Plants can contribute to a variety of dermatoses. The Toxicodendron genus, which includes poison ivy, poison oak, and poison sumac, is a well-known and common cause of allergic contact dermatitis (ACD), but many other plants can cause direct or airborne contact dermatitis, especially in gardeners, florists, and farmers. This article provides an overview of different plant-related dermatoses and culprit plants as well as how these dermatoses should be diagnosed and treated.
Plant dermatoses affect more than 50 million individuals each year.1,2 In the United States, the Toxicodendron genus causes ACD in more than 70% of exposed individuals, leading to medical visits.3 An urgent care visit for a plant-related dermatitis is estimated to cost $168, while an emergency department visit can cost 3 times as much.4 Although less common, Compositae plants are another important culprit of plant dermatitis, particularly in gardeners, florists, and farmers. Data from the 2017-2018 North American Contact Dermatitis Group screening series (N=4947) showed sesquiterpene lactones and Compositae to be positive in 0.5% of patch-tested patients.5
Plant Dermatitis Classifications
Plant dermatitis can be classified into 5 main categories: ACD, mechanical irritant contact dermatitis, chemical irritant contact dermatitis, light-mediated dermatitis, and pseudophytodermatitis.6
Allergic contact dermatitis is an immune-mediated type IV delayed hypersensitivity reaction. The common molecular allergens in plants include phenols, α-methylene-γ-butyrolactones, quinones, terpenes, disulfides, isothiocyanates, and polyacetylenic derivatives.6
Plant contact dermatitis due to mechanical and chemical irritants is precipitated by multiple mechanisms, including disruption of the epidermal barrier and subsequent cytokine release from keratinocytes.7 Nonimmunologic contact urticaria from plants is thought to be a type of irritant reaction precipitated by mechanical or chemical trauma.8
Light-mediated dermatitis includes phytophotodermatitis and photoallergic contact dermatitis. Phytophotodermatitis is a phototoxic reaction triggered by exposure to both plant-derived furanocoumarin and UVA light.9 By contrast, photoallergic contact dermatitis is a delayed hypersensitivity reaction from prior sensitization to a light-activated antigen.10
Pseudophytodermatitis, as its name implies, is not truly mediated by an allergen or irritant intrinsic to the plant but rather by dyes, waxes, insecticides, or arthropods that inhabit the plant or are secondarily applied.6
Common Plant Allergens
Most of the allergenic plants within the Anacardiaceae family belong to the Toxicodendron genus, which encompasses poison ivy (Toxicodendron radicans), poison oak (Toxicodendron pubescens,Toxicodendron quercifolium, Toxicodendron diversiloum), and poison sumac (Toxicodendron vernix). Poison ivy is the celebrity of the Anacardiaceae family and contributes to most cases of plant-related ACD. It is found in every state in the continental United States. Poison oak is another common culprit found in the western and southeastern United States.11 Plants within the Anacardiaceae family contain an oleoresin called urushiol, which is the primary sensitizing substance. Although poison ivy and poison oak grow well in full sun to partial shade, poison sumac typically is found in damp swampy areas east of the Rocky Mountains. Most cases of ACD related to Anacardiaceae species are due to direct contact with urushiol from a Toxicodendron plant, but burning of brush containing Toxicodendron can cause airborne exposure when urushiol oil is carried by smoke particles.12 Sensitization to Toxicodendron can cause ACD to other Anacardiaceae species such as the Japanese lacquer tree (Toxicodendron vernicifluum), mango tree (Mangifera indica), cashew tree (Anacardium occidentale), and Indian marking nut tree (Semecarpus anacardium).6 Cross-reactions to components of the ginkgo tree (Ginkgo biloba) also are possible.