Poison ivy (Toxicodendron radicans) leaf shape variability: Why plant avoidance-by-identification recommendations likely do not substantially reduce poison ivy rash incidence

Urushiol is the natural product produced by poison ivy (Toxicodendron radicans) that is responsible for millions of cases of delayed contact allergenic dermatitis in North America each year. Avoidance of poison ivy plant material is the clinically recommended strategy for preventing urushiol-induced dermatitis symptoms. However, poison ivy leaf shape is anecdotally notoriously variable, thereby confounding accurate poison ivy identification. This study focused on quantitative analyses of poison ivy and a common poison ivy look-alike (American hog peanut) leaf shape variability in North America to empirically validate the high degree of poison ivy leaf shape plasticity.Poison ivy and American hog peanut records were scored for seven attributes of compound leaf shape that were combined to produce a total leaf complexity score. Both the mean and the distribution of poison ivy total leaf complexity scores were significantly greater than that of American hog peanut. Non-metric multidimensional scaling analyses corroborated a high degree of poison ivy leaf shape variability relative to American hog peanut. A poison ivy accession producing frequent palmate penta-leaflet compound leaves was evaluated using linear regression modeling.Poison ivy total leaf complexity was exceedingly variable across a given latitude or longitude. With that said, there was a small but significant trend of poison ivy total leaf complexity increasing from east to west. Palmate penta-leaflet formation was significantly correlated with a stochastic leaflet deep-lobing developmental process in one unusual poison ivy accession.The empirically-validated poison ivy leaf shape hypervariability described in this report likely confounds accurate poison ivy identification, thereby likely resulting in many accidental urushiol-induced clinical allergenic dermatitis cases each year.

Automated summary

Urushiol, produced by poison ivy, causes millions of cases of delayed contact allergenic dermatitis in North America annually. Avoiding poison ivy plant material is recommended to prevent symptoms. However, the variable leaf shape of poison ivy makes accurate identification difficult.