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

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.