Toxicodendron plants can be more easily identified and avoided with knowledge of their characteristic leaf patterns. The most dependable way to identify poison ivy and poison oak species is to look for plants with 3 leaves, giving rise to the common saying, “Leaves of three, leave them be.” Poison sumac plants have groups of 7 to 13 leaves arranged as pairs along a central rib. Another helpful finding is a black deposit that Toxicodendron species leave behind following trauma to the leaves. Urushiol oxidizes when exposed to air and turns into a black deposit that can be seen on damaged leaves themselves or can be demonstrated in a black spot test to verify if a plant is a Toxicodendron species. The test is performed by gathering (carefully, without direct contact) a few leaves in a paper towel and crushing them to release sap. Within minutes, the sap will turn black if the plant is indeed a Toxicodendron species.13Pruritic, edematous, erythematous papules, plaques, and eventual vesicles in a linear distribution are suspicious for Toxicodendron exposure. Although your pet will not develop Toxicodendron ACD, oleoresin-contaminated pets can transfer the oils to their owners after coming into contact with these plants. Toxicodendron dermatitis also can be acquired from oleoresin-contaminated fomites such as clothing and shoes worn in the garden or when hiking. Toxicodendron dermatitis can appear at different sites on the body at different times depending on the amount of oleoresin exposure as well as epidermal thickness. For example, the oleoresin can be transferred from the hands to body areas with a thinner stratum corneum (eg, genitalia) and cause subsequent dermatitis.1
Compositae Family
The Compositae family (also known as Asteraceae) is a large plant family with more than 20,000 species, including numerous weeds, wildflowers, and vegetables. The flowers, leaves, stems, and pollens of the Compositae family are coated by cyclic esters called sesquiterpene lactones. Mitchell and Dupuis14 showed that sesquiterpene lactones are the allergens responsible for ACD to various Compositae plants, including ragweed (Ambrosia), sneezeweed (Helenium), and chrysanthemums (Chrysanthemum). Common Compositae vegetables such as lettuce (Lactuca sativa) have been reported to cause ACD in chefs, grocery store produce handlers, gardeners, and even owners of lettuce-eating pet guinea pigs and turtles.15 Similarly, artichokes (Cynara scolymus) can cause ACD in gardeners.16 Exposure to Compositae species also has been implicated in photoallergic reactions, and studies have demonstrated that some patients with chronic actinic dermatitis also have positive patch test reactions to Compositae species and/or sesquiterpene lactones.17,18
In addition to direct contact with Compositae plants, airborne exposure to sesquiterpene lactones can cause ACD.14 The pattern of airborne contact dermatitis typically involves exposed areas such as the eyelids, central face, and/or neck. The beak sign also can be a clue to airborne contact dermatitis, which involves dermatitis of the face that spares the nasal tip and/or nasal ridge. It is thought that the beak sign may result from increased sebaceous gland concentration on the nose, which prevents penetration of allergens and irritants.19 Unlike photoallergic contact dermatitis, which also can involve the face, airborne ACD frequently involves photoprotected areas such as the submandibular chin and the upper lip. Davies and Kersey20 reported the case of a groundsman who was cutting grass with dandelions (Taraxacum officinale) and was found to have associated airborne ACD of the face, neck, and forearms due to Compositae allergy. In a different setting, the aromas of chamomile (Matricaria chamomilla) have been reported to cause airborne ACD in a tea drinker.21 Paulsen22 found that ingestion of chamomile tea can induce systemic ACD in sensitized individuals.
Alstroemeriaceae, Liliaceae, and Primulaceae
Florists are exposed to many plant species and have a high prevalence of ACD. Thiboutot et al23 found that 15 of 57 (26%) floral workers experienced hand dermatitis that cleared with time away from work. The Peruvian lily (Alstroemeria, Alstroemeriaceae family), which contains tuliposide A, was found to be the leading cause of sensitization.23 Tulips (Tulipa, Liliaceae family), as the flower name suggests, also contain tuliposide A, which along with mechanical irritation from the course tecta fibers on the bulbs lead to a dermatitis known as tulip fingers.24,25 Poison primrose (Primula obconica, Primulaceae family), cultivated for its highly colorful flowers, contains the contact allergen primin.6 A common clinical presentation of ACD for any of these culprit flowers is localized dermatitis of the thumb and index finger in a florist or gardener.
Plants That Cause Irritant Reactions
Cactuses
Although the long spines of the Cactaceae family of cactuses is a warning for passersby, it is the small and nearly invisible barbed hairs (glochids) that inflict a more dramatic cutaneous reaction. The prickly pear cactus (Opuntia species) is a good example of such a plant, as its glochids cause mechanical irritation but also can become embedded in the skin and result in subcutaneous granulomas known as sabra dermatitis.26
Stinging Nettle
The dermatologic term urticaria owes its namesake to the stinging nettle plant, which comes from the family Urticaceae. The stinging nettle has small hairs on its leaves, referred to as stinging trichomes, which have needlelike tips that pierce the skin and inject a mix of histamine, formic acid, and acetylcholine, causing a pruritic dermatitis that may last up to 12 hours.27 The plant is found worldwide and is a common weed in North America.
Phytophotodermatitis
Lemons and limes (Rutaceae family) are common culprits of phytophotodermatitis, often causing what is known as a margarita burn after outdoor consumption or preparation of this tasty citrus beverage.28 An accidental spray of lime juice on the skin while adding it to a beer, guacamole, salsa, or any other food or beverage also can cause phytophotodermatitis.29-31 Although the juice of lemons and limes contains psoralens, the rind can contain a 6- to 186-fold increased concentration.32 Psoralen is the photoactive agent in Rutaceae plants that intercalates in double-stranded DNA and promotes intrastrand cross-links when exposed to UVA light, which ultimately leads to dermatitis.9 Phytophotodermatitis commonly causes erythema, edema, and painful bullae on sun-exposed areas and classically heals with hyperpigmentation.
Pseudophytodermatitis can occur in grain farmers and harvesters who handle wheat and/or barley and incidentally come in contact with insects and chemicals on the plant material. Pseudophytodermatitis from mites in the wheat and/or barley plant can occur at harvest time when contact with the plant material is high. Insects such as the North American itch mite (Pediculoides ventricosus) can cause petechiae, wheals, and pustules. In addition, insecticides such as malathion and arsenical sprays that are applied to plant leaves can cause pseudophytodermatitis, which may be initially diagnosed as dermatitis to the plant itself.6