Clinical Importance
Western poison ivy (Toxicodendron rydbergii) is responsible for many of the cases of Toxicodendron contact dermatitis (TCD) reported in the western and northern United States. Toxicodendron plants cause more cases of allergic contact dermatitis (ACD) in North America than any other allergen1; 9 million Americans present to physician offices and 1.6 million present to emergency departments annually for ACD, emphasizing the notable medical burden of this condition.2,3 Exposure to urushiol, a plant resin containing potent allergens, precipitates this form of ACD.
An estimated 50% to 75% of adults in the United States demonstrate clinical sensitivity and exhibit ACD following contact with T rydbergii.4 Campers, hikers, firefighters, and forest workers often risk increased exposure through physical contact or aerosolized allergens in smoke. According to the Centers for Disease Control and Prevention, the incidence of visits to US emergency departments for TCD nearly doubled from 2002 to 2012,5 which may be explained by atmospheric CO2 levels that both promote increased growth of Toxicodendron species and augment their toxicity.6
Cutaneous Manifestations
The clinical presentation of T rydbergii contact dermatitis is similar to other allergenic members of the Toxicodendron genus. Patients sensitive to urushiol typically develop a pruritic erythematous rash within 1 to 2 days of exposure (range, 5 hours to 15 days).7 Erythematous and edematous streaks initially manifest on the extremities and often progress to bullae and oozing papulovesicles. In early disease, patients also may display black lesions on or near the rash8 (so-called black-dot dermatitis) caused by oxidized urushiol deposited on the skin—an uncommon yet classic presentation of TCD. Generally, symptoms resolve without complications and with few sequalae, though hyperpigmentation or a secondary infection can develop on or near affected areas.9,10
Taxonomy
The Toxicodendron genus belongs to the Anacardiaceae family, which includes pistachios, mangos, and cashews, and causes more cases of ACD than every other plant combined.4 (Shelled pistachios and cashews do not possess cross-reacting allergens and should not worry consumers; mango skin does contain urushiol.)
Toxicodendron (formerly part of the Rhus genus) includes several species of poison oak, poison ivy, and poison sumac and can be found in shrubs (T rydbergii and Toxicodendron diversilobum), vines (Toxicodendron radicans and Toxicodendron pubescens), and trees (Toxicodendron vernix). In addition, Toxicodendron taxa can hybridize with other taxa in close geographic proximity to form morphologic intermediates. Some individual plants have features of multiple species.11
Etymology
The common name of T rydbergii—western poison ivy—misleads the public; the plant contains no poison that can cause death and does not grow as ivy by wrapping around trees, as T radicans and English ivy (Hedera helix) do. Its formal genus, Toxicodendron, means “poison tree” in Greek and was given its generic name by the English botanist Phillip Miller in 1768,12 which caused the renaming of Rhus rydbergii as T rydbergii. The species name honors Per Axel Rydberg, a 19th and 20th century Swedish-American botanist.
Distribution
Toxicodendron rydbergii grows in California and other states in the western half of the United States as well as the states bordering Canada and Mexico. In Canada, it reigns as the most dominant form of poison ivy.13 Hikers and campers find T rydbergii in a variety of areas, including roadsides, river bottoms, sandy shores, talus slopes, precipices, and floodplains.11 This taxon grows under a variety of conditions and in distinct regions, and it thrives in both full sun or shade.