What causes the rash?

Poison ivy is a common cause of contact dermatitis, an allergic reaction to something that comes in direct contact with the skin. This condition can be quite unpleasant but does not pose serious health risks. Prevention is better than treatment but can be hard to achieve.

Poison ivy is one of many plants that produce a resin called an urushiol that can cause an allergic rash. Related plants include the familiar poison oak and sumac, which are found in different geographical distributions. (Poison ivy is more common in the eastern U.S. and poison oak in the Southeast.) The rashes that each of these plants produce have a similar appearance. In addition, the same urushiols are also found in the mango, cashew, and gingko trees. In the case of mangos, peeling the fruit prevents dermatitis. People who press the whole fruit, including the rind, against their skin can develop a severe reaction around the mouth. Those downwind from burning vegetation containing one of the offending plants can also develop widespread allergic reactions.

Identifying poison ivy

Although it is often recommended that people learn to recognize the poison ivy plant ("Leaves of three, leave them be"), in practice, this is hard to do, since poison ivy and its relatives are often mixed in with other vegetation and not noticed until after the rash has begun. Keeping the skin covered in situations in which exposure is hard to avoid is the best way to prevent the problem.

More than half the population can react to the poison ivy resin if they are exposed to it.

What are the signs and symptoms of the poison ivy rash?

The poison ivy rash usually starts one or two days after exposure, though the delay between contact and its onset can be longer, up to several days. This may lead to confusion over where the exposure took place. The first signs of the rash are curved lines of red, itchy bumps or blisters. These continue to appear for many days, depending on how much resin touched the skin at a given point. This makes it seem as though the rash is "spreading," although the fluid in blisters is just part of the allergic reaction and contains no chemicals or bacteria. It also makes it appear that there may still be poison ivy in clothes and/or on pets. Although this is theoretically possible, repeated washing of these often produces no improvement.

Poison ivy is not contagious, neither from one person to someone else nor from one part of the body to another.

Many references emphasize that animals can carry the poison ivy resin. There's no doubt this is true, but its practical significance may be limited. The first sign of poison ivy, after all, is usually a curved line of rash on the skin. Unless your dog is shaped like a curved line, your poison ivy is more likely to have come from a stem or leaf which dragged against the skin, not from your pet.

What is the treatment for poisoning from these plants?

The best approach to poison ivy dermatitis is prevention. Washing with soap and water can help reduce the severity of the rash, but this is often impractical because it has to be done at once. (After 10 minutes, only 50% of the resin is removable, and by 30 minutes only 10%.)

Once it begins, the rash will usually clear on its own by 14-21 days. Treatment is directed at controlling the itching. Oral antihistamines (like diphenhydramine [Benadryl]) may help the itch somewhat, but often they do no more than make people drowsy. Cortisone creams, whether over-the-counter or by prescription, are only helpful if applied right away, before blisters appear, or much later, when the blisters have dried up. Compresses with cool water or Burow's solution (available without prescription) can help dry the ooze faster.

When the rash is severe, such as when it affects the face or causes extensive blistering, oral steroids (for example, prednisone) can help produce rapid improvement. This course of therapy should be maintained, often in decreasing doses, for 10-14 days or even longer in some cases, to prevent having the rash rebound and become severe again. Patients who are given a six-day pack of cortisone pills often get worse again when they complete it, because the dose was too low and administered for too short a time.

Folklore, medical and otherwise, endorses many other agents, from aloe leaves to tea bags to meat tenderizer as treatments for poison ivy and related plant poisonings. Though these remedies are generally harmless, they are of questionable value.

How can contact with these plants be prevented?

Poison ivy and its relatives are often hidden among other vegetation. Even if you know exactly what they look like, it is very hard to avoid coming in contact with them. Although wearing long pants and long sleeves in warm weather may be uncomfortable, it is important to do so when you might be in contact with plants you can't see, whether you are gardening in the backyard or hiking in the woods. So-called "barrier creams" may help a bit but are not very effective.

When pulling up weeds, those who may be allergic should make sure to tuck sleeves into gloves at all times, since sleeves tend to ride up the forearms and leave wrists and forearms exposed.

If you think you may have been exposed to poison ivy, wash the skin with cool water as soon as possible. After half an hour, however, this is no longer likely to prevent the reaction. As discussed above, washing pets and clothing may also be of limited help.

Attempts to desensitize people by giving them poison ivy by mouth or by injection were tried in the past but proved to be ineffective and potentially dangerous.

Poison Ivy, Oak & Sumac At A Glance

• Many people are susceptible to the rashes of poison ivy, oak, and sumac.
• The sap oil, called urushiol, causes the skin rash.
• Poison ivy is not contagious.
• Washing the oily sap from the skin with water and soap immediately can help prevent the rash.
• Avoiding direct contact with the plants can prevent the rash.

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