Older dogs and cats are the most rapidly growing segment in many veterinary clinic populations. In part, this is because better
and more sophisticated treatment for primary medical conditions has lead to longer-living pets. Many of these older animals
will be affected by non-specific signs associated with cognitive aging. The proportion of dogs and cats with cognitive decline
will increase as they age (e.g., clients will note behavioral changes for more 16-year-old dogs than for those 10 years old).
Changes in non-specific behavioral signs associated with aging may not accurately reflect the degree of true cognitive change
or "dementia" that the pet is experiencing, but these signs are excellent indicators of client distress. Client distress potentially
leads to euthanasia of the pet. We can now do better.
Common client complaints and signs of cognitive decline: The most common changes noted by clients are:
1. Disorientation. The dog or cat seems to get lost in the house, in his or her room, or confused when outside. The pet may
become increasingly distressed within each episode of disorientation early on in the progression of the cognitive changes,
and less so as the changes become more pronounced.
2. Alterations in social and environmental interactions. As cognitive decline progresses affected dogs and cats interact less
with their canine and feline housemates, play less, overall, ignored favored toys, and withdraw from clients, often refusing
interaction with them. If forced to interact, dogs and cats can become completely withdrawn, or agitated and more distressed,
possibly to the extent of becoming aggressive as a means to decrease interaction. When greeted, affected pets appear not to
recognize clients. This profound alteration in client interaction and affect is the change that most distresses clients.
3. Changes in diel/sleep-wake cycle. Affected dogs and cats may no longer exhibit standard sleep-wake cycles, instead, pacing
+/- vocalization during the night. Cats sleep often during the day as a normal behavior, so these changes may be most noticeable
for dogs. Increased vocalization that is repetitive and monotonic is the most common complaint of clients with aging cats.
The most distressing aspect of changes in diel for the clients is that they cannot comfort their pets when they pace and vocalize.
Early in the progression of the condition diel changes may only be manifest as increases in time spent sleeping, which, unfortunately,
may be considered a "normal" aging change.
4. Changes in elimination behaviors. Clients often described cognitive changes associated with eliminative behaviors as "a loss
of housetraining." It is likely that changes in memory and learning associated with true housetraining (e.g., the ability
to inhibit elimination unless provided with an appropriate substrate) are affected as a result of cognitive changes, but it
is important to note that these dogs are not incontinent. The pets either appear to "forget" to eliminate when taken to their
normal locations and substrates, and then eliminate anywhere they are when the need is urgent, or they have reduced inhibition
and will eliminate wherever they are once they reach a certain threshold stimulus. The extent to which cognition is involved
in inhibition of volitional behaviors is largely unexplored in dogs and cats but appears to be important in humans.
The importance of a complete medical work up and why reliance on non-specific signs, alone, is problematic for diagnosis:
Diagnosis cannot be made on the basis of non-specific signs, alone. For example, cognitive changes associated with age can
also be signs of separation anxiety. When signs of separation anxiety appear in older dogs they may be associated with anticipatory
anxiety. Failure of function or behaviors associated with anxiety are not uncommon in older dogs. In one of the first studies
on older dogs, 13 of the 26 dogs 10 years or older were diagnosed with separation anxiety (i.e., the behaviors occurred only
in clients' absence), and six were attributed to breakdown of housetraining that did not meet the criteria to make a diagnosis
of separation anxiety (i.e., "cognitive dysfunction"). Older pets have changing physical and emotional needs; accommodating
these needs and treating the dogs with anti-anxiety medications can help modulate symptoms, although the course of whatever
the underlying condition is may be inexorable.
Table 1: Organ system changes attendant with aging that may mask or mirror cognitive changes:
Accordingly, cognitive dysfunction is best defined by the following conditions: a change in interactive, elimination, or navigational
behaviors, attendant with aging, that are explicitly not due to primary failure of any organ system, and that are not consistent
with the definitional criteria for any anxiety disorder. Organ system dysfunction that could contribute to the non-specific
signs discussed are found in Table 1. The minimum recommended database required to exclude any of these systemic conditions
is found in Table 2.
When veterinarians are faced with client complaints about non-specific signs, they should ask if those signs meet this definition
for cognitive dysfunction. For example, if the non-specific sign of elimination in the house occurs in an older dog only when
the client is absent, the sign is much more likely to be associated with separation anxiety than with cognitive dysfunction
or decline. If the sign occurs regardless of whether the client is present and there are no systemic organic reasons for the
elimination, it is likely that the elimination is associated with cognitive decline.
Table 2: Minimum database required for aging pets
It should be obvious that physical ability, or lack thereof, could confound how the client perceives the extent to which the
complaint is behavioral or physical. A check sheet should be completed by clients at every annual visit. This way, not only
will the clients watch for signs that may be early indicators of impending cognitive decline, but the physical contribution
to the cognitive changes will be assessed in a routine manner along with the behavioral component. By having the clients complete
such check sheets each time the dog or cat is seen, changes will be noted in a more accurate manner than one that depends
on the client's memory, alone. This check sheet can also be used to assess whether the animal has improved post-treatment.
Changes in neurochemistry. It should be noted that the diagnosis of cognitive dysfunction or decline is a phenotypic one based
on how the behaviors look. This does not imply that any specific underlying neurocellular mechanism is responsible for the
changes noted. Recent advances have indicated that afflicted dogs may have variable amounts of cellular and neurochemical
changes that are, themselves, rooted in different molecular processes. Age-related declines are generally associated with
vulnerability of the cholinergic neurons. Such vulnerability could be the level of the cholinergic neuron, itself, the neurotrophic
support system, cytoskeleton alteration, target loss, and vascular dysfunction. We know so much about canine behavioral changes
that occur with age because dogs are good neuroanatomical and neurobehavioral models for humans because the non-specific behavioral
changes associated with aging are shared by humans and dogs.
Changes in brain pathology that aged dogs share with aged humans with a diagnosis of one form of human dementia (Alzheimer's
disease) include (1) thickening of the meninges and dilation of the ventricles, (2) age-related gliosis, (3) vascular changes,
(4) diffuse plaques, and (5) amyloid deposition. Dogs do not seem to form the true neurofibrillary tangles that humans form.