We are finally having some nicer weather in central Indiana, allowing people to be out puttering in their yards. Understandably, this has resulted cases of contact dermatitis showing up in my office, most of which were caused by poison ivy. Poison ivy is one of three plants in Indiana in the genus Toxicodendron. This genus also includes poison sumac, commonly found in central Indiana, and poison oak that is an infrequent offender.

The physical appearance of the poison ivy plant is highly variable, though it always has leaves in sets of three (see illustration). I learned a memory aid in Boy Scouts to remember what it looks like - "leaflets three let it be, berries white a poisonous sight." The white berries can sometimes be seen in the 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 a thick vine attached by small red tentacles to trees or other structures.

The rash of poison ivy, like most contact rashes, results from our immune systems reacting to a foreign substance on the skin. The substance binds to skin cells, is recognized by the immune system as a something foreign to the body, and then attacked. This causes the typical rash, known by the medical term, "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 active for years after a plant dies. It can also adhere to animal fur and clothing, explaining how people who have not been outdoors can break out with the typical rash.

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. Fifteen to thirty percent of people require numerous repeated exposures to poison ivy before they have any reaction at all. It's interesting to note that Native Americans, who have lived around poison ivy 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 rubbed along the skin. Areas of skin covered with clothing are generally spared unless the victim rubs the oil on skin that was covered (important safety 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? The variable appearance is due to the amount of oil that the skin is exposed to. If an area is exposed to a large amount of oil, it will break out soon 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 off immediately with soap and hot 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 and on 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 used particularly 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 prescribe antibiotics as well.

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. Consider your clothing contaminated and wash it as soon as possible.