No. 81: May-Jun 1992
The term "folie a deux" is used to describe an induced psychosis; it is usually reserved for human-human interfaces. Folie a deux generally occurs when two or more people are very close emotionally. But some people living alone do develop close emotional bonds with their pets, and apparently, vice versa:
"Ms. A, an 83-year-old widow who had lived alone for 15 years, complained that the occupant of an upstairs flat was excessively noisy and that he moved furniture around late at night to disturb her. Over a period of 6 months, she developed delusionary persecutory ideas about this man. He wanted to frighten her from her home and had started to transmit 'violet rays' through the ceiling to harm her and her 10-yearold female mongrel dog. Ms. A attributed a sprained back and chest pains to the effect of the rays and had become concerned that her dog had started scratching at night when the ray activity was at its greatest. For protection, she had placed her mattress under the kitchen table and slept there at night. She constructed what she called an 'air raid shelter' for her dog from a small table and a pile of suitcases and insisted that the dog sleep in it. When I visited Ms. A at her home, it was apparent that the dog's behavior had become so conditioned by that of its owner that upon hearing any sound from the flat upstairs, such as a door closing, it would immediately go to the kitchen and enter the shelter."
(Howard, Robert; "Folie a Deux Involving a Dog," American Journal of Psychiatry, 149:414, 1992.)