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Diseases and Conditions
Poison ivy rash
From MayoClinic.com
Special to CNN.com

Introduction

The advice, "Leaves of three, let them be," is familiar to many people, with good reason. It's a reminder to stay away from plants that feature three leaflets to a stem, such as poison ivy.

Poison ivy and poison oak, another plant with leaflets of three, are common causes of a skin irritation called contact dermatitis. Poison sumac, which has many leaflets to a stem, is another offender. Contact with poison ivy can cause a red, itchy rash consisting of small bumps, blisters or swelling.

Most people are sensitive to poison ivy and these other plants to some degree. The irritating substance is the same for each plant, an oily resin called urushiol (u-ROO she-ol).

Rashes caused by poison ivy and its cousins generally aren't serious, but they certainly can be bothersome. Treatment for poison ivy mostly consists of self-care methods to relieve the itching until the reaction goes away.

Signs and symptoms

Signs and symptoms of a poison ivy rash include:

  • Redness
  • Itching
  • Swelling
  • Blisters

Often, the rash has a linear appearance because of the way the plant brushes against your skin. But if you come into contact with a piece of clothing or pet fur that has urushiol on it, the rash may be more diffuse.

The reaction usually develops a day or two after exposure and can last up to three weeks, even with treatment. In severe cases, new areas of rash may break out several days or more after initial exposure. This may seem like the rash is spreading. But it's more likely due to renewed contact with the oily resin or to the rate at which your skin absorbed the urushiol.

Your skin must come in direct contact with the oil from the plant in order to be affected. Spreading blister fluid through scratching doesn't spread the rash, but germs under your fingernails may cause a secondary infection.

Causes

Poison ivy is a common weed-like plant found across the United States. It may grow as a bush, plant or vine. Poison oak can grow as a low plant or bush, and its leaves resemble oak leaves. The leaves on both poison ivy and poison oak typically grow three leaflets to a stem. Poison sumac may be a bush or a tree. It has two rows of leaflets on each stem and a leaflet at the tip. The smooth edges of its leaves distinguish it from its harmless sumac relatives.

These plants contain an oily substance called urushiol. When your skin touches the leaves of a poison ivy plant, it may absorb some of the urushiol made by the plant. It takes only a tiny amount of urushiol to cause a reaction, but direct contact is essential.

The resin can spread on your body if you accidentally rub it onto other areas of your skin. For example, if you walk through some poison ivy then later touch your shoes, you may get some urushiol on your hands, which you may then transfer to your face by touching or rubbing.

You may also develop a reaction indirectly if you touch urushiol left on an item, such as clothing, firewood or even a pet's fur (animals usually aren't affected by urushiol). Burning poison ivy, poison oak or poison sumac can also cause a reaction because the smoke contains the oil.

Urushiol can remain allergenic for years, especially if kept in a dry environment. So if you put away a contaminated jacket without washing it and take it out a year later, the oil on the jacket may still cause a reaction.

A poison ivy rash itself isn't contagious. Blister fluid doesn't contain urushiol and won't spread the rash. In addition, you can't get poison ivy from another person unless you've had contact with urushiol on that person.

When to seek medical advice

See your doctor if any of the following occur:

  • The reaction is severe or widespread.
  • The rash affects sensitive areas of your body, such as your eyes, mouth or genitals.
  • Blisters are oozing pus.
  • You develop a fever greater than 100 F.
  • The rash doesn't get better within a few weeks.

Complications

Scratching a poison ivy rash with dirty fingernails may cause a secondary bacterial infection. This might cause pus to start oozing from the blisters. See your doctor if this happens. Treatment for a secondary infection is generally with antibiotics.

Treatment

Poison ivy rashes typically go away on their own within one to three weeks. In the meantime, you can use self-care methods and over-the-counter medications to relieve signs and symptoms. If the rash is widespread or results in a large number of blisters, your doctor may prescribe a corticosteroid, such as prednisone.

Prevention

The best way to prevent an allergic reaction is to avoid contact with poison ivy and other poisonous plants. These suggestions may help:

  • Remove poison ivy. In your backyard, you can use an herbicide to get rid of poison ivy or use heavy gloves to carefully pull it out of the ground. Afterward, remove and wash your gloves and hands thoroughly. Don't burn poison ivy or related plants because the urushiol can be carried by the smoke and cause irritation.
  • Take precautions outdoors. When hiking or engaging in other activities that might expose you to poison ivy, try to stay on cleared pathways. If camping, make sure you pitch your tent in an area free of poisonous plants. Keep pets from running through wooded areas so that urushiol doesn't accidentally stick to their fur, which you then may touch.
  • Clean anything that may be contaminated. Wearing long pants, socks, shoes and gloves will help protect your skin, but be sure to wash your clothing promptly with detergent — in a washing machine, if possible — if you think you've come into contact with poison ivy.

    In addition, wash any other contaminated items, such as outdoor gear, garden tools, jewelry, shoes and even shoelaces, as soon as possible. If you must wait to wash any contaminated items, seal them up in a plastic bag or container to avoid contamination of other items. Dry cleaning will also get rid of urushiol.

  • Wash your skin. Gently washing off the harmful resin from your skin within five to 10 minutes after exposure may help avert a reaction. After an hour or so, however, the urushiol has usually penetrated the skin and washing won't necessarily prevent a reaction, but it may help reduce its severity.

Self-care

Once a rash has broken out, the following may help to soothe itching and swelling:

  • Over-the-counter high-potency corticosteroid creams, such as hydrocortisone, especially within the first few days
  • Calamine lotion
  • Creams containing menthol, such as Sarna
  • Oral antihistamines, such as diphenhydramine (Benadryl, others), which may also help you sleep better
  • Cool-water tub soaks with over-the-counter colloidal oatmeal (Aveeno)
  • Cool, wet compresses for 15 to 30 minutes several times a day

May 24, 2006

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