Poison Ivy, Oak, and Sumac Diagnosis

  • Diagnosis

    Poison ivy, poison oak, and poison sumac are plants that produce an allergic skin reaction (contact dermatitis) in some people who come into contact with them. Poison ivy, poison oak, and poison sumac are the most common causes of allergic reactions in this country. Every year, millions of Americans develop the characteristic red, itchy, blistering rash after being exposed to any one of the varieties of these poisonous plants belonging to the genus Rhus. Most of the time, poison ivy, poison oak, and poison sumac are more bothersome than serious. Occasionally, however, the itching is so severe that it disrupts sleep, or the swelling (particularly when outbreaks occur on the face) is so intense that it is disfiguring.

    Poison ivy grows as a vine that wraps around trees or snakes along the ground; it also grows as a shrub. Poison oak takes the shape of a low shrub in the eastern US and a high shrub in the western US. Poison sumac typically grows as tall as a shrub or tree. Most poisonous plants have three leaflets per leaf. Poison sumac, however, can have as many as 13 leaflets per leaf.

    • Poison Ivy (Toxicodendron radicans) grows as a stand-alone bush or as a vine. Each leaf has three leaflets: a bigger one in the center, flanked by two smaller ones. In the fall, the leaves turn a dull red color.
    • Rydberg’s Poison Ivy (Toxicodendron rydbergii) grows as a bush that is about three feet high. Most plants have the classic three-leaflet formation.
    • Eastern Poison Oak (Toxicodendron toxicarium) grows as a bush in sandy soil and in areas that have been burnt by a fire. This variety also has three leaflets.
    • Western Poison Oak (Toxicodendron diversilobum) grows as a thick bush or vine at most elevations — from sea level to 5,000 feet. It too has three leaflets.
    • Poison Sumac (Toxicodendron vernix) grows in standing water in peat bogs or swamps. Each leaf has as many as 13 leaflets.

    Urushiol — an oil in the sap of poison ivy, poison oak, and poison sumac — is what causes the allergic reaction. The allergy can be triggered directly by touching the plant or indirectly by touching something (a pet, a piece of clothing, a garden tool) that has touched the plant. Poison ivy, poison oak, and poison sumac can also be picked up from the air if the plants are being burned. The rash itself does not spread, nor can you catch it from another person’s rash.

    The offending substance that causes the rash is urushiol, a clear or slightly yellow oil in the sap of the plant genus Rhus, to which poison ivy, poison oak, and poison sumac belong. Similar substances occur in several other plants and trees, including the Japanese lacquer tree, the cashew nut tree (nutshell), the mango (rind, leaves, sap), and the fruit or pulp of the female ginkgo. Just one billionth of a gram is all that is needed to cause relentless itching in some people. People are not born with a sensitivity to poisonous plants, but can develop one after the first exposure (which usually does not cause a rash). As many as 85% of the population will get a rash after the second brush with a poisonous plant.

    In order to catch poison ivy, poison oak, or poison sumac, you need to touch the oil that has oozed from a broken plant root, stem, or leaf. Keep in mind that the oil can stay active for up to five years on any unwashed surface, including pets, tools, sports equipment, sneakers, shoelaces, rugs — even dead plants — and so on.

    The rash of poison ivy, poison oak, and poison sumac is characterized by red bumps that itch and blister Figure 01. The red bumps usually come on within 12 to 48 hours of exposure, and can affect any skin surface, although the soles of the feet and palms are usually spared. You may experience some swelling and tenderness around the affected area. Only the parts of your body that have touched the sap will break out. If you have brushed by one of these plants, the rash might appear streaky. The bumps may blister, crust, and scale after a few days. The rash usually clears up after a few weeks.

    Click to enlarge: Poison ivy rash

    Figure 01. Poison ivy rash

    Poisonous plants can be found all over the country, but certain species predominate in particular geographic regions Figure 02. Poison ivy (Toxicodendron radicans) is almost ubiquitous in the US. The variety Rydberg’s poison ivy (Toxicodendron rydbergii) is most common in New England and certain parts of the mid-Atlantic and Rocky Mountain regions. As the name implies, eastern poison oak (Toxicodendron toxicarium) abounds in eastern states, but also dwells in parts of the midwest. Western poison oak (Toxicodendron diversilobum) is prevalent in western states, and poison sumac grows in the east and midwest.

    Click to enlarge: Geographical distribution of poisonous plants in the US

    Figure 02. Geographical distribution of poisonous plants in the US

    Most rashes develop during warmer months. Spring and summer are prime times for poison ivy, poison oak, and poison sumac reactions. This is when the plants are loaded with sap and bruise easily. However, you can get a rash in the winter — particularly if you are burning wood that has urushiol on it.

