Contact Dermatitis
March 25, 2015
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Contact Dermatitis by Jonathan Schreiber, MD, PhD

“Leaves of three let it be.”  Most of us have probably heard this, and many of us have had first hand experience with poison ivy.  The rash that develops after contact with this plant is extremely uncomfortable and can last up to two weeks from the initial exposure.

Toxicodendron radicans, formerly known as Rhus toxicodendron produces an oil called urushiol, which is the substance that causes the allergic reaction.  The itching and rash generally develop a day or two after the exposure.  The onset of symptoms can be delayed even more in some cases.  Redness and clusters of blisters, which can range in size from a few millimeters to several centimeters, develop at the affected sites.  The rash is often spotty, or in lines created by streaks of oil that get transferred to a person’s skin as he or she walks by the plant, brushing against it.

If you know you will be in an area that has poison ivy, it is best to minimize exposed skin in order to avoid contact with the plants.  Keep in mind that if the toxic oil gets on your clothing, you can develop the rash from contact with the affected clothing.  This includes shoes and shoe laces.  The oil is quite stable and can last for months, and even years on inanimate objects.  Oil can also be transferred to people from pets that have been in contact with the plants.

After a suspected exposure, immediately wash the affected areas with soap and water.  Also be sure to wash all clothing, including shoes, that may have been in contact with the plants, as you don’t want to re-expose yourself later on.

Despite your best effects, a rash may still develop.  The rash can be treated with hydrocortisone and/or calamine lotion.  Antihistamines such as benadryl (diphenhydramine), allegra (fexofenadine) and claritin (loratatine) can help decrease the symptoms.  Keep in mind that diphenhydramine can cause drowsiness, so do not take this medication if you need to remain alert.  Cool compresses can help alleviate the symptoms.  If you develop significant swelling or trouble breathing, you should seek medical attention right away.  Physicians can prescribe medications much stronger than those you can get over the counter to treat your symptoms.

You cannot spread poison ivy from one area of your body to another, or to another person.  The rash only develops where urushiol contacts the skin.  You can only spread the rash by moving the oil from one area to another, but once you have washed off the oil, you are no longer in danger of contaminating new areas.

Even if you believe you are not allergic to poison ivy, it is best to avoid the plant, as you can develop the allergy after repeated exposures.  Some people are also allergic to mangos and cashew nuts, as the rind of the mango fruit, and the shells of cashew nuts contain chemicals similar to those in the urushiol.