December 9, 2012

Nature's Christmas Tree??

Not long ago while walking through Ohio's Springville Marsh State Nature Preserve I noticed this small tree:

My first thought was that it looked like fun Christmas ornaments were hanging from the tree, moving lightly in the breeze.

Immediately after that thought, however, my brain registered "Poison Sumac" and the notion of hanging the fat golden clusters of berries on a tree inside a house was quickly rejected. You see, I have a long history of being allergic to urushiol, the oily allergen that is characteristic of Poison Sumac, Poison Ivy, and Poison Oak. What a family! 

This family (Anacardiaceae) is commonly called the cashew family, and it includes mangos and pistachios as well as cashews. All of these contain urushiol or very similar compounds, but in smaller quantities than the related species that occur in North America. Some people are quite allergic to pistachios and the skin of mangoes for this reason, and cashews have to be peeled and roasted to destroy the irritant.

Poison Sumac (Toxicodendron vernix) fortunately is not very common. It only grows in wet areas, and other places I've seen it in Ohio include Cedar Bog, Cranberry Bog in Buckeye Lake, and Brown's Lake Bog. It has a compound leaf with 7-13 leaflets, so the poison ivy/poison oak adage "leaflets three, let them be" doesn't work. I don't have a great picture of its reddish pink fall color, but this gives the general idea:

It really is a bad idea to touch any part of this plant. The urushiol is found in all parts of it, and Poison Sumac causes an even worse contact dermatitis than the other two North American members of its genus. In fact, some botanists have called it the most toxic plant in the United States.

So what is urushiol, and why does it cause the itchy, blistery, seeping rashes? It is an organic compound that can have several chemical formulas depending on the composition of its side chains. Once it has been absorbed by the skin it is recognized by cells that are part of the immune system. These cells then migrate to lymph nodes where they present the urushiol to T-lymphocytes and send them to the skin. Once in the skin, the T-lymphocytes produce the compounds which cause the rash.

Lots of factual information, old wives' tales, and home remedies can be found in A Field Guide to Poison Ivy, Poison Oak and Poison Sumac by Susan Carol Hauser. As for most of the home remedies, the book basically says that if you use them the rash will clear in about two weeks; without them it will last 14 days! Surprisingly, the book says that if you rinse with copious amounts of water soon after exposure the urushiol will be washed away, and adding soap has no additional effect. I'm not sure I believe that; I always use soap if I know I've been exposed, and I think I'll continue to do so!

Apparently the word urushiol comes from Japanese, where it is represented by this character:

The Japanese have long used the toxic sap from the Chinese Lacquer Tree (T. vernicifluum) in art work, musical instruments and furnishings. After heat treatment, the urushiol-laden sap hardens and forms a durable, glossy finish which is way less toxic than the untreated material but some people still may be sensitive to it. If you search for "Urushi Lacquer" on Ebay you can see all sorts of possibly toxic products that you can buy!

What I'd like to know is why humans (and possibly some other primates) have immune systems that react in such an extreme way to this compound, while birds and other animals eat berries and other parts of the Toxicodendron genus without any problem. And what, if any, purpose does urushiol serve for the plants that produce it? 

At any rate, I definitely won't be getting out the gold spray paint any time soon, a la Martha Stewart, to turn these toxic berries into decorations for my house!

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