The Effects of Aging on Dogs

Like us, dogs don’t stay young forever – they age. While some aspects of getting old may not be much fun, getting old is not all bad. Each stage of life has its joys, pleasures and drawbacks. Middle age for a dog, which is between 5 and 9 years of age, is a kind of gray zone during which the dog is busily engaged in the process of life without any particular physical or mental deterioration to hamper him. But somewhere towards the end of middle age, dogs start acting and feeling their age.

The effects of the aging process are both physical and mental. Physically, structural and functional changes occur in virtually all organ systems throughout the body, affecting vision, hearing, stamina, susceptibility to drugs and locomotor activity. Mental changes are secondary to decreasing brain size and a reduced number of brain cells. In some cases, canine Alzheimer-like changes hasten deterioration. Aging does not affect all dogs in precisely the same way. Some dog breeds, and some individuals, are more successful agers than others. Some dogs, at the age of 10 years, may have no noticeable physical or mental incapacitation. Others of the same age, however, are already handicapped by age-related internal organ failure, failing senses or orthopedic problems.

Age-Related Physical Changes

• The Kidneys. Kidney function in dogs is often impaired in old age. With advancing age, blood flow to the kidneys decreases, there is a loss of filtering cells (nephrons), and impairment in resorptive processes in the nephrons. The result of all this is a failure of the kidneys to concentrate urine, so that older dogs with this type of deterioration will necessarily have to drink more and, consequently, produce larger amounts of more dilute urine. It is extremely important to make sure that such dogs have constant access to water so that they do not go into kidney failure. Some special kidney diets that contain low quantities of high quality protein can help sustain dogs in the borderline kidney failure.
• The Liver. Although some tests of liver function show progressive deterioration with age, most dogs survive to a ripe old age without this loss affecting them in any noticeable way. However, in some dogs, fat accumulates in the liver, sometimes secondary to other diseases such as diabetes mellitus (sugar diabetes) and hyperadrenocorticism. This can result in an increased size of the liver with higher levels of liver enzymes in the blood. Liver cirrhosis is also a disease of the older dog because of its chronic and progressive nature.
• Thyroid Glands. Hypothyroidism has been reported to be the most common endocrine disease in the dog. Most cases are breed-related, with an early onset (2 to 5 years), but in other instances, hypothyroidism does not cause problems until the dog is aged. Hypothyroidism will cause increased shedding, bilateral hair loss, a dry lusterless coat, increased susceptibility to infections, weight gain, and heat-seeking behavior, to name a few of the clinical signs.
• Adrenal Glands. The adrenal glands are affected in various ways by aging. The glands produce hormones involved in the regulation of blood sugar, electrolytes and stress, and serve other functions. Elderly patients under continued stress can suffer adrenal exhaustion. The opposite, hyperadrenicorticism, is a relatively common endocrine disorder of middle aged and older dogs. The latter causes signs such as muscle weakness, potbelly, hair loss, increased thirst, and increased urine production. If hyperadrenocorticism is diagnosed, it can be treated.
• Pancreas. Diabetes mellitus is usually a disease of the older dog. Complications associated with this disease include increased thirst and urine output, wasting away of muscle, and liver disease. This type of diabetes can be controlled using dietary control and insulin, if necessary.
• Pituitary Gland. Reduced production of growth hormone is supposed to be one of the main reasons for the overall aging process. In people, but not yet in dogs, injections of a growth hormone are given to delay the aging process.
• Musculoskeletal System. While young dogs appear strong, well-muscled and can run like the wind, older dogs usually show muscle wasting and are often handicapped by arthritis or intervertebral disc disease. Analgesics and, if indicated, various surgical procedures can bring many dogs relief.
• Cardio-respiratory System. As you might expect, both components of the heart and lung system are affected adversely with increasing age. A particularly common cardiac disease of older dogs is one in which the margins of the heart valves thicken (endocardiosis). This condition leads to cardiac murmurs and, functionally, to cardiac insufficiency. Meanwhile, aging affects the lungs, such as thickening of the walls of the small airways, leading to reduced efficiency of gaseous exchange.
• Special Senses. Dogs’ eyesight becomes poorer as they get older, due to age-related changes in the eye itself and in the processing of visual images centrally. The most common ocular aging change of all, lenticular sclerosis, in which the pupil of the eye appears grayish, does not significantly affect vision at all. Cataracts, however, which are also more common in elderly dogs, do impair vision, particularly when the dog is in bright light and his pupils are constricted.

Dogs’ hearing deteriorates progressively with age so that many older dogs appear not to hear you when you issue commands, and they do not respond to outside sounds that formerly would have aroused them. Loss of hearing can be either peripheral, due to changes in the ear itself or, as with failure of vision, related to changes in central processing.

• Central Nervous System. Dogs’ brain weight decreases with age primarily because of neuronal death in the cerebral hemispheres. Functionally, there is decreased production and increased destruction of central neurotransmitters. If canine cognitive dysfunction is involved, there are plaque-like accumulations of beta-amyloid in the brain.
• Behavioral Changes. Because of general central nervous system changes mentioned above, dogs progressively slow down mentally as they age. They become less interested in things around them, less reactive to things going on, spend more time sleeping, and tend to walk whereas before they might have run. “Normal” aging changes in dogs are not usually incapacitating but merely produce a gradual decline in mental function, which can seem quite appropriate. Dogs with canine cognitive dysfunction, however, may become disoriented, have reduced interactions with people and other animals, suffer sleep disturbances, and eventually become incontinent. Affected dogs can be significantly helped by treatment with a monoamine oxidase inhibitor called seligiline [Anipryl ®]. Seligiline can produce a quick turnaround in the majority of dogs with canine cognitive dysfunction and stands to provide affected dogs with a better quality life and longer life expectancy.