POISON IVY/OAK/SUMAC INFORMATION

How does Poisoning Occur?

Poison oak, poison ivy and poison sumac have the poisonous sap (urushiol, pronounced You-Roo-Shee-All), in their roots, stems, leaves and fruit. The sap is released when the plant is bruised, making it easier to contract Rhus-dermatitis in the spring and early summer when leaves are tender. The sap may be deposited on the skin by direct contact with the plant or by contact with contaminated objects, such as shoes, clothing, tools and animals. Severe cases have occurred from sap-coated soot in the smoke of burning plants. Because urushiol is inside the plant, brushing against an intact plant will not cause a reaction. But undamaged plants are rare because "Poison oak, ivy and sumac are very fragile plants," says William L. Epstein, M.D., professor of dermatology, University of California, San Francisco. Stems or leaves broken by the wind or animals, and even the tiny holes made by chewing insects, can release urushiol.

Can I spread it by Scratching?

"Rhus plants (poison ivy, poison oak, poison sumac) are the most common cause of allergic contact dermatitis in the US. Rhus plants contain the potent antigen urushiol, which will sensitize 60% to 80% of the persons who are exposed to it. (It) may be carried on the fur of pets, clothing, shoes, toys, tools, or other objects and then transferred to the skin. Approximately 24 to 36 hrs after a sensitized person is exposed to the urushiol, a blistery, itching rash develops. Usually within 15 minutes of contact, the urushiol binds to skin proteins. If it is washed off with soap and water before that time, a reaction may be prevented. After the antigen is fixed, however, it cannot be transferred to other areas. Scratching or oozing blister fluid cannot spread the antigen to other areas of the body or to other persons. New lesions that appear a few days after the primary lesions represent less sensitive areas or areas where less antigen was deposited, not spreading of the antigen. The course of the reaction usually is 12 to 18 days.

Developing a Rash

The first time urushiol touches the skin, an allergic reaction seldom occurs. With repeat exposure, sensitivity to urushiol develops. Sensitivity varies from person to person. About 85 percent of people develop an allergic reaction when adequately exposed to poison oak. Only about 15 percent seem to be resistant.

While sensitivity to poison ivy, oak, and sumac tends to decline with age, people who reach adulthood without developing a sensitivity have a 50 percent chance of developing an allergic reaction to poison oak, ivy and sumac. Children who have reacted usually find that their sensitivity decreases by young adulthood. People who were once allergic to poison ivy may even lose their sensitivity entirely later in life.

Nearly one-third of forestry workers and firefighters who battle forest fires in California, Oregon and Washington develop rashes or lung irritations from contact with poison oak, which is the most common of the three in those states.

You can get the oil on your skin by:

"Poison oak, ivy and sumac are very fragile plants," says William L. Epstein, M.D., professor of dermatology, University of California, San Francisco. Stems or leaves broken by the wind or animals, and even the tiny holes made by chewing insects, can release urushiol. Reactions, treatments and preventive measures are the same for all three poison plants. Avoiding direct contact with the plants reduces the risk but doesn't guarantee against a reaction.

Urushiol can stick to pets, garden tools, balls, or anything it comes in contact with. If the urushiol isn't washed off those objects or animals, just touching them, for example, picking up a ball or petting a dog, could cause a reaction in a susceptible person. (Animals, except for a few higher primates, are not sensitive to urushiol.)

Urushiol that's rubbed off the plants onto other things can remain potent for years, depending on the environment. If the contaminated object is in a dry environment, the potency of the urushiol can last for decades, says Epstein. Even if the environment is warm and moist, the urushiol could still cause a reaction a year later.

"One of the stories I tell people is of the hunter who gets poison oak on his hunting coat," says Epstein. "He puts it on a year later to go hunting and gets a rash [from the urushiol still on the coat]."

Almost all parts of the body are vulnerable to the sticky urushiol, producing the characteristic linear (in a line) rash. Because the urushiol must penetrate the skin to cause a reaction, places where the skin is thick, such as the soles of the feet and the palms of the hands, are less sensitive to the sap than areas where the skin is thinner. The severity of the reaction may also depend on how big a dose of urushiol the person got.

Quick action is needed because urushiol can penetrate the skin within minutes, there's no time to waste if you know you've been exposed. "The earlier you cleanse the skin, the greater the chance that you can remove the urushiol before it gets attached to the skin," says Hon-Sum Ko, M.D., an allergist and immunologist with the FDA's Center for Drug Evaluation and Research. Cleansing may not stop the initial outbreak of the rash if more than 10 minutes has elapsed, but it can help prevent further spread.