Delusions of Parasitosis

Etiology

Most commonly, delusions of parasitosis present secondary to a primary psychiatric disorder.  These disorders include schizophrenia, bipolar disorder, depression, anxiety, obsessive-compulsive disorder, and illness anxiety disorder.[4][5]

Secondary delusions of parasitosis can have several medical causes. These causes include:[6][7][8]

• Medical illnesses such as hyperthyroidism, B12, and folate deficiencies, neuropathy, and diabetes
• Neurologic conditions such as dementia, stroke, multiple sclerosis, encephalitis, meningitis, and post-surgical complications from neurosurgery
• Substance abuse including methamphetamine use, alcohol withdrawal, and acute cocaine use (described as "cocaine bugs")
• Infectious causes such as HIV, tuberculosis, leprosy, and syphilis
• Medication side effects from medications such as topiramate, ciprofloxacin, amantadine, steroids, ketoconazole, and phenelzine 

Epidemiology

Delusions of parasitosis are relatively uncommon.[6] Psychiatrists and dermatologists rarely see the disorder as patients do not believe that their symptoms are a delusion, but instead based in reality.[6] Previous studies have found different rates of incidence ranging from 1.9 to 27.3 cases per year per 100000 people.[9][10] The average age of the patient at diagnosis was found to be between 57 to 61.4 years of age on previous studies, suggesting an increased prevalence with increased age.[10] Some studies found a greater incidence in females than males, but this gender prevalence did not appear in other research.[11][10]

Pathophysiology

The pathophysiology behind delusions of parasitosis is poorly understood. A hypothesized mechanism includes a possible increase of extracellular dopamine within the striatum of the brain that could be the result decreased functionality of dopamine transporters to facilitate the transport of the neurotransmitter.[12] Favorable patient responses to dopamine antagonists as a treatment for the disorder bolster this theory.[13]

History and Physical

The most common symptoms typically presented by patients include pruritus, rashes, stinging, or formication with symptoms lasting six months or longer and having no physical findings present.  Patients may have had previous trials of topical dermatologic medications and/or antibiotics in attempts to treat the illness.  Patients may also have their pets evaluated to rule out fleas or mites with concerns that the pets may have transferred those to the patient.  Patients can provide exhaustive histories revolving around false descriptions of parasites, the timing of infestations, and even prior trials of attempting to have pest control treatment of their homes.  Other studies also demonstrated that patients might draw family members or friends into their delusions in 15 to 25 percent of cases resulting in a folie à deux.[14] Patients presenting with complaints and symptoms of delusions of parasitosis often will maintain functionality but frequently develop feelings of frustration and helplessness.[1]

Physical findings in patients with delusions of parasitosis can be helpful in some cases.  Patients may present with excoriations or scars from previous attempts that patients undertook to remove “organisms” using various objects or their own fingernails. Patients can also present with irritant dermatitis as they will sometimes turn to alternative treatments and home products in an attempt to remove the “organism.”[6]  Absence of physical findings will demonstrate lack of any puncture, sting, bites or specific rashes that would indicate an alternative diagnosis.  Punctures, stings, bites or specific rashes would be more indicative of parasitic or other organisms causing symptoms.

Evaluation

A thorough evaluation in patients suspected of having delusions of parasitosis begins with taking a thorough history with an emphasis on the origin of symptoms and previous negative workups if present.[4] Further evaluation should initially focus on ruling out an organic parasitic or organism infection.  Practitioners should focus on recent travel, exposure to friends or family members with similar symptoms, as well as household pets.[4][15] Practitioners should also pay attention to the possibility of bed bugs being present and causing symptoms. Routine laboratory testing may include a complete blood count with an emphasis on increased eosinophils.  Increased eosinophils may be indicative of an actual parasitic or other organism causing symptoms due to the body’s immune response.[16]  It is also essential to evaluate for secondary causes of the patient's symptoms; this may include an evaluation for thyroid disorders, vitamin deficiencies, neurologic disorders, and ruling out infectious causes such as HIV, tuberculosis, syphilis, and leprosy.[7][17] Referral for evaluation by a dermatologist can also confirm the diagnosis and assure the patient that practitioners are not ignoring their fears and frustration.

Treatment / Management

Once the clinician has made a diagnosis of delusions of parasitosis, the primary consideration in treatment is to build a strong rapport with the patient and maintain a good relationship.  Patients can feel frustrated and disrespected and untrusting of practitioners who do not take the time to listen to their complaints or symptoms, which can lead to noncompliance with follow up care and visiting alternative practitioners for second opinions.  Practitioners should focus on being objective and acknowledging the patient's symptoms, not dismissing their symptoms or feelings, and understanding the effect this can have on their daily life.  Agreeing or disagreeing with the patient overtly in the office about their symptoms can be a difficult decision to make.  The most accepted approach involves taking a neutral approach by stating that there are no organisms visible to the practitioner at this time but that they may have been present before.[1] It is imperative for practitioners not to dismiss patient complaints, but at the same time being careful not to perpetuate the delusion that can further bolster and strengthen it making treatment more difficult.[6](B2)

Rejecting a psychiatric origin of symptoms is common in patients.  Patients are often reluctant to take medications for the management of symptoms.  Recommendations are that practitioners tell the patient that symptoms are secondary to altered chemicals in the brain in the hope that they are more likely to comply with the use of medications.  Patients will often need reassurance that antipsychotic drugs are not being used for the treatment of schizophrenia or other psychotic conditions, as this can be a turn-off for patients.  Patients need education regarding alternative uses of antipsychotic medications through an explanation of the mechanism of action of the medication.  Also indicating to patients that studies have shown that patients have had success with the proposed treatment can improve compliance.

