The term delusional parasitosis (DP), also known as parasitic delusion or imaginary parasitosis, was coined by Karl Ekbom in 1938 to describe a new psychiatric disorder characterized by the false and fixed belief that the body is infested by living pathogens, despite the absence of any medical or microbiological evidence to justify it.1,2

Its incidence rate varies across series and it is more frequent in women.3–6 The predominant age of onset is in the sixth decade of life.2 Patients do not recognize themselves as mentally ill, so their first consultation may occur in multiple specialties, such as primary care, emergency medicine, infectious diseases, or dermatology.3

DP can be classified as primary or secondary. In the primary form, no triggering cause is identified. The secondary form is induced by different causes: use of a toxic substance, such as drugs or medications; a psychiatric disorder, such as schizophrenia or depression; or an organic neurodegenerative, endocrine, or infectious disease, among others. It is mandatory to rule out the onset of dementia or substance abuse as causes of DP, as these have been described as potential triggers. Another way to classify DP is into ectoparasitic forms, when symptoms are located on the skin, and endoparasitic forms, when internal organs or orifices are affected.3,7

For the diagnosis of DP, the patient must meet the DSM-5 diagnostic criteria for delusional disorder, shown in Table 1.

Symptoms are described as tingling or tactile sensations due to the presence of pathogens. Skin lesions include erosions, lichenification, excoriations, crusts, and scars, which are self-inflicted by the patient. There are clinical signs that allow suspicion of the disease. In the “matchbox sign,” patients bring the clinician samples collected by themselves in which they believe the pathogen is present, as shown in Fig. 1. In the “image sign,” patients show photographs on their mobile phone taken by themselves, depicting skin lesions and presumed pathogens.

Fig. 1.

Samples provided by patients as the “matchbox sign.”.

Differential diagnosis should include dermatological diseases such as nodular prurigo; other psychodermatoses such as dermatitis artefacta, psychogenic excoriation, or chronic pruritus8; and psychiatric disorders such as schizophrenia, somatic-type delusional disorders, or psychogenic depression.

The most historically used treatment has been pimozide, which acts by blocking dopamine D2 receptors and has an antipruritic effect. At present, second-generation antipsychotics such as olanzapine, risperidone, or aripiprazole are used as first-line treatment.9

This disease is difficult to manage medically because patients usually reject psychiatric treatment.2 Correct multidisciplinary coordination is necessary to ensure appropriate management.2,3

We present a series of 25 patients diagnosed with DP by the dermatology department of a single center.

A total of 25 patients with DP were diagnosed and evaluated (Table 2). Mean age was 55 years and median age was 53 years, with a range of 34–79 years. A total of 82% of patients were women, with a female-to-male ratio of 5:1. The distribution by age range was 32% between 18 and 49 years, 48% between 50 and 64 years, and 20% aged 65 years or older. Symptom duration was highly variable, ranging from 24h to 4 years before consultation. Most patients were referred from the emergency department (84%).

Urine toxicology tested positive in 32% of all cases, with cocaine positive in 100%. In the group younger than 50 years, toxicology was positive in 80% of those tested. Relevant medical comorbidities were described in 50% of patients, most of them psychiatric: depression, dysthymia, anxiety, personality disorder, and trichotillomania. One case had a history of real parasitosis: toxocariasis in 2015, with currently negative PCR. The shared psychiatric disorder folie à deux was described in 8 cases (32%). A total of 44% accepted psychiatric assessment with joint follow-up between both specialties.

Symptoms were exclusively ectoparasitic in 14 cases (56%), mixed in 8 cases (32%), and endoparasitic in 3 cases (12%). The “matchbox sign” appeared in 56%. The “image sign” appeared in 28%. Fifty-six percent of cases used toxic self-cleaning substances such as anti-lice products, disinfectant, or bleach. Dermatological lesions were self-inflicted by scratching, with the intention of eliminating the pathogens. Papules, plaques, and even ulcers were described in accessible areas and at different stages of evolution. Fig. 2 illustrates some of the lesions presented by the patients.

Fig. 2.

Skin lesions presented by the patients.

