The weather is finally starting to break a bit allowing many of our readers to get back out in the yard to prepare for fall. This has resulted in a lot of rashes showing up in our office. Most of these rashes were caused by poison ivy, one of three plants in Indiana in the genus Toxicodendron. This genus also includes poison sumac, and poison oak.
The physical appearance of the poison ivy plant is highly variable, though it always has leaves in sets of three (see illustration). A memory aid from my days as an Eagle Scout lets me recall what it looks like – “leaflets three let it be, berries white a poisonous sight.” The white berries can sometimes be seen in wintertime. The plant is small and low to the ground when young. As it grows, it can be found in various sizes all the way up to thick vines attached to trees or other structures by small red root-like structures.
The rash of poison ivy, like most contact rashes, results from the reaction of the body’s immune system to a foreign compound on the skin. The compound binds to skin cells and then attacked by the immune system. When dealing with poison ivy, sumac or oak, it causes a typical rash, known as “rhus dermatitis.”
In the case of poison ivy, oak and sumac, the offending chemical is the plant resin or oil urushiol. Interestingly, urushiol is also found in mangos and the shells of cashew nuts. This oil can remain in the environment for years after a plant dies.
To develop rhus dermatitis, you must be sensitized to urushiol. This means you had to have had a prior exposure to the resin to activate your immune system. The typical rash then develops on subsequent exposures. Between 15 to 30 percent of people require numerous repeated exposures to urushiol before they have any reaction at all. It’s interesting to note that Native Americans, who have lived around these plants for centuries, react the least of any race.
The initial rash usually occurs 24 to 48 hours after exposure to urushiol. It appears as redness with blisters, usually found in a line where the plant brushed the skin. Areas of skin covered with clothing are generally spared unless the victim transfers the oil from clothing to skin that was covered (important health tip to males – if you’ve been clearing brush, always wash your hands with soap and water before urinating).
People often have the misconception that fluid from the blisters can spread the rash. However, once the oil is washed off the skin with soap and water, the rash can no longer spread. Patients often wonder if it’s not contagious, how do they keep developing new areas of involvement? This depends on the amount of oil the skin is exposed to. If an area is exposed to a large amount of oil, it will break out sooner after contact. Areas that get a smaller dose may not break out for up to two weeks after the exposure. They also might be getting repeated exposures from clothing they were wearing or from pets that might have the oil on their fur. The entire course of the rash may last up to a month or so if left untreated.
Treatment of rhus dermatitis is based on the severity of the rash. If you know you have touched poison ivy, wash the area of contact immediately with lots of soap and warm water. Minor rashes usually respond well to cool compresses and either topical or oral diphenhydramine (Benadryl®). Over the counter 1% hydrocortisone cream applied three to four times a day can also speed resolution, though this should not be used around the eyes or mouth and very sparingly on children as well as areas of the body that have thin skin.
For more severe cases, a trip to the doctor is often needed. We usually prescribe a steroid cream, ointment or perhaps steroid pills. Pills are usually prescribed if the rash is found on the face or around the eyes. Occasionally scratching the rash may cause infection with skin bacteria. If this happens a doctor may also prescribe antibiotics.
As always, an ounce of prevention beats a trip to the doctor. Know what the plant looks like. When you’re in an area with possible poison ivy, wear protective clothing and gloves. Avoid rubbing your skin with clothing or gloves that have come into contact with plant material. Take a hot soapy shower or bath as soon as possible. Wash your clothing in hot water as well.

Dr. John Roberts is a licensed medical physician. He writes a weekly column exclusively for Sagamore News Media publications.