Poison Oak

Poison oak dermatitis is the most common skin allergy caused by contact with plants in the western United States.  Contact with poison oak in those who have become allergic to it results in an intensely itchy, red, bumpy, and often blistery rash which may even cause significant swelling and weeping in the affected areas.  Lines or streaks of red bumps or blisters are almost diagnostic of plant contact dermatitis.  Serious symptoms may include swelling of the face, mouth, neck, genitals, or eyelids and widespread, large blisters that ooze large amounts of fluid.

The rash is caused by urushiol, an oil found in all parts of the poison oak plant, in people who are allergic to it.  People generally don’t react upon initial exposure; therefore, young children rarely get poison oak dermatitis.  But the plant is such a strong allergen that just about everyone with an intact immune system will become sensitized with sufficient or repeated exposure(s).

The rash typically develops between 8 and 48 hours after contact with the plant, however, it can appear up to 15 days later.  The time between onset and development is determined by the amount of allergen coming in contact with the skin, the thickness of the exposed skin and the sensitivity of the individual.  The more urushiol you come in contact with the more severe the reaction is.  A severe reaction to a small amount of urushiol can develop in people who are highly sensitive.  The more allergic one is the less allergen it will take and the rash will develop sooner.  The rash will continue to develop in new areas over several days, but only on parts of the skin that have touched urushiol directly or where urushiol was spread by touching.

Once you have bathed, you cannot spread the rash on yourself or give it to someone else. The rash may seem to be spreading, but it is either still developing from earlier contact or you have touched something that still had urushiol on it. In severe cases of poison oak dermatitis, the rash may seem to be spreading all over the body.  This is an “id reaction” and is mediated by the immune system; i.e., other areas are breaking out in sympathy with the main severe areas of the rash.  Direct contact with the blister fluid or the rash will not cause someone else to break out.  The rash is not contagious.

Most poison oak rashes can be treated at home.  It is important to wash the area immediately after contact with the plant.  Often, the early symptoms of itching can be abated by strong prescription topical steroids. Moderate to severe cases require treatment by a physician, who may prescribe corticosteroid pills, topical steroids, or injections of cortisone.

In mild cases wet compresses, cool baths and calamine lotion can be used. Antihistamines are of no value.

Without treatment the rash usually lasts about to three weeks.  However, in people who are very sensitive to urushiol the rash can take up to 6 weeks to clear.

The best way to prevent the rash is avoidance.  Poison oak has leaves that are shaped somewhat like oak leaves.  The underside of the leaves are a much lighter green than the surface and are covered with hair.  It grows as a large standing shrub or vine and three leaves are commonly found in close proximity.

When contact with the plant is unavoidable, heavy clothing, such as long pants, a long-sleeved shirt or jacket, and vinyl gloves along with a barrier cream or lotion may help protect the skin.