The Poison Plants of Summer: Poison Ivy, Oak and Sumac
Jul 26, 2013
Summer is in full swing. You know the signs. Dad fires up the grill, mom finds her green thumb, your kids fetch their ball in the weeds…and the whole family comes down with an itchy rash. That’s right, the poison plants are back. And this summer they promise to send 40 million Americans to the doctor.
The three most common culprits—poison ivy, poison oak, and poison sumac—are native to America and caught European explorers by surprise. Captain John Smith wrote, “The poisoned weed is much in shape like our English ivy, but being touched, causeth redness, itching, and lastly, blisters.”
Pocahontas could have warned him. After all, Indian warriors coated arrow tips with poison ivy, and medicine men rubbed its leaves on infections in an effort to break open swollen skin. Physicians in the Colonies learned something new and expanded the plant’s use to the treatment of herpes, eczema, arthritis, warts, ringworm, and rattlesnake bites.
Today we know the rash is an irritating allergic dermatitis. Upon exposure, the immune system revs up, attacking the plant oil and damaging your skin. The few who don’t react should still watch out. Sensitization can occur anytime, making a fool out of cousin Eddie when he rubs the leaves up and down his arm just to prove he “ain’t allergic.”
The offending substance in the oil is urushiol (oo-ROO-shee-awl), a yellowish chemical inside the leaves, stems, and roots of the plant. Because the oil is inside, undisturbed leaves won’t harm you. However, if the plants become dry, insect-chewed, or otherwise damaged, the oil leaks to the surface where it comes into contact with humans and other animals.
As it turns out, only primates are allergic. Dogs, cats, cattle, and sheep don’t break out, but they can pass the oil to their human keepers. Clothing and tools also spread the oil, and since urushiol remains allergenic for years, unsuspecting spring gardeners can get the rash from last season’s gloves and shovels.
If you come into contact with poison ivy, you should wash well with plenty of soap and water. Unfortunately, this won’t always prevent the rash because the oil bonds fast to human skin and is nearly impossible to remove within 30 minutes of exposure. This also means you won’t spread the rash by touching affected skin. Poison ivy does have the illusion of spreading, but this is because areas with the greatest exposure break out first, and because thinly-skinned body parts react more quickly than thick skin.
Once you have the rash, it typically lasts 2 to 3 weeks as the damaged skin is replaced by new cells. All you can do is treat the symptoms and prevent infection. For mild cases, over-the-counter hydrocortisone cream and oral antihistamines—like Benadryl—work well to control inflammation and itching. Topical antibiotic ointment may prevent bacterial infection when the skin is broken open from repeated scratching.
If you have widespread rash, face or genital involvement, or signs of infected skin, it’s time to see your doctor. In these cases, prescription steroids or antibiotics might be necessary.
Prevention by avoidance is best. All three poison plants have compound leaves, most commonly with three leaflets. My Grandma Bert used to say, “Leaves of three, let them be!” and she was right.
The leaves are smooth and glossy, colored with a summer green that transitions to autumn shades of orange and scarlet. Poison ivy is a stout weedy vine that climbs trees east of the Rocky Mountains. Poison oak is larger, more shrub-like, and found west of the Rockies. Poison sumac grows in swampy areas of the Southeastern United States where it can reach heights of twenty feet.
IvyBlock, and other medications containing the chemical bentoquatam (BEN-toe-kwa-tam), prevents the rash by providing a barrier on the surface of exposed skin. It is available over-the-counter and approved for ages 6 and up. It should be applied at least 15 minutes prior to expected exposure and reapplied every 4 hours to remain effective.
So get out there and enjoy your summer. Fire up the grill. Tend the garden. Play ball. But don’t forget my grandmother’s advice. Otherwise, you might end up in the waiting room, scratching your itchy skin with those other 40 million Americans.
Dr Mike Patrick is an Assistant Professor of Pediatrics at the Ohio State University College of Medicine and Medical Director of Interactive Media for Nationwide Children's Hospital. Since 2006, he has hosted the award-winning PediaCast, a pediatric podcast for parents. Dr Mike also produces a national podcast for healthcare providers—PediaCast CME, which explores general pediatric and faculty development topics and offers free AMA PRA Category 1 Credit™ to listeners.
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