The most common cause of skin problems in the homeless is infections and infestations, and this patient has a scabies infection. Scabies usually involves the hands (including the palms and interdigital spaces) and feet as well as “folds” of the body (neck, axillary folds, waist, and upper thighs). Scabies also has a predilection for the areola/breast in women and the male genitals (glans, penile shaft, and scrotum). Head involvement is rare except in young children. The rash associated with scabies consists of pink papules and macules, some with scale or serous crust, as well as some linear macules that are the burrows of the mites. The patient likely became infested when he stayed with the friend with a rash. After infestation, the rash may take up to a month to become clinically obvious.
Allergic contact dermatitis may involve the dorsal hands, finger web spaces, and skin on the volar wrists, particularly when the palms are spared, but more commonly presents with erythema, edema, and papulovesicles, which are not present in this patient. Lesions are frequently geometric and have sharp cut-offs and a linear component to the morphology. There is typically a delayed onset of 1 to 3 days after exposure.
Bedbugs (genus Cimex) are another parasite of humans, but while the scabies mite lives on the human host, the bedbug lives off the human body. The bedbug is active during the night and typically bites areas of skin that are not covered by nightclothes. The face (including the eyelid), neck, hands, arms, and waist may have evidence of bites in the morning or may be delayed by 7 to 10 days. The bites are itchy and may be small pink-red papules or urticarial plaques 1 to 2 cm in diameter. Bedbugs live in crevices of mattresses, furniture cushions, suitcases, and walls.
Tinea manuum is a dermatophyte infection involving the hands and feet. Tinea manuum characteristically involves only one hand (and two feet). The findings are characterized by a dry scale, and if chronic, nails can be involved. The diagnosis of tinea manuum is unlikely if the feet are not involved.
