Poison ivy, oak, and sumac

Allergy to poison ivy, oak, and sumac occurs following skin contact with the antigenic oleoresins (urushiols) of Toxicodendron species.[1]Rietschel RL, Fowler JF, Fisher AA. Contact dermatitis. Baltimore, MD: Lippincott Williams & Wilkins; 2001.[2]Guin JD, Beaman JH, Baer H. Toxic anacardiaceae. In: Avalos J, Maibach HI, eds. Dermatologic botany. Boca Raton, FL: CRC Press; 1999.[3]Zug KA, Marks JG. Plants and Woods. In: Adams RM. Occupational skin disease. 3rd ed. Philadelphia, PA: WB Saunders Company; 1999.[4]Dannaker C, Maibach HI. Poison ivy and oak dermatitis. In: Lovell CR. Plants and the skin. Oxford, UK: Blackwell Scientific Publications; 1993. These urushiols are found beneath the outer skin of the leaves and stems of the plant, as well as the fruit, but come to the surface following minor injury. They are nonvolatile and dry quickly but retain their antigenic potential for years. The greater the damage to the plant (e.g., crushing leaves on the skin), the more urushiol is released, and the greater the chance (and severity) of an allergic reaction.

Urushiol, a phenolic lipid, is a mixture of several closely related organic compounds. Each consists of a catechol, substituted with an alkyl chain of 15 or 17 carbon atoms (replacing 1 hydroxyl group).[2]Guin JD, Beaman JH, Baer H. Toxic anacardiaceae. In: Avalos J, Maibach HI, eds. Dermatologic botany. Boca Raton, FL: CRC Press; 1999.[6]Guin JD, Beaman JH. Toxicodendrons of the United States. Clin Dermatol. 1986 Apr-Jun;4(2):137-48. http://www.ncbi.nlm.nih.gov/pubmed/2941125?tool=bestpractice.com The exact mixture depends on the species of the plant. On contact with skin these compounds are oxidized to quinine intermediates that bind to skin proteins and become antigenic. The urushiol-protein complex penetrates the stratum corneum and attaches to epidermal antigen-presenting Langerhans cells. These cells process the antigen and travel to regional lymph nodes, where they present the antigen to naive clusters of differentiation 4 T-cells. These T-cells then proliferate into urushiol-specific T-memory and T-effector cells, which elicit allergic contact dermatitis. This process generally takes 7 to 10 days, but occurs within 24 to 72 hours of re-exposure to the allergen. A single exposure may sensitize an individual, and patch testing is thought to sensitize one third of patients. Typically, contact dermatitis only develops after a number of exposures.

Types of Toxicodendron species

Toxicodendron is a genus of 30 or so vines, shrubs, and small trees belonging to the Anacardiaceae family. The taxonomy has changed from the genus Rhus, which is similar but nontoxic. In the northern US, the three main Toxicodendron species of concern are:[1]Rietschel RL, Fowler JF, Fisher AA. Contact dermatitis. Baltimore, MD: Lippincott Williams & Wilkins; 2001.[2]Guin JD, Beaman JH, Baer H. Toxic anacardiaceae. In: Avalos J, Maibach HI, eds. Dermatologic botany. Boca Raton, FL: CRC Press; 1999.[3]Zug KA, Marks JG. Plants and Woods. In: Adams RM. Occupational skin disease. 3rd ed. Philadelphia, PA: WB Saunders Company; 1999.[4]Dannaker C, Maibach HI. Poison ivy and oak dermatitis. In: Lovell CR. Plants and the skin. Oxford, UK: Blackwell Scientific Publications; 1993. CDC National Institute for Occupational Safety and Health: plant identification Opens in new window

Poison ivy species American Academy of Dermatology: poison ivy, oak, and sumac Opens in new window Government of Canada: poison ivy Opens in new window

• Toxicodendron radicans: there are approximately nine subspecies of poison ivy in the US. Each subspecies has subtle variations, but plants are generally either vines or small shrubs and trail along the ground or are low climbers. The leaves are alternate with three pointed and glossy leaflets. The edges of the leaves are smooth but can be toothed. They are often reddish in color when they first emerge in the spring, green during the summer, and various shades of yellow, orange, and red in winter. The fruit is white-yellow and hangs in clusters. Poison ivy is often mistaken for Virginia creeper (Parthenocissus quinquefolia), which belongs to the grape family but is nontoxic.

• Toxicodendron rydbergii: this poison ivy is found in the northern and central US, growing as a small shrub. It often produces upright branches that may resemble tines on a fork.

Poison oak species American Academy of Dermatology: poison ivy, oak, and sumac Opens in new window

• Toxicodendron diversilobum: is known as western poison oak and is found from Baja California in northern Mexico to the southern parts of British Columbia in Canada. It is more commonly found as a shrub; the leaves are lobed or deeply toothed with rounded tips and have three to five leaflets. The leaves can be indistinguishable from white oak (Quercus alba). The fruit can be quite large.

• Toxicodendron toxicarium: the eastern poison oak is a slow-growing small shrub found in the southeastern US, often restricted to sandy soils and oak-pine forests.

Poison sumac species American Academy of Dermatology: poison ivy, oak, and sumac Opens in new window

• Toxicodendron vernix: poison sumac is found in the eastern US and southeastern Canada, where it grows as a large shrub or small tree. Unlike poison ivy and oak, it has 7 to 11 leaflets arranged in pairs with an additional single leaflet at the end. It grows in standing water such as in swamps, bogs, and river bottoms.