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How to find the cause of aggression in older dogs
Detect aggression with unfamiliar people or pets.
Older dogs do not always adjust well to change. If you bring unfamiliar people or pets around your older dog, it could become very anxious and display aggression toward that person or pet.

pick from the following.
[i] It may also start barking as if trying to get attention and show other aggression. Pay close attention to your dog's cues regarding aggression, including which behavior upsets and causes him to bark..
[ii] You should observe your dog and hear what it is thinking. The dog might be wary or aggressive and you might wonder what the dog is doing or saying..
[iii] Sign up for a dog safe weekly pass and see what changes occur within a few days. If your dog displays aggression because of fear for the strangers around your dog, stay away from pets you do not know..
[iv] Dogs with dementia (canine cognitive dysfunction) have trouble recognizing familiar faces. If your dog has dementia, it could become aggressive toward familiar people or pets because it can't remember who they are..
Answer:
[iv]