Poison Ivy
Poison ivy (Toxicodendron radicans) is a nuisance plant that grows throughout the continental United States. It grows in almost any type of soil, in both the shade and the sun. While it is most commonly found as a trailing vine, it can also form an upright shrub, and can climb trees, boulders, or walls to heights of 15 meters (50 feet). Its seeds are an important winter food for many types of birds.
Poison ivy's oil causes an itchy, blistering rash in most people who come in contact with it. All parts of the plant contain the oil, although the leaves are the most easily bruised and are therefore the most likely to cause the rash. The oil is sticky and will cling to (and be spread on) skin, clothing, tools, and animal fur. It is also spread in smoke when the plant is burned. In fact, irritation from poison ivy smoke is a major cause of temporary disability in forest fire fighters.
The active ingredient of the oil is urushiol (you-ROOSH-ee-ol). Urushiol is absorbed quickly into the skin. The itching and blistering that results is not due to direct damage done by urushiol, but by the allergic reaction mounted by the immune system. Relatively few people are actually immune to the effects of urushiol, although sensitivity varies and can change over time. Washing the oil off immediately after contact can help reduce the likelihood of developing a rash. In recent years, a clay-based lotion has been shown to help prevent the rash by binding to the urushiol before it can penetrate the skin.
Rashes last approximately two weeks. Some people find relief from the itching and blistering by applying calamine lotion or the mucilaginous sap of jewelweed (Impatiens capensis). Hot water can provoke a short-lived, intenseirritation followed by a longer period of relief. Prescription corticosteroid creams are used for severe cases.
Poison ivy.Recognizing the plant is the best way to avoid it. The three leaflets of poison ivy are from 3 to 15 centimeters long, smooth to slightly indented at the edges, shiny and reddish in spring but becoming a glossy to dull green in summer. "Leaflets three, let it be; berries white, poisonous sight" is a handy way to remember the characteristic appearance of poison ivy.
Poison oak, which grows in California, Oregon, and Washington, has a somewhat similar appearance, while poison sumac grows as a shrub and has a compound leaf and drooping clusters of green berries (unlike other sumacs, which have upright clusters or red berries). All three plants are members of the family Anacardiaceae, many of whose members—including mango and cashew—also contain skin irritants in some plant parts.
Defenses, Chemical; Lipids; Poisonous Plants.
Bibliography
Darlington, Joan R. Is It Poison Ivy: Field Guide to Poison Ivy, Oak, Sumac and Their Lookalikes, 2nd ed. Durham, NH: Oyster River Press, 1999.
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