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Toxicodendron radicans (syn. Rhus toxicodendron, Rhus radicans; Poison ivy) is a plant in the family Anacardiaceae. The name is sometimes spelled "Poison-ivy" in an attempt to indicate that the plant is not a true Ivy (Hedera). It is a woody vine that is well known for its ability to produce urushiol, a skin irritant that causes an itching rash for most people, technically known as urushiol-induced contact dermatitis.
Additional recommended knowledge
Poison-ivy is subject to frequent taxonomic reclassification and confusion; it is currently divided into eastern and western species in the genus Toxicodendron. At least six distinct subspecies of Toxicodendron radicans are recognized. Complicating identification and taxonomy are the fact that the species (even a particular subspecies) can be highly variable in growth habit and leaf appearance.
Habitat and range
It grows throughout much of North America, including all Canadian provinces except Newfoundland (but not the territories) and all U.S. states except Alaska, and is normally found in wooded areas, especially along edge areas. It also grows in exposed rocky areas and in open fields and disturbed areas. The plant is extremely common in suburban and exurban areas of the Middle Atlantic and Southeastern United States. It rarely grows at altitudes above 1,500 meters (5,000 ft). The plants can grow as a shrub up to about 1.2 meters (4 ft) tall, as a groundcover 10–25 centimeters (4–10 in) high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may at first be mistaken for tree limbs.
It is more common now than when Europeans first entered North America. Real estate development adjacent to wild, undeveloped land has engendered "edge effects,", enabling poison ivy to form vast, lush colonies in such places. It is listed as a noxious weed in the U.S. states of Minnesota and Michigan.
The leaves are ternate with three almond-shaped leaflets. The berries (actually drupes) are a grayish-white color and are a favorite winter food of some birds. Hence the mnemonic,
"Leaves of three, let it be; berries white, danger in sight."
Another version is:
"Leaves of three let it be; if it's hairy, it's a berry."
Effects on the body
The reaction caused by poison-ivy, urushiol-induced contact dermatitis, is an allergic reaction. Around 15% to 30% of people have no allergic response, but most if not all will become sensitized over time with repeated or more concentrated exposure to urushiol. Note that reactions that worsen over time may reflect an anaphylactoid semiology and can therefore be dangerous, even life-threatening.
For those who are affected by urushiol, it causes a very irritating rash. In extreme cases, corticosteroids can be needed to treat rashes and severe itching. The first symptom of contact is a severe itching of the skin that develops into reddish colored inflammation or non-colored bumps, and then blistering of the skin occurs. In severe cases, clear fluids ooze from open blistered sores. Once the urushiol poison has had contact with the skin, it is quickly bound to the skin.
The oozing fluids released by itching blisters do not spread the poison. The appearance of a spreading rash indicates that some areas received more of the poison and reacted sooner than other areas. The blisters and oozing result from blood vessels that develop gaps and leak fluid through the skin; if the skin is cooled, the vessels constrict and leak less. If poison ivy is burned and the smoke then inhaled, this rash will appear on the lining of the lungs, causing extreme pain and possibly fatal respiratory difficulty. If poison ivy is eaten, the digestive tract, airway, kidneys or other organs can be damaged.
Understanding why new lesions may develop for two weeks (studied on forearm) after one exposure was made clear by a Univ of Miami scientist: larger amounts have earliest onset and largest reaction, smallest produce a delayed reaction. The overall severity 'progresses' with the combined active lesions. Therefore, the last new lesion should occur at two weeks after last exposure, the total rash (untreated) may go on for 3-4 weeks.
Urushiol oil can remain active for several years, so handling dead leaves or vines can cause a reaction. In addition, oil transferred from the plant to other objects (such as pet fur) can cause the rash if it comes into contact with the skin.
People who are sensitive to poison-ivy can also experience a similar rash from mangoes; the skin of mangoes has a chemical compound similar to urushiol. 
Similar reactions have been reported occasionally from contact with the related aromatic sumac or Japanese lacquer tree.
Confusion with other plants
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Toxicodendron_radicans". A list of authors is available in Wikipedia.|