Toxic plant encounters in the great outdoors

July 19, 2017 The Tablet

Spending time outside and with nature benefits us mentally and physically. While outdoors, whether hiking, biking, playing or gardening, contact with certain plants may result in skin reactions, called phytodermatitis. This review covers the more common toxic plant encounters in B.C. for which pharmacists may be called upon for advice.

Handling or brushing against stinging nettle (Urtica diocica and Urtica gracilis) causes tiny ‘stinging’ hairs (trichomes) to break at their tip, exposing needle-like points. The points penetrate the skin causing mechanical irritation and release chemicals including histamine, acetylcholine and serotonin. Immediate intense burning and stinging ensues. This may be followed in minutes to hours by urticarial rash, pruritus, vesicles, numbness and/or tingling. It is not surprising that the term ‘urticaria’ comes from the genus name Urtica. Initial decontamination and symptomatic and supportive treatment may be found in Tables 1 and 2. Most symptoms subside within 1 to 2 hours (36 hours for sensitive individuals).

Poison ivy and poison oak contain a highly allergenic oleoresin called urushiol found in most plant parts. Allergic contact dermatitis follows direct contact with the injured plant or indirect contact with urushiol on hands, clothing, shoes, tools and pets or particulate matter from burning plants landing on the skin. Up to 50 to 70% of adults may become sensitized following exposure. 

Symptoms are delayed 6-72 hours (up to three weeks) and include intense pruritus, swelling, streaks of erythema, papules or vesicles; commonly on the hands, arms, legs and face. Vesicles may rupture leaving a crust. In severe cases bullae develop. Vesicular fluid contains no urushiol and, therefore, is not responsible for spreading symptoms. See Tables 1 and 2 for initial decontamination and treatment. Medium to high potency topical corticosteroids for two weeks are most effective. Lower potency corticosteroids are recommended for facial and anogenital regions. Severe symptoms or symptoms involving > 10-20% body surface area require systemic (oral) corticosteroids. For adults, prednisone 40 mg daily x five days then tapered over the next 10 days is the recommended treatment. Short courses (e.g. five days) of corticosteroids are associated with rebound dermatitis, thus patients should be counseled on the importance of completing the full course of treatment. 

Complications include transient hyperpigmentation, secondary bacterial infections and rarely erythema multiforme. Black glossy macules, called black spot poison ivy, develop rarely when urushiol on the skin oxidizes and turns black. Lesions peel away in days to weeks without scarring. Cross-reactivity may occur with cashew nut shell, mango skin, gingko fruit and other plants.

Skin contact with certain chemicals in plants followed by sun exposure results in skin eruptions called phytophotodermatitis. In B.C., plants containing photosensitizers include the giant hogweed and cow parsnip. All parts of the plant exude a sap containing photosensitizing psoralens. Psoralens readily penetrate the epidermis, but require activation by long wave ultraviolet light before binding to DNA and causing cellular damage. All those exposed through touching or brushing against the plant or through indirect contact (e.g. pets, other objects) and then exposed to sunlight may develop the reaction. Prolonged exposure to sunlight and moist conditions may worsen the reaction.

Within 24-48 hours a burning, erythematous and edematous rash in a linear, crossing or bizarre pattern may develop. The next 24-48 hours may see progression to painful blistering, partial or full thickness burns. Pruritus is uncommon. Complications include infection, hyperpigmentation and photosensitivity lasting months to years.

Decontamination and treatment is similar to poison ivy and poison oak, though sun exposure should be avoided for 2-3 days (see Tables 1 and 2). Topical antiseptics are recommended for blistered areas to prevent infection. Topical or oral corticosteroids may be indicated. Severe cases may require analgesics, wound debridement, burn dressings and skin grafting. Topical hydroquinone cream has been used for hyperpigmentation.

Plants of the genus Euphorbia include houseplants, garden perennials and common weeds. Skin contact with the highly irritant milky sap (latex) found in the stems, leaves and fruits has resulted in pruritus, painful erythema, vesicles, bullae, hyperpigmentation and, in severe cases, burns. Symptoms may begin immediately and slowly progress, or be delayed for up to 24 hours. Eye exposure commonly occurs in gardeners from sap spraying in the eye or touching/rubbing the eye with contaminated fingers. Symptoms include immediate irritation and burning, pain, lacrimation, photophobia, blepharospasm and conjunctivitis. Effects may worsen over the first two days leading to keratitis, uveitis, corneal ulceration, increased intraocular pressure, temporary changes in vision and rare cases of permanent blindness though many of these cases involved secondary bacterial infection. Early treatment with adequate flushing of the eye, an ophthalmic exam and topical cycloplegics, corticosteroids and/or antibiotics usually results in resolution within 4-14 days. 

Prevention of phytodermatitis includes learning to recognize plants, avoiding them and wearing protective clothing (vinyl gloves for poison ivy and poison oak) if contact is necessary. Giant hogweed is considered an invasive plant by the Invasive Species Council of BC. Removal by professionals is recommended. For those handling Euphorbia plants, eye protection is recommended as well as proper hand washing to prevent finger to eye exposure. 

For more information on toxic plant exposures call the Poison Control Centre at 604-682-5050 or 1-800-567-8911.

References available upon request at

Table 1: Decontamination following dermal exposure to toxic plants

General Plant specific
  • Immediate gentle washing of skin with soap and cool water (avoid rubbing or scratching). 
  • Wash hands well and under fingernails.
  • Remove exposed clothing. 
  • Wash exposed fomites (clothing, shoes, tools, pets) with soap and water. 
  • Stinging nettle: remove irritant hairs from skin via adhesive tape.
  • Poison ivy and oak: washing most effective within 10 minutes; beneficial up to 2 hours; always decontaminate regardless of the time. 
  • Giant hogweed: avoid swimming and exposure to the sun for 2-3 days, thereafter use sunblock with high SPF.
  • Urushiol may remain active on fomites for many years.


Table 2: Symptomatic and Supportive treatment for phytodermatitis

  • Pruritus and/or inflammation
  • Cool water or medicated soaks or baths (Burow’s solution, colloidal oatmeal)
  • Cooled calamine lotion
  • Topical corticosteroids
  • Oral antihistamines
  • (Topical antihistamines are NOT recommended)
  • Weepy vesicles or bullae
  • Dressings of water, normal saline or Burow’s solution
  • Pain
  • Analgesics

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