatic diseases tied for 4th. In cats, the top four fatal diseases in order include cancer, renal failure, cardiovascular disease, and diabetes mellitus. These facts are important when educating your clients and staff on the early warning signs of serious life shortening common diseases in the elderly pet and in determining what tests should be included in a geriatric health screening. Considerable research in the area of interrupting the aging process is ongoing. Technologies such as gene splicing may someday allow each of us to live longer. Until then, exercise, senior diets and antioxidant supplements are generally recognized as valuable anti-aging strategies.