The most widely used treatment was pimozide at a dose of 1mg/day in 48% of patients. Atypical antipsychotics, specifically paliperidone, quetiapine, and aripiprazole, were used for the treatment of 3 cases. In patients with cocaine-positive urine toxicology, treatment consisted of detoxification. Among treated patients, the outcome was variable: 43% achieved clinical improvement, with complete cure in half of them. In the remaining patients, symptoms persisted despite treatment. Follow-up was lost in 11 of the 25 cases, as they did not attend scheduled visits.

The incidence rate of DP is variable, ranging from 0.2 to 23.6 per 100000 inhabitants/year.3–6 The predominant sex is female, with a female-to-male ratio of 3–5:1.10,11 An association between female sex and advanced age has been described, attributed to neurological and cutaneous changes. Male sex is associated with young patients who have used toxic substances, with a more acute onset of symptoms.12 Cocaine is one of the main substances related to the appearance of DP due to drug use.13 Psychiatric comorbidity is very frequent, depression being the most common concomitant disease.1 The shared psychotic disorder folie à deux is characteristic, and its frequency ranges from 5% to 25% of cases.1,10,12 Another aspect related to DP is social isolation, which is considered either a risk factor for this condition or a consequence of it.13

Patients report pruritus and a sensation of being infested by “bugs” moving through the body.12 Exclusively ectoparasitic symptoms are the most common. The “matchbox sign” is the most frequently described sign.1,12,14–17 The “image sign” is becoming more frequent as a result of new technologies.3 Toxic substances and dangerous medications may be used as means of disinfection.18 Suicide attempts have been described, although they are very uncommon.19

It is difficult to achieve appropriate therapeutic adherence, as patients do not recognize themselves as mentally ill and therefore do not accept direct referral to psychiatry. When joint follow-up between dermatology and psychiatry is proposed, acceptability increases.17 Therefore, treatment should be initiated from the dermatology department.16,20 Despite the lack of studies with scientific evidence, antipsychotics are accepted as the drugs of choice.12 At present, pimozide is no longer the first option because of its high risk of adverse effects, including extrapyramidal effects and cardiac alterations.

The latest clinical practice guidelines propose second-generation antipsychotics as the treatment of choice because of their greater efficacy and better safety profile.21 The minimum effective dose is recommended, and the drug should be maintained for at least 1 year once symptoms have disappeared and reintroduced if symptoms recur. Amisulpride, olanzapine, and risperidone are included as first-line treatments, with aripiprazole and risperidone left as second-line treatments.21 Aripiprazole has also been proposed as first-line therapy with promising results.22 It is an atypical antipsychotic and partial agonist of D2 receptors, a function that differs from that of other second-generation antipsychotics. It has a better cardiometabolic profile and a lower risk of extrapyramidal effects and hyperprolactinemia, with some antidepressant effect. It should be considered a powerful therapeutic option because of its efficacy and good safety profile. Olanzapine is associated with a higher risk of metabolic disorders, weight gain, and dyslipidemia, whereas risperidone may increase prolactin levels.23Table 3 describes the doses to administer and possible adverse effects, as well as other considerations regarding the proposed second-generation antipsychotics. In any case, antipsychotics are the treatment of choice, and it is recommended that they be offered to the patient using the term neuroleptics and emphasizing that they are prescribed for their antipruritic or sedative action.

In Fig. 3, we propose a diagnostic and therapeutic algorithm to follow when this disease is suspected, with the aim of guiding management and optimizing clinicians’ actions.

Fig. 3.

Algorithm for suspicion and management of DP.

Compared with other series described in the literature, our study stands out because of its detailed clinical description of the lesions observed in DP and reviews the epidemiological characteristics of a broad case series diagnosed in a single center.24–26 This is the first description of the management of DP from a psychodermatology unit.

In conclusion, dermatological lesions and a detailed clinical history are essential to guide suspicion of DP. The signs described are highly characteristic and bring us closer to the diagnosis. Follow-up and management are truly complex, with a high rate of loss to follow-up among patients. Second-generation antipsychotics are the treatment of choice, and the dermatologist is often the physician responsible for initiating the drug. Ultimately, DP is a clinically relevant entity that dermatologists should suspect and learn to treat, as it is an underdiagnosed and undervalued disease.

None declared.

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