Be careful about that friendly little puppy you pet, the purring cat you stroke or anyone who wants to shake your hand after coming inside from a hike or gardening.

There's a decent chance they could be carriers who've come in contact with poison ivy, a plant that spreads easily and is hard to kill. It does its damage by secreting a gooey sap called urushiol (you-ROO-shee-all).

An estimated 90 percent of Americans are susceptible, and poison ivy afflicts a whopping 55 million of us each year. That includes hundreds of thousands of children who get it at camp or on hikes or riding horses or playing in the backyard.

People who take all sorts of steps to avoid getting the blistery rash, like Crystal Force, 60, of Smyrna in the US state of Geogia, are surprised to learn that the plant that causes so much itching and pain is closely related to cashew and pistachio nuts, whose shells, vines and leaves also can cause severe reactions.

"I've had (poison ivy) all my life," Force said. "I'm an avid gardener, so it's inevitable I'm going to come in contact with it, though I'm always looking. I just had to ask for a dose pack of prescription cortisone tablets that will make it better."

Cortisone didn't exist in the days of Captain John Smith, much to his physical irritation. The leader of the Jamestown expedition in 1609 saw poison ivy immediately and noticed it was similar to English ivy - until he touched it. Writing home, he reported it "cause the redness, itchings and lastly blysters."

Untreated, the blisters can last several weeks. And the pain, Force said, "is terrible," causing pus-filled sores that can become infected, requiring antibiotics.

Even those lucky enough to keep the oil at bay can get sick if neighbors decide to burn dead bushes or poison ivy-wrapped trees, said Atlanta dermatologist Rutledge Forney.

"Smoke from burning poison ivy can affect your lungs, cover your whole face and body, and really cause problems," Forney said. "It happens with firefighters who breathe it. It's the oil in the smoke that causes the irritation."

With summer's official start on June 21, pediatricians, dermatologists and allergists are expecting a sharp spike in the number of cases.

"Poison ivy cases always rise right around now," Forney said. "Typically, the worst season is through the summer, but the danger doesn't go away then."

That's because the sap can still cause "contact allergic reactions" five years after the plants have died.

"When you come in contact with this stuff, you have about 15 minutes to wash it off with soap and water," said Dr Dennis Spangler, head of the 17 Atlanta Allergy & Asthma Clinics in the area.

"Over the next 24 to 48 hours, your body builds up a sensitivity, then a rash develops, people itch it, sores result and the only thing that works is prescription steroids."

Some doctors prescribe a five-day course of steroids, but Dr Kathleen Sheerin, an Atlanta allergist, said that's often not long enough. She usually prescribes a two- to three-week course of Prednisone tablets daily.

"Five days will suppress it, but it may start popping back up," she said. "If you take it longer, that's unlikely, unless you're exposed again. If you don't take steroids long enough, it would have to be a new exposure."

Poison ivy rash and blisters can't be spread from one part of the body by scratching your wrist, and then touching your leg, for example, but it should be treated quickly, Spangler said. Open sores often become infected, which then require antibiotics.

"You can rub alcohol and calamine lotion and none of that makes any difference," Spangler said. "Very mild topical steroid creams you can buy over the counter can be effective if it's really mild. But if it's worse, you'll need a prescription of Prednisone pills or a topical cream."

Dr Mark Moncino, an Atlanta pediatrician, said he has seen "a ton" of children with painful rashes and blisters.

"If someone comes in contact with poison ivy, there are two options: Do nothing, or come in and probably leave with a prescription," he said. "Oral steroids will usually knock out the worst of it within 24 to 48 hours, but you've got to stay on it."

Camp counselors and others usually have learned the hard way that there's a big difference between the three-leaf plants and four-leaf clovers.

Amy Bram, director of Camp Kingfisher at the Chattahoochee Nature Center near Roswell, Georgia, said workers scour camp areas, carefully removing poison ivy plants with gloves.

"We also avoid every kind of burning because you can inhale it and get quite sick from that," she said. "We tell them the old saying, 'leaves of three, let them be,' and that's been working for us."

But what about people like birders who just can't stay out of the woods?

Birders who are looking up often step in it or touch it, so Carol Lambert, 57, past president of the Georgia Ornithological Society, strongly advises people before hikes to take plenty of water - not just for drinking, but for washing exposed areas.

"I show them what poison ivy looks like, the leaves and vines. A lot of people don't know the vines grow high on trees," Lambert said.

At a glance

55 million Americans a year develop allergic reactions to poison ivy, poison sumac and poison oak; poison ivy results in 1.5 million emergency room visits a year.

Poison ivy is a woody plant with three-leaf clusters, thus the adage, "leaves of three, let them be." Leaves can be smooth or notched. The top of the leaves is smooth, the underneath, fuzzy. Plants produce yellow-green flowers and white berrylike fruit.

About 90 percent of people will develop an allergic reaction to poison ivy.

Poison ivy produces an oil called urushiol. It's found on leaves, stems and roots and flows in spring and summer. One-billionth of a gram can cause a rash.

Poison ivy is not contagious, but the oil that causes itching can be spread by contact. If you've been exposed, don't touch anyone until you've thoroughly cleaned yourself.

Sources: American Academy of Dermatology, American Academy of Family Physicians

(Shanghai Daily September 12, 2007)