Days are getting longer, the temperatures are rising, and stay-at-home restrictions are easing. As a result, many of us are spending more time outdoors. Hikers are hiking, mountain bikers are biking, and gardeners are gardening. Unfortunately, these activities are not risk-free when it comes to your skin. Depending on where you live, you may inadvertently have a close encounter with poison oak, poison ivy, or poison sumac and end up an annoying, unsightly, and very itchy rash.
Why does poison ivy cause itchy rashes?
Plants of the genus Toxicodendron (previously classified as the genus Rhus) can cause an itchy rash in many people who come in contact with them. The most commonly encountered plants of this genus include poison ivy, poison oak, and poison sumac.
Even though all three of these plants can cause the rash, it is commonly referred to as poison ivy dermatitis, particularly east of the Rockies. It is often referred to as poison oak in the Western US. The medical name for the condition is Toxicodendron dermatitis or Rhus dermatitis. For ease of reading, the term poison ivy will be used instead of listing all three plants repeatedly.
The rash is a type of allergic contact dermatitis (a classic type IV cell-mediated hypersensitivity reaction). It is triggered when the allergens in oily sap resin, urushiol, is transferred to the skin. These allergens are small and able to quickly penetrate the outer layer of the skin called the epidermis. The presence of these allergens on the skin triggers the immune response that results in the rash.
It is important to know that urushiol is found in all parts of the plants, including the leaves, stems, and roots. It can even be found in dead branches. And, it can be transferred to clothes, the fur of pets, and other objects such as gardening tools that come in contact with the plants. Transfer of the resin from these objects to human skin can also trigger the immune response.
The epidemiology of poison ivy dermatitis
Urushiol is highly allergenic and many people are sensitized to it. In fact, it is estimated that ~50 million people annually require medical treatment after being exposed to urushiol. Although anyone can be affected, certain professionals are at particular risk, including,
- forestry workers
Poison ivy affects children and adults of all ages, although there is some evidence that the allergic response can decreases with age if people are not repeatedly exposed. Also, it affects all ethnicities and skin types.
In the U.S., poison ivy is more common east of the Rockies and poison oak west of the Rockies. Poison sumac is found mainly in the southeastern part of the country.
Identification of the plant: the key to avoiding contact
Plants in the genus Toxicodendron commonly encountered by people who enjoy the outdoors. Therefore, it is important to learn how to identify them. Their appearance can vary depending on a number of factors, including:
- growth cycle of the plant
- region of the country
- season of the year
The old adage “leaves of three, let them be” can be a helpful way to remember how to differentiate poison ivy and poison oak from other plants that are safe to touch.
1. Poison ivy
The poison ivy plant has leaves that grow in groups of three. The leaves are pointed at the tip and the middle leaf may be slightly larger than the other two side leaves. They are green in color in the spring but turn reddish to orange in color in the fall. Poison ivy can grow as both a vine and a shrub. It may have a small cluster of white berries in the spring that can last throughout the winter.
2. Poison oak
Poison oak is more typically found as a shrub, although it can sometimes be found as a vine. Its leaves, which vaguely resemble oak leaves, also occur in groups of three similar to poison ivy. The leaves are green in the spring but turn orange to red in the fall. Poison oak flowers may be yellow or green and have greenish-yellow or white berries.
3. Poison Sumac
Poison sumac is less like the previous varieties in that it is a woody shrub with stems that contain 7 -13 leaves arranged in pairs. The berries also vary and may be pale yellow or cream-colored. Poison mainly grows in wet swampy or boggy areas.
Reference for plant ID and photos): https://www.cdc.gov/niosh/topics/plants/identification.html
The poison ivy dermatitis rash
After being exposed to the urushiol within the oily sap of the plant, a red rash may appear within a few hours, especially if you’ve had poison ivy dermatitis before. In previously unexposed individuals, it may take up to 2 or 3 weeks to develop the rash.
The rash appears as red, intensely itchy bumps or fluid-filled vesicles typically arranged in a characteristic linear or streak-like pattern identifying where the plant contacted the skin. If contact from the oil is a result of touching a piece of clothing or, touching a pet that was romping in the woods and as a result had oil on its fur, the rash may appear more spread out.
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If not promptly removed, the oil resin can be spread to other areas of the body by scratching. In addition, the rash may appear to be spreading over time because the eruptions will occur first in the areas of most intense exposure to the allergens and later in areas with less intense exposure.
The rash itself is not contagious. This is not an infection, it is an allergic reaction.
If left untreated, a poison ivy rash may last for several weeks. Those who have previously suffered from the rash tend to heal in a few days. Those who have never had the rash seem to take longer to heal.
Poison ivy prevention and treatment
As is the case for many conditions, prevention is the best cure:
- Avoid the plant if possible and learn how to identify poison ivy in all of its seasonal and geographic variations. Remember: “leaves of three, let it be.”
