What is it?
Delusional parasitosis is a delusional disorder in which people believe that they are infested with parasites, bugs, or insects when they in fact are not. The affected often say that they feel like these creatures are crawling on or under their skin. They tend to collect “evidence” of their infestation and bring it to their doctor when they seek help: this evidence usually consists of things like textile fibers that have landed on their skin. The affected are often very insistent on being correct about being infested.
Delusional parasitosis is quite rare and the exact rates of occurrence are not known, but the typical patient is a white woman older than 50 years.
What causes it?
The cause and risk factors of delusional parasitosis are not fully understood. For diagnostic purposes it is, however, divided into three categories. In the primary one, the disorder is presented alone and is not accompanied by any other mental dysfunction. In the secondary functional one, the disorder is associated with a mental condition like schizophrenia or depression. In the secondary organic one, the disorder is brought about by a medical condition or the use of substances. Physical illnesses that can lead to delusional parasitosis include cancer, tuberculosis, vitamin B12 deficiency, and diabetes. Substances that can contribute to its development include cocaine and metamphetamine.
What happens during it?
The conditions and substances listed above are associated with delusional parasitosis because they can produce crawling sensations on the skin. While most people can brush these off as illusionary, some become fixated on them.
Our brains have delicate systems that produce hierarchies of probability given the things we know. For example, you probably find it very likely that the room you sit in right now does actually exist and will continue to do so in the future, but you may also acknowledge that it is possible, although very unlikely, that it is an illusion of your mind or that a meteor will destroy it after five minutes. It is thought that delusional disorders, delusional parasitosis included, arise when these predictive systems go wrong and they assign too much value to explanations that are in reality quite unlikely.
How can it be treated?
The people who suffer from the secondary forms of the disorder are treated by addressing the underlying problem. People with primary delusional parasitosis are very hard to treat, as they don’t want to accept that they suffer from a psychological rather than a physical condition. Because of this the caregiver should be very careful with how they approach the persons beliefs and not be condescending or imply that the person is mentally ill. Skillfully conducted psychotherapy can be effective, and some medications have also shown some usefulness.
Sources: http://emedicine.medscape.com/article/1121818-overview#a5, https://en.wikipedia.org/wiki/Delusional_parasitosis, https://en.wikipedia.org/wiki/Delusional_disorder, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3676875/