What causes poison ivy or poison oak rash?

Poison ivy and poison oak rashes are caused by allergy to a substance called urushiol found in these plants.  Since urushiol is found in all parts of the plant (leaves, stems, and roots), contact with any of these will induce a rash.  Urushiol can even reach your skin indirectly when you touch items such as clothing, tools, or pets that have been exposed.  These objects may remain capable of transmitting the urushiol to humans for years until they are washed with soap and water. In contrast, human skin (usually hands) will transmit urushiol for only a very short time.

What does poison oak look like?

Poison oak is the plant found in California, while poison ivy is found in most other states.  They look very much alike. Both have three leaflets arising from a node on the stem.  A few other perfectly safe plants also have leaflets in clusters of three, but to play it safe, it is wise to avoid all plants with this feature. Thus the old adage “leaflets three” let it be.

A black deposit is produced within a few minutes after crushing poison oak leaves or stems on a light-colored surface.

How long does it take before the rash appears?

Usually, the first exposure to the plant does not cause a skin rash. When it does, however, it takes one to two weeks to appear. In contrast, subsequent exposures will cause a rash to occur much sooner (within hours or a few days).

Is everybody allergic to poison oak?

While some lucky people never become allergic to poison oak, most persons become sensitized at some time and remain allergic.  This may occur after the first exposure in some people or only after many exposures in other people.

Does the blister fluid? Cause the rash to spread?

No. the rash appears to spread because the time required for the rash to appear depends on the amount and duration of plant juice contact.  Areas with less contact will develop a rash later, giving the false impression that the rash is spreading.

Does the rash ever become infected?

Yes. If small pus-filled blisters or yellow-colored crust occur, this may be a sign of bacterial infection which should be treated with antibiotics.

Can I prevent the rash?

Washing with mild soap and water within ten minutes of exposure will usually prevent the rash.  If you touched the plant with your hands, wash the hands especially thoroughly since your hands can spread the juice of the plant to other parts of the body.  Simple washing with water and mild soap will also effectively remove any plant juice from clothing, pets, or tools. Strong soaps are unnecessary.

Unfortunately, there is no way to reliably desensitize persons allergic to poison oak without producing very unpleasant side effects.

TREATMENT

Your doctor will mark the appropriate treatments depending on the severity of the rash.

Never apply to the skin creams, lotions, ointments, or sprays containing benzocaine (examples: Solarcaine and others), antihistamines (examples: Benadryl, Caladryl, and others), or zirconium.  These frequently produce allergic rashes.

Mild to moderate rash:

Soaks: When the skin is weeping, oozing or crusted (or if bathing causes stinging or burning), use this technique:

1.     Lie in a comfortable position, usually in bed, with a waterproof material over the mattress to prevent wetting the mattress.

2.     Obtain dressings, which do not need to be sterile.  These may be 2-4 inch wide Kerlix, soft gauze, or soft linen such as old sheeting or pillowcases, handkerchiefs, or shirts.

3.     Moisten the dressings by immersing them in a solution of two Domeboro Blueboro tablets or packets per quart of lukewarm water. Alternatively, you may use a solution of ¼ cup of vinegar per quart of water.

4.     Gently wring them out so they are sopping wet, but not dripping. Wrap loosely several layers on affected skin so that rapid drying does not occur. Cover with a towel.

5.     Remove after ____ minutes. If dry, it may be necessary to re-moisten dressings to remove adherent crusts. Re-apply with the following frequency: __________.

6.     Discard or launder dressing material daily

7.     Do not treat more than one-third of the body at a time in order to avoid excessive chilling.

Or:

Baths: take baths with Aveeno colloidal oatmeal added to bathwater (one cup to ½ tub), 2-3 times per day.

Antihistamines: Itching can be reduced with the use of oral antihistamines.  There are many effective medications in this class.  These are typically started at a low dose and gradually increase until relief is obtained or intolerable side effects occur, which ever occurs first.  Drowsiness is the most common side effect at higher doses. However, this and other side effects sometimes resolve after a few days even while continuing to take the medication.

Caution must be used when driving or performing sensitive tasks.  Use of alcohol should be avoided or limited due to increased side effects and Never used before driving. Caution must also be used if other medications that can reduce one’s alertness are used (for example Valium, Librium, Xanax, and barbiturates. The combination of these medications may cause decreased alertness).

• Start with this antihistamine _____________ at this dose _____.
• If still itching and no side effects occur after ___ days, increase the dose to ______.
• Continue this process to a maximum dose of ___________.
• Once the itching is controlled, continue taking the medication at the lowest effective dose until the rash is gone.

Calamine lotion:

This is non-prescription item also reduces itching and is soothing.  Apply as often as needed.

Medicated (cortisone-type) cream or ointment:

Immediately after bathing and also one other time during the day, apply ___________________ to the involved skin. Wait until after the blisters have subsided before starting this treatment.

Severe with marked edema and blisters

Follow the marked instructions above. You will also be give and oral steroid medication.

Oral steroid: Prednisone, a cortisone-type medication taken by mouth, is dramatically effective in treating a severe rash.  It’s safe to take for a short period (three weeks). If taken for too short a time, a “rebound” effect may occur when it is stopped, where the skin suddenly gets worse. Thus it is important to precisely follow the instructions given.  If you have a peptic ulcer, high blood pressure, or diabetes, you should inform your doctor.

Improvement in your rash should be prompt and steady.  It depends on getting enough steroids. If you don’t improve steadily, please call your doctor so treatment can be modified.

©2011 Park City Dermatology. Last updated March 24, 2011