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What is the itchy rash on this boys legs?

A 14 year old boy develops an itchy rash on his legs after walking through the woods. He was wearing shorts so his legs were unprotected.

What is your diagnosis?

poison ivy image

14 year old boy develops itchy rash on legs after walking through the woods. He was wearing shorts so his legs we unprotected. 


Poison ivy dermatitis

Get ready, here it comes. If you haven't seen this already, you will be seeing it shortly. Poison ivy dermatitis is a type IV hypersensitivity reaction against the oil in certain rhus containing plants. This includes poison oak, poison sumac, and poison ivy dermatitis. Remember the old adage, leaves of three, let it be. In fact, the first time you encounter poison ivy dermatitis, you become sensitized. It is only on subsequent exposures that you actually develop the cutaneous manifestation. Clinically, there are multiple, very pruritic vesicles that occur after exposure to plants outdoors. Usually, hiking through the woods or going up to the cabin. Patients also think that the fluid inside the lesions can spread, but this is not true. Actually, there is an enzyme in the skin that breaks the oil down in approximately 4 hours. So really, the rash you see is from areas that were exposed and rubbed during the first 4 hours. The fluid itself has nothing to do with the transmission of poison ivy dermatitis, the only exception being if the oil gets on animal fur and/or clothing. Any clothing contacting the oil should be washed in very hot water with detergent. Animals should be washed with a good detergent shampoo, and I defer to a veterinarian for additional information on this.


  1. The rash occurs after outdoor exposure.
  2. Oftentimes the blisters are in a linear pattern, suggestive of the "brushing of the leaves as one passes the plants".
  3. There are over-the-counter oil blockers (Ivy-Block) that can be applied on the skin BEFORE exposure to prevent the development of the rash. There is also a wash to use immediately after exposure (Zanfel) that washes away the oil preventing it from triggering the allergic reaction. They claim it also reduces pruritus at any time after exposure. I’ve had several anecdotal reports on both of these products. Check with your friendly pharmacist for additional details.
  4. The reaction is about 18 days. So if you are going to treatment poison ivy dermatitis, cover the patient for three weeks. I have also seen Steroid/Medrol Dosepaks, and they work great for five or six days and then once the patient discontinues the medication, the rash flares up. For localized areas, I use a topical class 1 gel kept in the refrigerator to be applied 1-3 times daily while symptomatic. In widespread severe cases, I will use prednisone for 18 days. Generally speaking, in adults, I will use 40 mg in the morning for 6 days, 20mg in the morning for 6 days and finally 10 mg in the morning for 6 days. In children weighing between 60 and 120 pounds, I will use 20 mg in the morning for 6 days, 10 mg in the morning for 6 days and finally 5 in the morning for 6 days.


Charles E. Crutchfield III, MD
Clinical Adjunct Associate Professor of Dermatology
At the
University of Minnesota Medical School
Medical Director, Crutchfield Dermatology

Impressive poison ivy rash, notice the “tell-tale” linear blisters near the wrist
case of poison ivy on leg

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