Plant dermatitis (also called ‘phytodermatitis’) is caused by reaction to skin contact with certain plants.
It is not always obvious which plant is responsible for a flare-up of the dermatitis (also known as eczema). Sometimes a rash may develop without direct contact with the plant; the juices may be on clothing or the fingers. A plant cause for dermatitis is suspected when there is a linear or streaky pattern. It is nearly always asymmetrical.
The rash may take various forms. Some plants only cause a rash if the sun is shining on the skin at the same time i.e. photocontact dermatitis. Big blisters appear on exposed parts which have been in contact with the plant. Anyone can get this kind of plant dermatitis, which settles leaving brown marks (pigmentation), which may last several months. When caused by perfume, it is known as ‘berloque dermatitis’.
Other plants cause a rash only in certain people who have developed an allergy to them, i.e. allergic contact dermatitis. A number of cases of very allergic contact dermatitis resulting in facial swelling and blistering of exposed parts are seen every year due to Rhus trees (Toxicodendron succadaneum), which are specimen trees with lovely autumn colours. Primula obconica and chrysanthemum can also cause allergic rashes. Not everyone is allergic to these plants, but those who are may even get rashes from pollen carried in the wind.
Allergic contact dermatitis to a plant is not contagious and the blister fluid does not spread the rash. The rash appears between four hours and 10 days after exposure to the plant, depending on individual sensitivity and the amount of contact. Sometimes more rash appears after treatment has begun due to blood stream spread of the allergy to other areas (autoeczematisation).
Compositae dermatitis images supplied by Dr Shahbaz A. Janjua
What is the treatment for plant dermatitis?
Plant dermatitis is self limiting. It clears up without treatment, as long as exposure to the plant is avoided. Topical steroids, and sometimes by oral steroids may be necessary to treat the rash. If there are blisters, compress the areas for 15 minutes twice daily with a mixture of a tablespoon of white vinegar in a litre of water. Ice packs or cold showers will temporarily relieve itching. Steroids are less effective when the skin is blistered. Avoid soap as it irritates.
The only way to prevent plant dermatitis is to avoid contact with the responsible plant, when it has been identified. Your dermatologist may be able to organise allergy testing (patch tests). We do not test for reactions to the most strongly allergenic plants because the tests could themselves create a new allergy. Many plants also cause irritant dermatitis and so patch tests must be performed and interpreted by an expert.