In the U.S., there are five different types of poison ivy, poison oak, and poison sumac. All contain the exact same substance that causes the blistering, itchy rash most of us get on contact with these plants. Its called toxicodendrol, a sticky sap in the leaves and branches of the plants. If youre sensitized (allergic) to this sap (and more than half of us are), the rash breaks out after about 12-72 hours, but only on areas which directly contacted the resin. This is called rhus dermatitis. The rash is usually red, raised, and often blistered at the central most areas. Because people usually brush by the plants, the rash may have a "streaky," linear, or patchy pattern, unlike stings which are usually single and round.
The rash spreads only by spread of the invisible sap on the skin, not by leaking blister fluid from the rash itself. That blister fluid is a product of your own body, and contains no poison ivy sap. The reason the rash often continues to break out in new areas beyond 72 hours is that people unknowingly continue to contact it --- from unwashed clothes (especially shoes and laces), sporting or gardening equipment, pets, etc. Anything that comes in contact with the plant can carry active sap for months! The sap is so potent, you can pass it from object to object a dozen times just by light contact, and the last object can still cause an intense rash. People leave an invisible "trail" of toxicodendrol where they sit, touch, sleep or bathe. They end up re-exposing themselves over a period of weeks. So dont worry about your oozing blisters being contagious --- just start washing everything you may have touched!
Avoiding contact with the plant is the most important preventive measure. The site map below will help you recognize the different types of poison ivy/oak/sumac in your geographic area. If youve already come in contact with one of these, start washing everything --- soap, detergents and rubbing alcohol are all effective. This dramatically reduces the spread to yourself and others. In fact, if you wash with soap and water within 15 minutes after contact, you may not even get a rash.
Once you have the rash, it lasts 1-4 weeks. Treatment is directed at the intense itching, and shrinking your rash. Treating the itching is especially important, since scratching can lead to skin infection. Be sure to clip your kids fingernails short!
There are some lotions that have been shown to be effective barriers to toxicodendrol (see Recommended Products section). If applied frequently, they can prevent the typical skin rash even with heavy contact --- but washing after contact is still important.
Far more powerful agents are available by prescription for the rash and itching of poison ivy. Here are some good reasons to seek care from a physician:
More than poison ivy on our Self Care Home Page
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