Poison Ivy is one of the burdens of American terrier work -- it's out there and you will get it if you work enough hedgerows.
Poison ivy, poison oak and poison sumac are all in the Rhus family of plants, and these plants produce more cases of contact dermatitis than all other substances combined. Plant-caused rashes can affect any part of the body, but commonly affect the forearms, face and back of the hands.
Poison ivy skin rashes are not contagious -- only the toxic oil of the plant (urushiol) causes the poison ivy reaction, and generally it has to be on your skin for an hour or more before it takes effect.
You can reduce or eliminate your chance of getting poison ivy in the field by wiping off your arms with soap and water if they have been dragging around in poison ivy during a dig. A very easy way to do this is with a packet of "handywipes" designed to clean off oils, such as Kimberly-Clark's Professional Heavy-Duty Hand Cleaning Wipes.
Tecnu is a special outdoor skin cleanser that removes Poison Oak and Ivy Oils from your skin. This product stops the irritant from spreading, and can also decontaminate pets and tools. Small bottles are sold at many camping stores and pharmacies, such as Walgrens; just apply with a small hankerchief..