Out of around 15,000 annual inquiries to the Danish Pest Infestation Laboratory, about fifty were about animals that do not exist. Among these, in particular delusional parasitosis is the one to dominate. People with this disease are convinced that they are haunted by troublesome animals (which other people cannot see or make probable). Doctors call delusional parasitosis, or imaginary animals, a monosymptomatic psychosis – i.e. a real mental illness with a single prominent symptom. The disease may occur alone or be part of other diseases.
It is part of the disease that the patients are very persistent in their efforts to get rid of their animals and often have a history of many unsuccessful inquiries to physicians, zoologists, public authorities, etc. on their problem. Their reactions in this area are understandable enough, but the animals they want help controlling do not exist.