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Poison Ivy

Poison Ivy - Rhus Dermatitis

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Updated May 15, 2014

Poison ivy is known in medical terms as Rhus Dermatitis which is a type of contact dermatitis. As the name implies, a contact dermatitis is an irritation of the skin caused by contact with a specific irritant. In the case of poison ivy, the irritant is called urushiol which is a resin found in the plants in the Anacardiaceae family and the Rhus genus. Plants included in this classification are poison ivy, poison oak, and poison sumac. Also included are the cashew nut tree, mango tree, Japanese lacquer tree, and marking nut tree.

Poison Ivy
The appearance of poison ivy, oak, and sumac varies by regions and season. Poison ivy leaves are most likely to be in groups of three and notched, although they can be smooth edged. Poison ivy is usually found east of the Rocky Mountains growing as vines or shrubs.

Poison Oak
Poison oak leaves come in groups of three, five, or seven. They are smaller than poison ivy leaves and have smooth, rounded edges. Poison oak is usually found west of the Rocky Mountains as a small bushy plant or climbing vine.

Poison Sumac
Poison sumac has seven to thirteen leaves on one stem angled upward. They are smooth edged, oval and about 10 cm long. Poison sumac is found in boggy areas in the south.

Interesting Facts About Poison Ivy
In the United States poison ivy, poison oak, and poison sumac cause more cases of contact dermatitis than any other agents combined. Rhus dermatitis accounts for 10% of the US Department of Agriculture and Forestry Services lost time injuries. Twenty-five million to 40 million Americans require medical attention after being exposed to one of these plants.

Poison Ivy occurs from contact with the leaf or internal parts of the stem or root of the plant. Eight to 48 hours after exposure to urushiol the characteristic rash appears. This rash is typically red, contains blisters, and is in a linear or circular pattern. Urushiol can be found under fingernails, on clothing, and on tools unless it is deliberately removed. The resin itself can be active and cause a new rash for up to 3 weeks after exposure. Urushiol is not found in blister fluid and not responsible for spreading the rash. If untreated, the rash usually resolves in 3 weeks.

Treatment of Poison Ivy
The most common sites on the body for poison ivy are exposed areas on the arms, legs, and face. The intensity of the rash varies depending on the sensitivity of the person, and the amount and extent of exposure.

  • Washing the skin with soap and water inactivates and removes the resin. Washing is most effective if it is done within 15 minutes of exposure.
  • Cold, wet compresses are effective in the blistering stage. They should be used for 15 to 30 minutes several times a day for the first 3 days.
  • Steroid creams or ointments are helpful to reduce redness and itching. Hydrocortisone can be used on the face, but is usually not strong enough for more than mild cases on the arms or legs. Typically, a prescription strength steroid is needed for these areas.
  • Oral steroids are used for severe cases of poison ivy but must be used for at least a week.
  • Short, cool tub baths with colloidal oatmeal (Aveeno) can be soothing and help control inflammation.
  • Calamine lotion helps control itching but used too long can cause excessive drying of the skin and more inflammation.
  • Antihistamines help reduce itching and the older types such as diphenhydramine (Benadryl) help encourage sleep.
  • Any exposure to the eyes or eyelids or the development of a honey-colored crust should be evaluated by a health care provider.

Prevention of Poison Ivy

  • Desensitization is not effective either by chewing leafs or having commercially prepared extracts injected.
  • The most effective prevention is using a barrier to protect the skin. Clothing serves as an effective barrier but since the urushiol remains on the clothing, it must be removed carefully and laundered without contacting the skin.
  • Urushiol can penetrate latex gloves but not rubber gloves.
  • A lotion containing 5% quaternium-18 bentonite (IvyBlock) can be applied to the skin and provides a barrier for 4 to 8 hours. It must be washed off and reapplied for continued exposure.
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