IMPROVED veterinary care and nutrition and safer home environments are allowing more and more dogs to live well into their senior years. Indeed, an estimated 25-50% of pet dogs in Europe are more than seven years old.
While we are all grateful to have our pets living longer lives, we are seeing a higher incidence of diseases associated with ageing than we were seeing several decades ago. We are also seeing a higher incidence of behavioural problems that are related to ageing.
When are dogs classified as senior? Unlike humans who fall into nice neat categories with “elderly” being 60 to 74 year olds and “aged” being all those over 74, life stages for dogs vary with breed and size.
Dogs over 40kg are physiologically senior at six years of age, whereas most dogs under 40kg are considered seniors at age seven. Recent studies by Milgram et al indicated that beagle dogs had age-related brain changes as early as seven years of age.
Increased lifespan appears to be directly associated with an increase in age-related diseases such as cancer and renal failure and age-related changes in the endocrine, cardiovascular and musculoskeletal systems. Ageing is also associated with an increased likelihood of cognitive decline as well as the development of dementia-like disorders such as canine cognitive dysfunction (CCD).
Dogs show age-dependent decline in learning and memory, which varies as a function of task and in many respects parallels the pattern of cognitive decline seen in aged humans.
Behavioural changes in senior dogs with cognitive impairment can be classified into five general categories:
- disorientation in the home or garden;
- changes in social interactions with human family members;
- loss of house training;
- changes in the sleep-wake cycle;
- altered activity levels.
The first three categories clearly reflect losses of memory and learning. Changes in the sleep-wake cycle would appear to reflect disruption in the normal circadian rhythm; however, wild dogs usually show a crepuscular pattern of activity (active at dusk and dawn) and adaptation to the diurnal pattern of humans by domestic canines probably also represents a type of learning which can be lost with cognitive impairment.
In addition to learning ability, memory can also be compromised in older dogs. Forms of memory that appear to involve age-sensitive mechanisms include spatial memory (ability to remember the location of a food reward) and object-recognition memory (ability to recognise an object seen 10 to 120 seconds previously). However, the variability in performance of these tasks is extensive.
Older dogs appear to fit into one of three categories as determined on the basis of learning ability and memory testing. Dogs may be:
- unimpaired or successfully ageing;
- age-impaired;
- severely impaired (CCD).
Careful questioning of an owner is the best way for veterinarians to detect early signs of cognitive impairment in an older dog. Veterinarians are absolutely central in identifying and treating dogs with any level of cognitive decline.
How common?
Due to the ageing dog population, the prevalence of cognitive decline is increasing significantly. In one study by Neilson et al, the prevalence of cognitive and behavioural impairments was found to increase significantly with age.
At age 11-12 years, 28% of dogs exhibited impairment in one or more of the five categories listed above, while 10% exhibited impairment in two or more categories. By age 15-16 years, a huge 68% had impairments in one or more categories with 35% having impairments in two or more. A related study of dogs aged 11 to 14 years found that over a six to 18 month period almost all dogs either stayed the same or showed worse cognition over time.
A study by Landsberg et al compared the reasons for referral to behavioural referral practices of dogs aged either less than or more than nine years old. The results are shown in Table 1.
Whilst aggression is less common in senior dogs, anxieties and phobias are more common. It has been suggested that hearing loss, poor eyesight and a generalised overall decline of sensory abilities may factor into the increased incidence of anxieties seen in geriatric dogs, with destructive behaviour being a common sign of separation anxiety.
What changes occur?
Just like people, some dogs age without experiencing cognitive dysfunction, some show mild age-related cognitive changes (normal ageing) while others experience pathological ageing (CCD).
Most dogs show some age-dependent decline in learning and memory, which varies as a function of task and in many respects parallels the pattern of cognitive decline seen in aged humans.
In addition, aged dogs develop neuropathology that is similar to that seen in both successfully ageing humans and in patients with Alzheimer’s disease. Like humans, ß- amyloid protein is deposited in the ageing dog brain, it shows a selective brain distribution that changes as a function of age and accumulation appears to correlate with behavioural impairment.
Whilst it must be recognised that there is a huge variation in learning and memory ability in older dogs recent research has shown that:
- older dogs can typically learn simple skills to the same extent as younger dogs (although some older dogs can have profound impairment);
- simple associative learning such as visual discrimination (e.g. learning that one of two dissimilar objects covers a food reward) typically remains intact with age;
- more complex discrimination problems (e.g. discrimination on the basis of size or oddity) typically are problematic for ageing dogs with substantial impairment evident;
- prefrontal cortex-dependent tasks such as reversal learning (rewards given for animals shifting from responding to one object to responding to a second object) are also consistently impaired in older dogs (the prefrontal cortex of the brain is implicated in planning complex cognitive behaviors and decision making and working toward a defined goal);
- memory (especially spatial memory such as location of a food reward and object-recognition memory such as ability to recognise an object seen 10 to 120 seconds previously) is also compromised in older dogs;
- contrast sensitivity (a measure of how faded an image has to be before it is indistinguishable from a uniform background) deteriorates with age in the canine while form perception (e.g. distinguishing between a triangle or circle) is largely unaffected by age;
- disruption of the frontal or cerebellar regions can release the normal inhibitory controls on behaviour leading to nonfunctional repetitive behaviour instead of normal directed exploration (for example, when out in the garden a dog with non-functional repetitive behaviour may repeatedly walk or pace in circles whereas a dog with normal cognitive function will normally explore all areas of the garden using sight, hearing and smell).
In summary, older dogs have reduced learning ability and reduced maximal working memory capacity.
These deficits are due to the use of ineffective strategies and inflexibility in strategy modification; i.e. when compared with young dogs, aged dogs tend to commit more errors, require a longer training period and show reduced memory capacity.
These age-related deficits are not necessarily indicative of global cognitive dysfunction, as visuo-spatial learning and memory are impaired at an earlier age than object recognition memory.
The presence of circulating testosterone in ageing, sexually intact male dogs may slow the progression of cognitive impairment, at least among dogs that already have signs of mild impairment. Oestrogens would be expected to have a similar protective role in sexually intact female dogs but results in females have been inconclusive.
- continued next month
