What could be responsible for this pruritic eruption?

An 80-year-old female presented with a several month history of pruritic eruption on her arms, chest, and upper back. She felt like something was crawling under her skin. She brings in material from her house, which she suspects could be the cause. 

What could be responsible for her pruritic eruption?

A. Scabies
B. Bed bugs
C. Body lice
D. Dermatitis herpetiformis 
E. Delusions of parasitosis 

(Answer and discussion on next page)

Answer: Delusions of parasitosis 

An extensive and exhaustive workup was unrewarding—suggesting the diagnosis of delusions of parasitosis, which is further supported by the lack of a primary lesion. Patients have no obvious cognitive impairment and abnormal organic factors are absent. True infestations and primary systemic diseases that cause pruritus are not involved. Primary skin lesions are not present. Physical examination may reveal no lesions, but only linear erosions with crusts, prurigo nodularis, and/or ulcers. 

Differential diagnosis. A biopsy will show evidence of insect bite reactions and dermatitis herpetiformis has evidence of pathology on hematoxylin and eosin stain initially before confirmation with immunofluorescence. 

Bonus question: How do delusions of parasitosis differ from formication? 

Answer: Delusions of parasitosis are distinct from formication. Formication refers to the cutaneous sensation of crawling, biting, and stinging. Patients with this condition can accept proof that they do not have an infestation.