Urticaria from stinging nettle
The stinging nettle (Urtica dioica) is a plant that grows in the United States, Eurasia, Northern Africa, and some parts of South America. It is commonly found in patches along hiking trails and near streams. The leaves are green with a characteristic tapered tip and bear tiny spines or hairs. The spines contain substances such as histamine, serotonin, and acetylcholine. Within seconds of contact with the stinging nettle, sharp stinging and burning will occur. Urticaria and pruritus may appear a few minutes later and may last up to 24 hours. The plant is eaten in some parts of the world and has been used as medicine.
The wood nettle (Laportea canadensis) is a relative of the stinging nettle that often grows in woodlands. Like the stinging nettle, the wood nettle leaves are covered with spines that sting when they come into contact with skin. However, the leaves are shorter and more oval shaped that the stinging nettle, and they lack the tapered tip that is characteristic for the stinging nettle. The reaction from the wood nettle is generally milder than that of the stinging nettle.
Plants can illicit different types of reactions in the skin: urticaria (immunologic and toxin mediated), irritant dermatitis (mechanical and chemical), phototoxic dermatitis (phytophotodermatitis), and allergic contact dermatitis.where anyone coming into contact with the hairs of the plant can be affected. Previous sensitization is not required. The reaction usually occurs immediately after exposure.
The allergic contact dermatitis seen with toxicodendron (poison ivy and poison sumac) appears 48 hours after exposure of a previously sensitized person to the plant. This type of delayed hypersensitivity reaction is known as cell-mediated hypersensitivity. Generally, no reaction is elicited upon the first exposure to the allergen. In fact, it may take years of exposure to allergens for someone to develop an allergic contact dermatitis.
The poison ivy plant can grow anywhere and is characteristically found in “leaves of three.” Skin reactions are often appear as linearly-arranged vesicles a few days after the exposure to the urushiol chemical in the sap of the plant. Poison sumac has red stems with 7-12 green, smooth leaves, and causes a similar skin reaction as poison ivy. It typically grows in wet areas.
Most stings are self-limited. Topical corticosteroid creams may be used if needed.
This case and photo were submitted by Susannah McClain, MD, of Three Rivers Dermatology in Coraopolis, Pa., and Dr. Bilu Martin.
Dr. Bilu Martin is a board-certified dermatologist in private practice at Premier Dermatology, MD, in Aventura, Fla. More diagnostic cases are available at mdedge.com/dermatology. To submit a case for possible publication, send an email to email@example.com.