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Toxicodendron combines the Latin root toxico meaning “poisonous” and the Greek root dendron meaning “tree,” and it is the genus that contains the Rhus genus members that are poisonous to the touch and are found in North American and the northern part of South America. In the United States, it is common with the exception of the states of Alaska and Hawaii, desert areas, and elevations over 4,000 feet (1,219 meters).
The members of the Toxicodendron group include Pacific poison oak (Toxicodendron diversilobum), Atlantic poison oak (Toxicodendron pubescens Mill.) eastern poison ivy (Toxicodendron radicans (L.) Kuntze), which has a number of subspecies, western (Toxicodendron rydbergii), and poison sumac (Toxicodendron vernix). They belong to the Sumac family, Anacardiaceae. Thus, poison ivy is not an true ivy, which belongs to the genus Hedera, and poison oak is not a true oak, which belongs to the genus Quercus.
This group of plants causes severe allergic contact dermatitis in both humans and dogs when they come in contact with the nonvolatile oil, toxicodendrol, also referred to as urushiol, if they are susceptible. Urushiol is also found in what some sources say are unrelated species, including Japanese lacquer trees and mangoes and cause similar effects. The term rhus dermatitis is also used to refer to poison ivy, poison sumac, and poison oak dermatitis. Roughly 50-70% of people will react to casual exposure, and in the United States, Toxicodendron dermatitis is the most frequent cause of contact dermatitis.
What happens to people who come in contact with Toxicodendron species depends both on their susceptibility and on how the contact occurs. Skin contact in those who are allergic to the plants usually results in lesions in 12 to 48 hours and may continue to manifest for several weeks. The first lesions are generally from direct contact, and the later ones from secondary contact. If, however, exposure comes from the smoke of burned plants, the eyes, lungs, and airways may be affected, and if it comes from homeopathic remedy made with Toxicodendron, it may have gastrointestinal effects.
Children are admonished with the rhyming reminder, “leaves of three: let them be” to help them avoid touching poison ivy and its relatives. While poison ivy and poison oak often do have three leaflets, they may have five or even seven. Poison sumac characteristically has between seven and 19 leaflets.