    Most people who come into contact with poison ivy, poison oak, or poison sumac will get the rash. However, sensitivity decreases over time. Not everyone who is exposed to a poisonous plant will have an allergic reaction. About 15% of the population has a resistance to them. And while sensitivity varies from person to person (some people are extremely sensitive and develop a terrible rash with extreme swelling), it generally diminishes with age. If you had poison ivy, poison oak, or poison sumac as a child, you may not get it as an adult.

    The distinct rash poison ivy, poison oak, and poison sumac produce are sufficient to make the diagnosis.

    Avoiding poisonous plants is your best defense. Learn to recognize the appearance of poison ivy, oak, and sumac. Poisonous plants can grow as vines or shrubs. Poison ivy and poison oak have three leaflets per leaf, whereas poison sumac can have as many as 13 leaflets per leaf. The leaves of poisonous plants may be dull or shiny, and are anywhere from one to five inches in length. The edges of the leaves may be pointy, lobed, or smooth. Though usually green, in autumn leaves can turn yellow or red; in spring they bear small green or yellow flowers.

    Wear and wash protective clothing. If you spend a lot of time outside (particularly in wooded areas), wear clothing that covers your arms and legs. Be sure to wash your clothes after spending time outdoors in the vicinity of poisonous plants, because the sap can get on them and stay on them for months.

    Lotion or deodorant can be used to block a reaction. A lotion available in most drug stores called bentoquatam (Ivy Block) can act as a barrier to poisonous plants. When applied 15 minutes before exposure, it keeps urushiol from being absorbed through your skin. . You can also try what members of the US Forestry Service do before they head into the woods: spray some deodorant on your arms and legs. Aluminum chlorohydrate (the active ingredient in deodorant) can keep urushiol from irritating your skin.

    Wash anything that may have touched a poisonous plant. People often get rashes after their pets bring urushiol into the house. Likewise, handling a Frisbee or a football that has landed in a patch of poison ivy, oak, or sumac can cause a rash. Touching towels or clothes that have urushiol on them can as well. Be sure to wash anything that may have come into contact with a poisonous plant. Wash exposed skin areas with soap and water right way. The longer you wait, the less effective washing is. If you’re in the woods, use water from a lake or stream. If you catch it within the first half hour, you may not have a reaction. If you have some rubbing alcohol handy, you can use it to inactivate urushiol.

  • Prevention and Screening

    Avoiding poisonous plants is your best defense. Learn to recognize the appearance of poison ivy, oak, and sumac. Poisonous plants can grow as vines or shrubs. Poison ivy and poison oak have three leaflets per leaf, whereas poison sumac can have as many as 13 leaflets per leaf. The leaves of poisonous plants may be dull or shiny, and are anywhere from one to five inches in length. The edges of the leaves may be pointy, lobed, or smooth. Though usually green, in autumn leaves can turn yellow or red; in spring they bear small green or yellow flowers.

    Wear and wash protective clothing. If you spend a lot of time outside (particularly in wooded areas), wear clothing that covers your arms and legs. Be sure to wash your clothes after spending time outdoors in the vicinity of poisonous plants, because the sap can get on them and stay on them for months.

    Lotion or deodorant can be used to block a reaction. A lotion available in most drug stores called bentoquatam (Ivy Block) can act as a barrier to poisonous plants. When applied 15 minutes before exposure, it keeps urushiol from being absorbed through your skin. . You can also try what members of the US Forestry Service do before they head into the woods: spray some deodorant on your arms and legs. Aluminum chlorohydrate (the active ingredient in deodorant) can keep urushiol from irritating your skin.

    Wash anything that may have touched a poisonous plant. People often get rashes after their pets bring urushiol into the house. Likewise, handling a Frisbee or a football that has landed in a patch of poison ivy, oak, or sumac can cause a rash. Touching towels or clothes that have urushiol on them can as well. Be sure to wash anything that may have come into contact with a poisonous plant. Wash exposed skin areas with soap and water right way. The longer you wait, the less effective washing is. If you’re in the woods, use water from a lake or stream. If you catch it within the first half hour, you may not have a reaction. If you have some rubbing alcohol handy, you can use it to inactivate urushiol.

Recommended Reading

Meet the Pharmacists

I'm Beth Isaac, PharmD. Welcome to PDR Health!

Check out my latest post on cholesterol drugs.

Poison Ivy, Oak, and Sumac Related Drugs

Poison Ivy, Oak, and Sumac Related Conditions