Parthenium dermatitis manifesting clinically as polymorphic light eruption and prurigo nodularis- like lesions with vasculitis-like picture on histopathology

Parthenium dermatitis is a widespread and distressing dermatoses in rural and urban India caused by the air borne allergen of the Compositae weed Parthenium hysterophorus. Parthenium dermatitis has been thought to be mediated solely by type IV hypersensitivity, but recently a combined immediate (type I) and delayed (type IV) hypersensitivity mechanism has been postulated in the initiation and perpetuation of parthenium dermatitis, especially in sensitized subjects with an atopic diathesis. Initially, the exposed sites of the body are involved. Later in the course of the disease, unexposed sites may get involved. Various clinical presentations have been described in parthenium dermatitis. Typically, it presents as an air borne contact dermatitis (ABCD) involving the eyelids and nasolabial folds Other presentations include a photodermatitis (essentially a pseudo photodermatitis), atopic dermatitis, seborrheic dermatitis, exfoliative dermatitis, hand dermatitis. Photosensitive lichenoid dermatitis and prurigo nodularis are rarer presentations. Uncommon presentations have been described in parthenium dermatitis. They include prurigo nodularis-like lesions and photosensitive lichenoid eruption. Three cases are presented, two of whom presented as polymorphic-like lesions and one as prurigo nodularis. All three patch tested positive to parthenium on Day 2. Prick testing was positive in two of the three patients. Parthenium dermatitis mimicking polymorphic light eruption has not been reported. Histopathology revealed vasculitis in the lesional skin in two of the patients. Although leukocytoclastic vasculitis has been reported earlier from the prick-tested site, this is the first report demonstrating the presence of vasculitis in lesional skin of parthenium dermatitis.