Reactions to poison ivy, poison oak, and poison sumac, which affect 10 to 50 million Americans a year,1 are classified as Toxicodendron dermatitis; 50% to 75% of US adults are clinically sensitive to these plants.2 Furthermore, people of all ethnicities, skin types, and ages residing in most US geographical regions are at risk.3 Allergenicity is caused by urushiol, which is found in members of the Anacardiaceae family.4 Once absorbed, urushiol causes a type IV hypersensitivity reaction in those who are susceptible.5
Cutaneous Manifestations
Toxicodendron dermatitis presents with an acute eczematous eruption characterized by streaks of intensely pruritic and erythematous papules and vesicles (Figure 1). Areas of involvement are characterized by sharp margins that follow the pattern of contact made by the plant’s leaves, berries, stems, and vines.6 The fluid content of the vesicles is not antigenic and cannot cause subsequent transmission to oneself or others.3 A person with prior contact to the plant who becomes sensitized develops an eruption 24 to 48 hours after subsequent contact with the plant; peak severity manifests 1 to 14 days later.7
FIGURE 1. Erythematous vesicular rash with secondary crusting in a patient with Toxicodendron dermatitis.
When left untreated, the eruption can last 3 weeks. If the plant is burned, urushiol can be aerosolized in smoke, causing respiratory tract inflammation and generalized dermatitis, which has been reported among wildland firefighters.2 Long-term complications from an outbreak are limited but can include postinflammatory hyperpigmentation and secondary bacterial infection.8 Rare reports of nephrotic syndrome also have appeared in the literature.9 Toxicodendron dermatitis can present distinctively as so-called black dot dermatitis.6
Nomenclature
Poison ivy, poison oak, and poison sumac are members of the family Anacardiaceae and genus Toxicodendron,6 derived from the Greek words toxikos (poison) and dendron (tree).10
Distribution
Toxicodendron plants characteristically are found in various regions of the United States. Poison ivy is the most common and is comprised of 2 species: Toxicodendron rydbergii and Toxicodendron radicans. Toxicodendron rydbergii is a nonclimbing dwarf shrub typically found in the northern and western United States. Toxicodendron radicans is a climbing vine found in the eastern United States. Poison oak also is comprised of 2 species—Toxicodendron toxicarium and Toxicodendron diversilobum—and is more common in the western United States. Poison sumac (also known as Toxicodendron vernix) is a small shrub that grows in moist swampy areas. It has a predilection for marshes of the eastern and southeastern United States.6,11
Identifying Features
Educating patients on how to identify poison ivy can play a key role in avoidance, which is the most important step in preventing Toxicodendron dermatitis. A challenge in identification of poison ivy is the plant’s variable appearance; it grows as a small shrub, low-lying vine, or vine that climbs other trees.
As the vine matures, it develops tiny, rough, “hairy” rootlets—hence the saying, “Hairy vine, no friend of mine!” Rootlets help the plant attach to trees growing near a water source. Vines can reach a diameter of 3 inches. From mature vines, solitary stems extend 1 to 2 inches with 3 characteristic leaves at the terminus (Figure 2), prompting another classic saying, “Leaves of 3, let it be!”12
FIGURE 2. Poison ivy consists of 3 terminal leaves.