- Wear protective clothing when gardening or hiking: high socks, long sleeves, and pants.
- The use of barrier creams to prevent poison ivy rash is controversial. These topical preparations are typically applied prior to exposure in an attempt to prevent the rash. A reduction of reactivity was seen in a small study with a 5% quaternion-18 bentonite lotion (bentoquatam).
2. Remove the resin as soon as possible after exposure
Although the rash itself is not contagious, spreading the oil from one person to another is certainly possible if the oil is not removed.
- Wash the exposed area with mild soap and water to remove any remnant oil resin from your skin as soon as possible. Be sure to clean under the fingernails to remove any resin that may remain under the nails.
- Wash all items that may have come in contact with the plant or its oils including clothing, patio furniture, gardening tools, gardening gloves, golf clubs, and even your pets. The oil does not have to come directly from the plant to cause a reaction.
3. A mild case of poison ivy dermatitis with home remedies
- Over-the-counter hydrocortisone cream applied twice daily may relieve the itch and minimize the reaction.
- An oral antihistamine may also relieve the itch.
- Apply calamine lotion to small blisters and oozing skin to help dry out the rash.
- Epsom salt soaks may also help dry out the rash.
- Do not open blisters as this may predispose the skin to infection.
- Cool compresses applied to itchy areas may be comforting.
If the rash is widespread or severe, affects the face or genitals, if blisters are oozing pus, or if fever develops, seek medical attention immediately.
4. Medical treatments for the poison ivy rash
- Topical Corticosteroid Therapy: high potency corticosteroid preparations applied directly on the rash is the mainstay of therapy for rhus dermatitis. To avoid the development of skin atrophy, the extended use of these high potency preparations should be avoided on certain areas such as:
- the face
- skin folds of the armpits and groin
- Systemic corticosteroid therapy: Patients with severe or widespread disease, or with a rash that involves the face or genitals, may benefit from oral steroid therapy. Although there is no standard dosing, clinical experience suggests that poison ivy may rebound when too short of a course is taken (such as a 6-day dose pack of methylprednisolone). A high dose (1 mg/kg/day up to a maximum starting dose of 60 mg) of oral prednisone therapy given over 2 – 3 weeks can dramatically improve poison ivy symptoms. For patients who cannot take oral medication, or cannot comply with the dosing schedule, intramuscular injections are another treatment option.
- Antibiotics may be necessary if bacterial infection of the underlying rash is suspected.
If it’s not poison ivy what else could it be?
- An allergic contact dermatitis from a different kind of plant: Rashes similar to the one caused by the genus Toxicodendron can also be caused by other plants. Plants in the genus Asteraceae, which include the flowering plants: daisies, sunflowers, dandelions, and asters can also cause a similar skin eruption. Plants in the genera Alliaceae (garlic), Allium (onion), and Myrtaceae (tea tree), amongst others, can also cause similar rashes.
- An allergic contact dermatitis from something other than a plant: Allergic contact dermatitis can also be caused by allergens that are not plants. Nickel and gold, metals typically found in jewelry, for example, are common causes of allergic contact dermatitis.
- Phototoxic reactions that may appear as red rashes with or without blistering may occur in response to exposure to photosensitizing plants and ultraviolet exposure. The most common plants to cause these types of reactions include those in the genus Rutaceae (lemons and limes) and Umbelliferae (celery, wild parsnip, and parsley.) Unlike poison ivy, these reactions will only appear on sun-exposed skin.
Poison ivy dermatitis: the bottom line
It’s best to avoid contact with the Toxicodendron plants that cause this annoying rash. But if you can’t or didn’t, move quickly to remove any resin on your skin by washing with soap and water. If the rash erupts anyway (and it is likely to), try home remedies first. But seek medical care for more serious cases.
See more articles by Fayne Frey, MD
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Originally published on May 17, 2020. Updated by the author and republished on June 3, 2021.
Fayne Frey, MD
Fayne Frey, M.D., is a board-certified clinical and surgical dermatologist practicing in West Nyack, New York, where she specializes in the diagnosis and treatment of skin cancer. She is a nationally recognized expert in the effectiveness and formulation of over-the-counter skincare products.
She is a frequent speaker in many venues where she captivates audiences with her wry observations regarding the skincare industry. She has consulted for numerous media outlets, including NBC, USA Today, and, the Huffington Post. and has also shared her expertise on both cable and major TV outlets.
Dr. Frey is the Founder of FryFace.com, an educational skincare information and product selection service website that clarifies and simplifies the overwhelming choice of effective, safe and affordable products encountered in the skincare aisles.
Dr. Frey is a graduate of the Weill Cornell Medical College and is a fellow of both the American Academy of Dermatology and the American Society for Dermatologic Surgery.