Shared Delusional Parasitosis in Two Families: Clinical Insights Into Folie à Deux and Folie à Trois

Abstract

Delusional parasitosis is a rare psychotic disorder characterized by individuals firmly believing that they are infested with parasites despite no medical evidence. It may be shared among close contacts-termed folie à deux when 2 individuals are affected or folie à trois when 3 individuals share the delusion. Delusional parasitosis' somatic focus often leads patients to seek dermatologists, causing delayed diagnoses and unnecessary antiparasitic treatments. Herein, we present 2 familial cases of shared delusional parasitosis. In both cases, patients exhibited the matchbox sign, presenting nonparasitic materials as "evidence" of infestation. Dermatological and psychiatric evaluations excluded organic causes, diagnosing primary delusional parasitosis. Treatment with antipsychotic medications led to symptom remission. Psychoeducation was critical in preventing relapse in secondary cases. Delusional parasitosis with shared delusions is often misdiagnosed, requiring dermatologists to recognize it early. A multidisciplinary approach that combines psychiatric care and psychoeducation is essential for effective management and for preventing the reinforcement of delusional beliefs.

Keywords: delusional parasitosis; folie à deux; matchbox sign; psychodermatology; shared psychotic disorder.

Figure 1.. The patient presented a pink box with suspected parasitic material, which was later identified as skin debris and textile fibers.

Figure 2.. Clinical presentation of the patient from family 1. (A) Frontal view showing self-induced haircut due to persistent scalp infestation delusion. (B) Lateral view revealing no visible scalp lesions.

Figure 3.. The patient provided a white napkin containing alleged parasites, which, on examination, revealed a psychodid fly.

Figure 4.. Microscopic view of the sample from Figure 3, showing a psychodid fly—a nonparasitic insect species.