First generation antipsychotics (FGA), such as pimozide were widely used in the treatment of delusions of parasitosis in the past but are no longer recommended as a first-line treatment due to adverse side effects.[7]  The opioid blocking mechanisms were theorized to improve symptoms of pruritus and formication making the medication an attractive choice[18]. The side effect profile of FGAs versus second-generation antipsychotics (SGA) makes a clear choice of not choosing an FGA as first-line treatment.(A1)

First line treatment for delusions of parasitosis consists of antipsychotics at lower doses that minimize side effects. If patients have a relapse of symptoms after discontinuation of antipsychotic medication, they should restart the therapy.[6]  Recommended second-generation antipsychotics include quetiapine, olanzapine, risperidone, and aripiprazole.  These medications should be employed at the lowest possible dose to prevent side effects including extrapyramidal symptoms, QT prolongation, and metabolic side effects.[18] Newer antipsychotics such as lurasidone, paliperidone, and brexpiprazole could be attractive medication choices due to their lower rate of side effects but do not have any significant evidence supporting their use in delusions of parasitosis, and their cost can be a drawback.[19][20][21][22](A1)

Differential Diagnosis

There are many diagnoses to take into consideration in the differential diagnosis for delusions of parasitosis.  Actual parasitosis must be ruled out based on a thorough history, physical exam, and laboratory testing.  Patients with an illness anxiety disorder can also fool physicians, and this diagnosis merits strong consideration.  Clinicians must also consider secondary causes from underlying psychiatric, substance abuse, or other medical conditions.

Misdiagnosis of delusions of parasitosis can occur when the patient has a cutaneous infection from either parasites or other organisms.

Delusions of parasitosis could also present in a patient having an underlying illness anxiety disorder, where patients have a preoccupation with disease and need evaluation by practitioners and often need constant support and reassurance.

Delusions of parasitosis can be secondary to many psychiatric illnesses, such as schizophrenia, bipolar disorder, post-traumatic stress disorder, obsessive-compulsive disorder, major depressive disorder, and generalized anxiety disorder.[4][5]  Primary delusions of parasitosis would not have accompanying psychiatric complaints and would be the chief complaint without other symptoms. Substance causes must also be considered, such as cocaine use disorder that can present with similar symptoms.[8] Alcohol use, alcohol withdrawal, and amphetamine abuse disorders can also cause secondary delusions of parasitosis.[8]

Nutritional deficiencies can cause secondary delusions of parasitosis.  B12 and folate are the two most common vitamin deficiencies that can lead to symptoms.[23][24]

Neurologic disorders are also a common cause of secondary delusions of parasitosis.[17] Head trauma, strokes, multiple sclerosis, atrophy, encephalitis, meningitis, and postoperative neurological changes could be the root cause of symptoms.[6]

Infections such as HIV, leprosy, and tuberculosis can also cause symptoms. Disorders of the thyroid and pancreas, such as diabetes mellitus, are a potential cause of symptoms.[7]

Reactions to medications can also cause secondary delusions of parasitosis.  Studies have found that the side effects of topiramate, ciprofloxacin, amantadine, steroids, ketoconazole, and phenelzine can cause formication symptoms.[25][26][27][28][29]

Prognosis

Delusions of parasitosis generally do not inhibit the patient's ability to function but rather serve as a hindrance and persistent symptom.  However, for some patients, delusions can decrease their life quality significantly.[1]  Patients generally have good response rates to atypical antipsychotic treatments.[18]  Patients can typically be tapered off medications over weeks and can respond well if symptoms reappear with the reintroduction of treatment.[6][5][30]  Studies have demonstrated successful response rates to antipsychotic treatment of delusion of parasitosis from 60 to 100 percent.[13] Treatment of secondary causes of delusions of parasitosis should focus on treating the underlying condition with consideration of possible adjunct antipsychotic treatment for the management of symptoms if needed.

Complications

Delusions of parasitosis can present a few common complications.  A delay in treatment can lead to superficial scarring on the patient’s skin and a possible decrease in function.[31] A small study reported that patients with delusions of parasitosis also had increased incidents of self-inflicted keratoconjunctivitis. Another significant complication in treating the illness is the side effects of antipsychotic medications, including extrapyramidal symptoms, QT prolongation, weight gain, and other metabolic effects.[21] The second generation, atypical antipsychotics are preferred compared to typical antipsychotics for a more favorable side effect profile.