No one knows exactly why dogs or humans decline in their abilities when they age. One theory suggests that as the genetic material (DNA) reproduces itself in each new cell, the successive transcriptions become less accurate, sort of like making copies of copies of copies on a photocopier, where each one gets progressively grainier and harder to read. Damage to the DNA can also come about due to natural radiation damage from cosmic rays and more terrestrial sources (such as breathing in air pollutants or fumes from certain solvents) which in turn might lead to faulty enzyme production. When this happens it will often result in cell deaths in the nervous system and elsewhere. Other theories of aging blame simple wear and tear, suggesting that various physical and neural systems break down from frequent use, and may break down even faster if they are put under stress. Other theories suggest that aging results from the accumulation of metabolic waste products in the cells or the increase in unstable chemicals (free-radicals) that interact with molecules in the cells and interfere with their functioning.
Regardless of the source of aging effects, the brain and nervous system of dogs (and people) change markedly as they age. Old dogs have smaller, lighter brains than young dogs. The change is quite significant and the older brain might be up to 25 percent lighter. It is important to note that this change is not necessarily due to brain cells dying off. Actually, we mostly lose parts of the nerve cells, the branches (dendrites and axon filaments) that connect with other nerve cells. These connections to other cells start to break down with age. If we could consider the brain as a complexly wired computer, it would be the same as if various circuits in the central processor simply stopped functioning because connections were broken. For the most part, it is the loss of these connections that reduces the size and the weight of the brain.
With age, there are also chemical changes occurring in the brain that affect behavior, memory, and learning. In dogs and humans the mitochondria, little strand-like structures in the nucleus of cells, are responsible for converting nutrients into energy. As dogs and humans age, mitochondrial efficiency decreases. The mitochondria begin to act as if they have become leaky, since they now begin to release "free radicals," chemicals that oxidize compounds essential for normal cell function. The loss of these compounds places the cell at risk. As the tissues degenerate, protein deposits called "amyloids" accumulate in the brain. High levels of amyloids, especially when associated with clusters of dead and dying nerve cells, are taken as part of the evidence that the individual is suffering from Alzheimer's disease. Studies conducted at the University of Toronto by a team of researchers including psychologist, Norton Milgram, have shown that dogs with high levels of amyloids in their brains have poorer memories and difficulties learning new material, especially if it involves more complex thinking and problem solving. This equivalent to Alzheimer's disease in dogs is called Canine Cognitive Dysfunction Syndrome.
Like Alzheimer's disease, physical evidence, found only in autopsies, reveals the same sort of degenerative brain lesions in dogs and humans. With age, dogs, like humans, naturally accumulate deposits of beta amyloids. This starch-like protein builds up, becomes waxy, and forms plaque. As plaque builds up, it clogs the brain and inhibits the transmission of signals from the brain. In both Alzheimer's and Canine Cognitive Dysfunction, the level of this accumulated plaque predicts the severity of the mental or cognitive impairment.
There are some noticeable changes in dogs with Canine Cognitive Dysfunction Syndrome, which, like Alzheimer's, are not a normal part of aging. The main symptoms can be easily summarized by the acronym DISH which is short for Disorientation, Interaction changes, Sleep changes, and House soiling.
Signs of Disorientation often include:
Symptoms of decreased Interaction skills include:
Typical changes in Sleep patterns include:
Sometimes symptoms include apparently forgetting Housetraining:
Observing such changes in a well-loved older dog can be very distressing for his owner and family, however the good news is that there are some things that can be dog to slow and perhaps even reverse the effect Canine Cognitive Dysfunction Syndrome. It has long been known that keeping your dog mentally active can help (see Building a Better Brain for Your Dog). However in my next blog entry I will tell you about a surprisingly simple way to make your dog's brain healthier and more efficient and to stave off the effects of aging on your dog's mental abilities.
Stanley Coren is the author of many books including: The Modern Dog, Why Do Dogs Have Wet Noses? The Pawprints of History, How Dogs Think, How To Speak Dog, Why We Love the Dogs We Do, What Do Dogs Know? The Intelligence of Dogs, Why Does My Dog Act That Way? Understanding Dogs for Dummies, Sleep Thieves, The Left-hander Syndrome
Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission.