Delusions of parasitosis. A psychiatric disorder to be treated bydermatologists? An analysis of 33 patients

S.F.ZOMER, R.F.E.DE WIT, J.E.H.M.VAN BRONSWIJK, G.NABARRO* ANDW.A.VAN VLOTENDepartment of Dermatology, University Hospital Utrecht, Heidelberglaan 100, 3584 CX Utrecht, the Netherlands*Department of Psychiatry, University of Amsterdam, the Netherlands

Accepted for publication 8 January 1998

Summary Delusions of parasitosis is a rare disorder in which patients have the false and fixed belief that they areinfested by parasites. It is a psychiatric disorder, but patients usually present to a dermatologist andreferral to a psychiatrist is almost always rejected. Treating a patient with delusions of parasitosisrequires patience and tact. The neuroleptic pimozide is the treatment of choice, but a significantproblem is convincing the patient to take the drug. We report a study of 33 patients (13 men and 20women) with delusions of parasitosis. The mean age at onset was 56·9 years and the mean durationof symptoms before attending the department of dermatology was 1·3 years. Pimozide (Orap) wasprescribed for 24 patients, but only 18 patients took it. Follow-up information was available for 18patients: five had full remission, four were less symptomatic, five were unchanged and four had diedof unrelated causes.

Delusions of parasitosis is a psychiatric disorder inwhich patients have a false and fixed belief that they areinfested by parasites. Negative findings during examina-tion cannot change this belief. It is not a phobia, becausein such cases there is a fear of infestation by parasites butaffected individuals know that this fear is irrational. Thedisorder was first described in 1894 by Thibierge,1 whonamed it acarophobia. Other names which have beengiven to this disorder include dermatophobia, parasito-phobia and entomophobia. In 1946 Wilson and Miller2

renamed the disorder delusions of parasitosis.In most cases delusions of parasitosis is a monosymp-

tomatic hypochondriacal psychosis, in which there areno other thought disorders and the delusions are notsecondary to another psychiatric illness.3 However,delusions of parasitosis has been reported in associationwith medical and psychiatric disorders including vita-min B12 deficiency, diabetes mellitus, drug abuse andschizophrenia.4,5 When the delusion is shared byanother person, usually a spouse, it is called folie adeux.5 Patients visit a dermatologist rather than apsychiatrist, because they are convinced that theyhave a dermatological problem. The suggestion ofreferral to a psychiatrist is nearly always rejected, andtherefore the dermatologist has the difficult task oftreating these patients.

Before the introduction of the neuroleptic pimozide,6

a very specific dopamine antagonist, delusions ofparasitosis was difficult to manage. Pimozide is thepharmacological treatment of choice,7 and full remis-sion has been reported in 50%7,8 of patients. Prior to theuse of pimozide, excluding patients with no treatment atall, the full remission rate was < 30%.8 We report ananalysis of, and follow-up data on 33 patients withdelusions of parasitosis.

Patients and methods

Between January 1982 and October 1996, 33 patientswith delusions of parasitosis were seen in our depart-ment. Their files were studied for factors common to thepatient cohort. To obtain information on follow-up ofthe patients their general practitioners and dermatolo-gists were interviewed. The results of treatment werecompared with data from the literature. Differencesbetween groups were statistically analysed usingStudent’s t-test.

Results

Thirteen men and 20 women were studied (ratio1 : 1·5). The overall mean age at the onset of symptomswas 56·9 years (range 16–89), with a median of60 years. There was no significant difference between

British Journal of Dermatology 1998; 138: 1030–1032.

1030 q 1998 British Association of Dermatologists

Correspondence: W.A.van Vloten.

the mean onset age in women and men (61·3 vs.50·2 years). At the time of the first consultation themean duration of the disorder was 1·3 years (range1·5–48 months), with a median of 1·0 year. Therewas no significant difference between the duration inwomen and men. Sixty-one per cent were married orcohabiting, 30% lived alone and 9% were widowed.Folie a deux was diagnosed in six couples (18%).

Before attending the clinic 18 patients (55%) hadvisited one other dermatologist, six patients (18%) hadvisited two or more dermatologists and nine patients(27%) had not consulted a dermatologist. Dermatolo-gists referred 12 patients (36%) to our clinic, 19 patients(58%) were referred by their general practitioner andtwo (6%) were referred by entomologists.

On physical examination, 20 patients (61%) did nothave any skin abnormalities. In 13 patients (39%)excoriations, erosions or erythema, especially on thearms and legs, were observed. Itching was mentioned by27 patients (82%). Most of the patients (75%) thoughtthat there was a reason for their infestation. Pets andfleas were mentioned most frequently (36%), followedby moving and home renovation (16%).

‘Proof ’ of the infestation, such as ‘specimens’ in amatchbox or on adhesive tape, was provided by 16patients (48%). These specimens included hair, fibres,crusts and flies. No parasites were found on microscopyof the specimens. In 45% of the patients in whom testswere performed, laboratory investigations (full bloodcount and biochemistry) were all normal.

Pimozide (Orap: Janssen-Cilag BV, Tilburg, theNetherlands) was prescribed for 24 patients (73%), ofwhom 18 took it. The dose varied from 1 to 5 mg daily.Improvement in the pimozide responders was notedafter 3–4 weeks. Pimozide was continued for 6 weeksbefore tapering. After their first visit 23 patients (70%)returned for follow-up. Four patients were seen by amedical social worker, and two of these were referred toa psychiatrist.

During initial treatment at the clinic, seven patientshad full remission and seven were less symptomatic.Eleven of these had taken pimozide. The results of initialtreatment at the clinic are shown in Table 1. There wasno significant difference between the groups regardingmean duration of the disorder.

Fifteen patients were lost to follow-up for variousreasons (e.g. patients had moved without leaving aforwarding address). Results for the 18 patients inwhom long-term follow-up information was availableare shown in Table 2. None of the patients with fullremission had maintenance treatment with pimozide.

Discussion

The diagnosis of delusions of parasitosis can often bemade on the basis of the history, but it is important tomake sure that the patient does not have a real infesta-tion. As patients almost always reject psychiatric refer-ral, the dermatologist must treat these patients. Thecurrent treatment for delusions of parasitosis is theneuroleptic pimozide (Orap),7 which has been shownto be superior to other treatments.5 The principaldifficulty in management is convincing patients totake the drug. When patients read on the instructionleaflet that the drug is used for psychiatric disordersthey are often reluctant to take it.

We tell the patients that some people are moresensitive to stimuli on their skin than others, and thatthe drug (pimozide) increases the threshold to thesestimuli. Their belief with regard to infestation is notchallenged. It is of no use attempting to convincepatients that they are not infested by parasites, becausetheir conviction is unshakeable. Negative results oflaboratory investigations are equally unconvincing.Investigations are only of value in excluding realinfestation.

In contrast with the literature,5,8 in which an overallmale/female ratio of < 1 : 2 is reported, in our studythere was not a marked predominance of women. Theoverall male/female ratio was 1 : 1·5. The collection ofspecimens of pseudoparasites in a matchbox or onadhesive tape as ‘proof ’ of infestation is regarded aspathognomonic5,9 of delusions of parasitosis, but in our

DELUSIONS OF PARASITOSIS 1031

q 1998 British Association of Dermatologists, British Journal of Dermatology, 138, 1030–1032

Table 1. Results of initial treatment of 33 patients with delusions ofparasitosis

Full remission Improvement No change

Pimozide 6 5 7No pimozide 1 2 12Total 7 (21%) 7 (21%) 19 (58%)

Table 2. Results of follow-up in 33 patients with delusions ofparasitosis

Mean duration ofNumber of patients follow-up ( years)

Lost to follow-up 15 –Died of unrelated causes 4 5·6Full remission 5 5·1Less symptomatic 4 4·9No change 5 1·4

study this was mentioned in only 50% of patients’ files.The mean duration of symptoms was surprisingly short(1·3 years, range 1·5–48 months). A mean duration of< 3 years has been reported previously,8 although inone study it was 8·8 months.5 The median (1·0 year) issimilar to that given in previous reports.8 In our studyseven patients had a duration of symptoms longer than2 years, and in one it was 4 years.

The outcome in our study was not as favourable asthat reported in other studies,8 in which a full remissionrate of 50% has been described. In our study only fivepatients (28%) had full remission at follow-up. It may bethat the patients seen at our hospital constitute a moreproblematic group. Of those whose symptoms wereunchanged, two patients are still being treated withpimozide. Pimozide was prescribed in relatively lowdoses of 1–5 mg daily, and it is possible that higherdoses might have been more beneficial.5,9 The durationof treatment with pimozide differs for individualpatients. When full remission is achieved pimozideshould be continued for at least 6 weeks before thedose is gradually reduced. The starting dose of pimozideis 1–2 mg daily, with gradual increases according to theresponse. Above 12 mg daily disturbing extrapyramidalsymptoms are more likely to occur. The therapeuticeffect of each dose increment should be evaluated

about every 2 weeks. At the start of the treatment it isadvisable to see the patient more frequently, in order tosupervise the effects closely and to maintain goodrapport with the patient. An alternative treatment isadministration of a neuroleptic (e.g. fluspirilene) bydepot injection.10

References1 Thibierge G. Les acarophobes. Rev Gen Clin Ther 1894; 8: 373–6.2 Wilson JW, Miller HE. Delusion of parasitosis (acarophobia). Arch

Dermatol Syphilol 1946; 54: 39–56.3 Munro A. Monosymptomatic hypochondriacal psychosis. Br J

Hosp Med 1980; 24: 34–8.4 Johnson GC, Anton RF. Delusions of parasitosis: differential diag-

nosis and treatment. South Med J 1985; 78: 914–18.5 Driscoll MS, Rothe MJ, Grant-Kels JM, Hale MS. Delusional para-

sitosis: a dermatologic, psychiatric and pharmacologic approach.J Am Acad Dermatol 1993; 29: 1023–33.

6 Riding J, Munro A. Pimozide in the treatment of monosympto-matic hypochondriacal psychosis. Acta Psychiatr Scand 1975; 52:23–30.

7 Lynch PJ. Delusions of parasitosis. Semin Dermatol 1993; 12: 39–45.

8 Trabert W. 100 years of delusional parasitosis; meta-analysis of1,223 case reports. Psychopathology 1995; 28: 238–46.

9 Lyell A. Delusions of parasitosis. Br J Dermatol 1983; 108: 485–99.

10 Frithz A. Delusions of infestation: treatment by depot injections ofneuroleptics. Clin Exp Dermatol 1979; 4: 485–8.

1032 S.F.ZOMER et al.

q 1998 British Association of Dermatologists, British Journal of Dermatology, 138, 1030–1032