Poison ivy, and related plants poison oak and poison sumac, causes more cases of allergic contact dermatitis in the
than any other known allergic substance. These plants contain a resinous sap material called urushiol that is responsible for the dermatitis. Reaction to these plants is more properly called Rhus dermatitis. United States
What parts of the plant should I avoid?
Rhus dermatitis comes from contact with the plant’s leaf. However, all parts of the plant contain the harmful sap, therefore even contact with the stems and roots will result in Rhus dermatitis.
How does the skin react to the plant?
Even light contact with the plant will result in redness at the area of contact. However, more often, linear wheals will appear from brushing the plant’s leaves. Linear and vesicular eruptions are highly characteristic in Rhus dermatitis. Other forms of eruption may appear depending on the type of contact with the plant, for example brushing against an animal’s hair that came into contact with the plant. Skin rashes will appear within eight hours of the contact. However, the reaction may be delayed for a week or more.
How is this prevented?
Avoidance of the plants is the best means to prevent the skin reaction. However, if contact with the plant occurs, wash the affected area right away. Washing the skin with any type of soap inactivates and removes the plant’s sap. But washing may not remove any of the sap if done beyond an hour after contact.
How is it diagnosed?
History and clinical examination alone is enough to establish a diagnosis. The linear and vesicular eruptions in Rhus dermatitis are highly characteristic and point easily to the cause. Patch testing is not necessary.
How can this be treated?
If blisters and skin redness appear, cold wet compresses will help to decrease inflammation. These should be applied for 15 to 30 minutes several times a day for one to three days or until the blisters and itchiness diminish.
Antihistamines may help to control itchiness. These include hydroxyzine and diphenhydramine.
Topical steroids also decrease inflammation, especially to areas with redness, and control itching. However, they have no effect on blisters.
Oral steroids such as prednisone are reserved for more severe and extensive reactions.