History
Patients must be queried about their symptoms, the duration of symptoms, and their belief about the etiology. Notably, Goddard [19] has described a seasonality to delusions of parasitosis, and Vila-Rodriguez et al discuss the facilitation of delusions of parasitosis resulting from Internet-based dissemination of the condition. [20]
The diagnosis and treatment of delusions of parasitosis (DoP) can be an involved clinical activity. Patients with DoP can resist suggestions that their condition is psychiatric rather than physical and refuse referrals for psychiatric care. In fact, in 35% of patients, the belief of infestation is unshakable. [21] In approximately 12% of patients, the delusion of infestation is shared by a significant other. This phenomenon is known as folie à deux (eg, craziness for 2) or folie partagé (ie, shared delusions). Variations in this are the conviction that a child, a spouse, or a pet is infested.
The condition of DoP is a monosymptomatic psychosis, a type of psychopathology relatively distinct from the remainder of the personality. If the condition has a defined pathologic or external cause (eg, scabies), it is not truly delusions of parasitosis. In investigating the history of a patient with such suspected delusions, other causes of itch must be investigated. To diagnose this condition, true infestations (eg, scabies), pediculosis, and primary systemic causes of pruritus must be excluded. [22] Examples include hepatitis, HIV infection, dermatitis herpetiformis, thyroid disease, anemia, renal dysfunction, neurologic dysfunction, and lymphoma.
Delusions of parasitosis are distinct from formication. Formication involves the cutaneous sensation of crawling, biting, and stinging. Formication does not involve the fixed conception that skin sensations are induced by parasites. Patients with this condition can accept proof that they do not have an infestation. Many cases of formication remain idiopathic.
The diagnosis of DoP should be made carefully. Iatrogenic delusional parasitosis, a case of physician-patient folie a deux, has been noted in which a physician made the diagnosis of delusions of parasitosis that was then carried in the medical record, although the patient in fact did not have DoP or actual infestation. [23]
Delusions of parasitosis can occur in isolation on the eyelids, which can result in blindness. [24] It can also present as a belief that the patient has a disseminated fungal infestation. Authors estimated that 1% of patients with DoP may believed that they are infected with fungi. [25]
Mimics of delusions of parasitosis
Other forms of psychiatric illness can mimic delusions of parasitosis. Such psychiatric illnesses are accompanied by signs of mental illness. Delusional parasitosis can be the presenting feature of dementia, in which case the delusions of parasitosis is actually secondary.
For example, patients with schizophrenia may think they are being attacked by insects as a manifestation of their paranoia.
A type of severe depression termed psychotic depression may cause the patient to believe he or she is contaminated or "dirty" because of insect infestation. Such a patient may have a depressed mood and a sense of helplessness, hopelessness, worthlessness, or excessive guilt. Often, these feelings are obvious at clinical presentation. [26]
Drug-induced delusions of parasitosis have been reported during treatment for Parkinson disease. [27] Gabapentin-induced delusions of parasitosis has been noted. [28]
Steinert and Studemund [29] reported a 45-year-old man who did not have a history of psychological pathology, who, after ingesting ciprofloxacin to treat an infection, was overcome with acute delusional parasitosis. He stopped taking the ciprofloxacin, and the delusions of parasitosis resolved altogether without utilization of an antipsychotic agent. Tran et al reported a patient who had delusions of parasitosis after receiving a therapeutic dose of mefloquine, [30] and Krauseneck and Soyka reported an association of delusions of parasitosis with pemoline drug therapy. [31]
Cases in which an etiology is defined are best classified as secondary delusions of parasitosis.
Guarneri et al [32] noted a patient who was thought to have delusions of parasitosis but who, in fact, had infestation with Limothrips cerealium; they termed the condition pseudo-delusory syndrome (ie, infestation with an uncommon insect).
Ghaffari-Nejad and Toofani [33] noted a case of secondary delusions of parasitosis in a patient with major depressive disorder who had delusions of oral parasitosis; the patient sensed lizards and small organisms in her mouth. [34]
Physical
Patients with delusions of parasitosis create their rash. They can present with no findings, erosions or ulcers with or without crusts or prurigo nodularis. They may evidence a dermatitis related to attempted treatments, which may include irritating or corrosive cleansers or harsh abrasive devices. Delusions of parasitosis involving the eyelids has been reported. [35]
