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Poison Oak is a shrub-like form of poison ivy and is a member of the cashew family, Anacardiaceae.
Poison Oak from the southeastern United States is called Rhus quercifolia, and its leaves are divided into three leaflets with dense hair and three to seven lobes. The poisonous fruit is white and berry-like and are somewhat hairy. On the Pacific coast, the poison oak variety is Rhus diversiloba. It is shrubby or sometimes a climbing plant with three-leaflet leaves and toothed leaves that are hairless. Both species are poisonous, just like apoison ivy. The plant is generally found at elevations under 4,000 feet (1,200 meters).
The chemical in poison oak is urushiol and it is found in all parts of the plant. When you touch it, the oil of the poison is absorbed into the skin and a rash results. The rash is the consequence of your body trying to fight off the poison.
You can get a reaction by touching any part of the poison oakplant and if your pet has wandered through it and you touch the pet, you can also get the infection. Interestingly, pets are immune to the condition. You can also get poison oak from touching your clothing if it contains even trace amounts of urushiol.
Once you have absorbed the poisonous oil into your skin and the rash appears, you will not spread the rash by scratching. This is because the rash is directly caused by your immune system, not from the poison itself. The oil that may ooze from the rash is not the poisonous urushiol, but an oil secreted from your own body. You may think that you are spreading the infection by scratching, but you are not.
The best way to avoid poison oak is to recognize it and, when you come home from a hike, wash everything you were wearing — including your shoes!