Pets, like people, can suffer from mental illness.
Dr. Todd Martin, a friend of mine who is a dentist in Chicago, called me a few weeks ago regarding Peyton, his 4-year-old Maine Coon cat.

Peyton was acting strangely. Suddenly, he stopped using his litter box, his pupils would dilate, he would seemingly stare off into space, the skin on his back would "ripple", and he was becoming aggressive.

Peyton was quickly becoming a pet that wasn't fun to be around. He had even begun lashing out with his claws at Martin, suddenly and unexpectedly.
Peyton's signs, especially in the absence of any apparent physical abnormalities, were classic for a mental condition occurring in cats known as hyperesthesia syndrome.
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Mental illness