The current status of the weed Parthenium hysterophorus L. as a cause of allergic contact dermatitis.

The dissemination of allergenic plants as horticultural introductions, or in the case of the weed Parthenium hysterophorus L. apparently by chance, can have undesirable results. This species of the Compositae family, native to the Caribbean region, has spread widely in India, causing allergic contact dermatitis from its content of sesquiterpene lactones, and has recently appeared in Australia. The clinical features consist of dermatitis of the exposed skin surfaces from direct contact with the weed, but resembling photodermatitis. In fact, contact dermatitis from the plant can coexist with light sensitivity. The plant does not show phototoxic activity but may contain an unidentified photoallergen. Since plant hairs and the pollen contain sesquiterpene lactones, they can probably cause airborne contact dermatitis. Attempts to control spread of the plant have so far not been successful.