The rash is caused by urushiol, an allergen found in all parts of the poison oak plant. 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.
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 exposure and development of a rash is determined by the amount of allergen contacting the skin, the thickness of the skin and the sensitivity of the individual. Exposure to large amounts of the allergen urushiol generally equates with more severe reactions, however, highly sensitive individuals may develop severe reactions to even minimal contact with the plant. The more allergic one is, the less allergen it will take to develop a rash and it will develop sooner. The rash will continue to develop in new areas over several days in parts of the skin that have touched the plant directly or where the allergen was spread from one area to another.
Once you have changed out of exposed clothing and bathed, you cannot spread the rash on yourself or give it to someone else. The rash may seem to be spreading, but it is usually still developing from previous plant contact or you have touched something that still has the allergen 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 you or someone else to break out. The rash is a manifestation of your allergy; it is not contagious.
Most poison oak rashes can be treated at home. Mild cases can be treated with wet compresses, cool baths, and calamine lotion. Antihistamines are of no value. It is important to wash the area immediately after contact with the plant as this will inactivate the allergen.
Moderate to severe cases require treatment by a physician. Often, the early symptoms of itching can be reversed with strong prescription topical steroids. Cortisone pills and injections of cortisone are used in more severe cases. Without treatment, the rash often lasts up to 3 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 undersides of the leaves area much lighter green than the surface and are covered with hair. It grows as a large standing shrub or vine and there are groups of three leaves in close proximity to each other.