Walking Well: Poison Ivy

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What's the big deal with poison ivy? Most of us get a little red rash with pimples and blisters for a few days. But my neighbor has spent six days in the hospital twice with very severe rashes, circulatory and breathing difficulties, was told to dispose of all possibly exposed clothes, and realizes that she could die if exposed again to poison ivy. Clearly, poison ivy has differing toxicity in different people.

About 75 percent of people in the United States have the rash at some time. Only 15 percent of the population will not develop the rash, while 20 percent are highly sensitve. Letters from afflicted hikers have asked how to deal with this sensitivity.

Nearly everyone knows that the poison ivy plant (climbing species: Toxicodendron radicans) has three shiny green leaves and is a climbing vine or bush, twining around trees. It develops yellowish-white berries later in the season and its leaves turn red and orange in the fall. There is a limited geographical distribution with the northern boundary in southern Vermont. The bad news is that another non-climbing species (Toxicodendron rydbergii) starts there and goes north, covering the rest of the AT. For the PCT, poison ivy is not seen in California but covers the Pacific Northwest. Poison oak and sumac also cause a problem and are found over the whole AT, as well California's PCT. Poison oak is a major problem in California, disabling one third of forest fire fighters because it is widespread and aerosolizes in fires.

The villain in poison ivy poisoning is urushiol, an oil similar to carbolic acid. It is the active agent in plant sap, which causes the contact dermatitis. The initial contact with the toxin does not cause a rash, but over time sensitization develops. Most cases are mild, but some are severe. Rash onset may occur as early as six to twelve hours after exposure, but is usually at 24 to 48 hours. It may begin as late as a week after contact. The rash may last about a week in mild cases, but in highly sensitive cases it can last two to three weeks and may require hospitalization.

It's a myth that only the leaves of poison ivy, oak, and sumac are poisonous. Stems, roots, flowers, and berries are also poisonous. The sap in these structures contains urushiol, and you can get poison ivy in the middle of winter when all it is is a vine and bare branches.

Nor is the poison ivy rash contagious: It is not spread by contact with blister fluid, only by contact with urushiol. This may occur when you brush against a plant containing urushiol or when you touch something (such as a pet's coat or gardening clothes) contaminated with urushiol. The rash may emerge over several days if the concentration of urushiol differs on various parts of the body.

Some people seem to be immune to poison ivy, because they never develop a reaction no matter how often they're exposed. But even a person who has apparent immunity may develop a rash if exposed to extremely high concentrations of urushiol or if they are continually exposed to it.