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Poison ivy, oak, and sumac are allergic skin rashes (or Rhus dermatitis) caused by the plants of the same name. All three plants secrete a potent, irritating oil known as urushiol that causes blistering and intense itching once it penetrates the skin.
The allergic rash of poison ivy, oak, and sumac is characterized by red, weeping blisters and severe itching. The rash usually appears within one to two days of initial contact with the plant oil, although it may take longer to appear in areas where the skin is thicker, and lasts from one to three weeks (longer in severe cases). It starts as itchy, inflamed red patches or streaks, and as the oil penetrates into the skin, blisters and small papules form.
Poison plant rash cannot be spread from person to person by contact with the rash itself or fluid from the blisters, and scratching does not spread the rash (although it can cause scarring and potential infection). Only urushiol oil can cause the rash.
Urushiol oil or resin is found in the leaves, roots, and woody parts (i.e., vines and stems) of the poison ivy, oak, and sumac plants. It is a clear substance that is released by the plant when it is cut or bruised. Leaves are bruised easily, especially in the spring, so even a gentle brush against a plant can cause the urushiol to seep out and onto the skin.
Urushiol can remain active for years. For that reason, even dead poison ivy, oak, or sumac plants must be handled with care. Plants should never be burned or shredded, as airborne particles can spread the oil to sensitive areas like the face and eyes and may potentially cause damage to lungs.
The three main sources of poison plant rash—poison ivy, poison oak, and poison sumac—are members of the Anacardiaceae, or cashew, family. While they are usually concentrated most heavily in a few specific regions of the country, all three have been found in locations throughout the United States. Identifying the plant, particularly if people live in a wooded area or have a lot of vegetation in their yards or neighborhood, is essential to preventing the rash.
Poison ivy, known as Rhus radicans or Toxicondendron radicans, is found throughout the United States. The plant grows in vines (typical in the Midwest, East coast, and South) or small bushes (in the North, West, and Great Lakes region), and has clusters of three leaves. (Hence the popular saying: "Leaves of three, let them be.") The leaves are red in the spring; green throughout the summer; and yellow, red, and orange in the fall when they also produce white berries.
Poison oak is a small shrub. The plant, which is also known as Rhus diversiloba or Toxicondendron diversilobum, is found in the western United States. Like poison ivy, poison oak leaves change color with the seasons. The plant also produces white berries in the fall.
The small, woody shrubs that are poison sumac are most common in the Eastern United States. Also known as Rhus vernix or Toxicondendron vernix, poison sumac differs in appearance from the three-leaf clusters of poison ivy and oak. It is feather-like in appearance, with two rows of leaves arranged on either side of a long stem, topped off by a long leaf at the tip. It can be distinguished from regular, non-poisonous sumac by its berries, which are green to white as opposed to the bright red berries of regular sumac.
Author Info: Paula Ford-Martin, Thomson Gale, Gale, Detroit, Gale Encyclopedia of Children's Health, 2006
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