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Answer: Urushiol

Appearance

“Leaves of three, let them be!” The distal cluster of leaves often look “wet and shiny” in the sun. Poison ivy and poison oak have this trait. You will see a stem with a larger leaf at the end and two smaller leaves shooting off the sides—these side leaves have pointed tips and are usually either notched (like mittens) or smooth on the edges. The plant is reddish in spring, green in summer, and yellow/orange in fall. It can have clusters of greenish-white berries through the spring and summer and can have green/yellow flowers.

Scenario

Picture this: a work-free day. You want to make the most of it!

You take a nice walk through the forest or spend a pleasant afternoon doing weekend warrior activities clearing out your garden. Maybe you arrange time with friends and enjoy an out-of-the-way picnic. It’s only later that you discover the blossoming of a terrible itchy, raised red rash with scant vesicles on areas of exposed skin. They threaten to torture you for days to weeks. You’ve been exposed to the silent but injurious toxin urushiol.

The most common plants in the Toxicodendron genus are poison ivy, poison oak (Rhus diversiloba), and poison sumac (Rhus vernix). All of them wield urushiol as defense from herbivores and can cause allergic reactions when human skin comes into contact with their sap. 

Distribution

Most of North America has a species of poison ivy, though it is not present in Alaska or Hawaii. It is found in forests, fields, wetlands along streams, roadsides, and even urban environments like parks and backyards. Poison oak is found both on the Pacific and Atlantic shores, and poison sumac is found primarily in the Eastern half of the United States.

Exposure

Most exposures happen with direct contact with the plant but may occur from fomites—your dog’s hair when petting it, yard tools, unwashed clothing, or working gloves.

When in contact with skin, the sap oil (urushiol) of these plants can cause an allergic contact dermatitis (phytodermatitis) reaction.

Skin reactions of urushiol contact vary but usually include an itching rash, swelling, and redness. They classically display a linear morphology of crisscrossing streaks.

Burning these poisonous plants produces smoke that contains urushiol and, though rare, can cause life-threatening pulmonary insult if inhaled.

Toxin

Urushiol is an allergenic oleoresin containing the allergens pentadecylcatechols or heptadecylcatechols. While urushiol itself is essentially benign, it causes a pronounced local T-cell-mediated immune response—a delayed hypersensitivity—which causes the vigorous local dermatologic inflammation.

Treatment

1. Corticosteroids are the mainstay of therapy, used to diminish the immune response and resulting inflammation. Antihistamines are typically used for itching. For mild cases, you can use manganese sulfate solution to reduce the itching.
2. After a known exposure, immediately rinse the exposed skin with rubbing alcohol, poison plant wash, or degreasing soap (such as dishwashing soap) or detergent and lots of water.
3. Make sure you advise patients that any materials, tools, and pets that may have been exposed must be thoroughly washed and decontaminated. The oils are persistent and can cause re-exposures (and reactions) weeks to months later if these fomites are not cleaned off.

Facts

• All parts of a plant with urushiol are poisonous.
• The oil is mostly colorless to watery yellow and has no odor. It is secreted from any damaged part of the plant.
• Pistachios also contain the toxin but do not seem to cause a rash.
• Cashews can occasionally have topical effects in sensitive individuals. The fruiting body has a three-layered pericarp where the middle layer has the heat-labile cardol and anacardic acid, which are chemically similar to urushiol. While these are destroyed by roasting, harvesters can develop allergic phytodermatitis from exposure to the oil.
• Mangos contain urushiol in their outer skin. Facial and lip dermatitis can occur when a sensitized individual bites into an unpeeled mango.

Selected Sources

• Poisonous plants: geographic distribution. Centers for Disease Control and Prevention website. Available at: https://www.cdc.gov/niosh/topics/plants/geographic.html.
• Poison ivy: an exaggerated immune response to nothing much. University of Massachusetts Amherst website. Available at: https://www.bio.umass.edu/micro/immunology/poisoniv.htm.