Allergy & Asthma-Kent H. DeYarman, MD

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Poison Oak Allergy

Poison Oak Allergy

What happens in poison oak allergy?


Poison oak allergy is a type of allergy known as contact dermatitis. It is quite different than allergies to pollen, food, bee stings, etc. Contact dermatitis is also known as Type IV delayed hypersensitivity. In contact dermatitis T lymphocytes (a type of white blood cell) become sensitized to a foreign material (in this case chemicals in poison oak known as urushiols) and recognize the foreign material for years to come. With subsequent exposure the sensitized T lymphocytes migrate to the area of poison oak exposure. Upon arrival they release materials that cause the redness, swelling, itching, and oozing we associate with poison oak. It usually takes 1-7 days (average 2 days) to develop the rash after exposure. The T lymphocytes continue to release their materials that cause the rash until they no longer detect urushiol. Consequently the rash may continue for weeks if not treated.


Who becomes allergic to poison oak?


We do not know why some people do or do not become sensitive to poison oak. Genetic predisposition plays a role but so does the amount of exposure. In western Oregon exposure is very high and many people become allergic to poison oak, perhaps as many as 75%. Older individuals are less likely to develop problems with poison oak. It is not uncommon to spontaneously lose sensitivity to poison oak.


There are many misconceptions about poison oak. Some pertinent facts are listed below:


*You cannot get poison oak “through the air.” You must physically touch the poison oak urushiol oil or be exposed to it in smoke from burning poison oak. Because it takes 1-7 days (and occasionally as long as 14 days) to develop the rash after exposure, it is often difficult to remember the actual exposure.


*All parts of the plant contain the urushiol oil that causes the rash. It is most prevalent in the leaves in the spring but can be found in the stem and the roots at any time of year. The concentration and exposure is highest if you are exposed to leaves that have been torn or bruised (mowing, weedeating) and if the plant penetrates the normal skin barriers (a twig snapping hard on the skin).


*You cannot spread the rash on yourself or to another person once you have washed with soap and water. Soap and water destroy the urushiol chemical. It is, however, important to clean all parts of the body including under the fingernails or the oil may still be present. You may also acquire poison oak from exposure to the oil on animals, clothes, tools, fishing rods, binoculars, cameras and other inanimate objects where it may remain active for months if not washed with soap and water. Tecnu makes products said to remove or inactive the oil better than soap though I am not aware of any scientific studies documenting this. It may seem like the rash is spreading even after washing with soap and water but the time from exposure to the rash depends on the sensitivity of the skin and the degree of exposure. In general the skin around the eyes, inner arms, legs, and genital areas are most sensitive and break out first. The palms are resistant to the rash but hands often transmit the urushiol oil to other areas before washing. It takes about an hour for the urushiol to become “fixed” to structures in deeper skin layers. After this occurs you cannot completely wash it off though you still cannot spread the rash after washing to other parts of your body or to other people. If you know you are working in poison oak a quick soap and water shower and a clean set of clothes every 45-60 minutes may prevent much of the rash. The type of soap does not matter though there are many myths still circulating about this.


Treatments available without seeing a doctor.


*Ivy Block is a barrier cream that when applied before exposure absorbs the urushiol oil and prevents the rash. This is the only FDA studied and approved barrier cream shown to be effective. It is fairly expensive and only partially effective. You still need to take other preventative precautions. Tecnu markets products said to remove the oil from the skin better than soap though I am not aware of FDA studies demonstrating this.


*You should not use nonprescription local anesthetics or topical antihistamines on the rash including those that say they are for poison oak. Most of these can easily cause allergic rashes by themselves, especially if put on irritated skin. Calamine (but not Caladryl) is OK as is Burow's solution, the latter being useful for oozing areas. You should not use bleach on the rash, a common practice in Oregon. It may numb nerve endings but may damage the skin.


*Non prescription cortisone creams are usually too weak to be effective but may be used for rashes around the eyes and on the face.


*Very cold or moderately hot water may provide temporary relief of itching and burning in some people.


*Oral antihistamines (such as Benadryl) may reduce itching but less so than for other types of allergic reactions and usually only at doses that cause significant sedation. Newer non sedating antihistamines such as Claritin and Zyrtec have minimal effect on poison oak.


What treatments are available through a doctor?


*Cortisone is the only medicine that makes a significant difference. Topical cortisones may help if they are very potent (such as Halog, Lidex, Diprolene, or Temovate). These cannot be used on the face or under skin folds and may be very expensive, making them practical only for outbreaks that involve small areas of the body. Oral cortisone pills or syrups (such as Prednisone, Medrol, Prelone) may be very useful and may be used safely in short courses. Cortisone shots (such as Kenalog, Depomedrol, etc) may be effective but we prefer cortisone orally rather than the injected forms because they tend to be more effective and do not linger for weeks to months in the body as cortisone shots do.


What about desensitization?


*Desensitization has not been found to be effective for poison oak and all desensitization materials have been removed form the market by the FDA. Though many people seemed to improve on desensitization programs in the past (with both shot programs and oral desensitization programs) many people also improved who were not receiving desensitization. No one has yet found a desensitization program for poison oak or other types of contact dermatitis that has been more effective in scientific studies than a placebo. This is in contrast to desensitization for other types of allergy (inhalant allergy, bee stings, some drug allergies) where desensitization has been shown to be clearly effective. Work is ongoing in this area and perhaps we will have effective desensitization in the near future.