I thnk it's poison ivy. Take a look at this:
http://www.poison-ivy.org/html/poison-ivy-odyssey.htm
This stuff can hit you in various sizes and shapes and spread from there. I've always been susceptible to it. Rats!
That's the stuff. The guy in the pics has a fair to middlin' case. The next step is massive swelling and cracking of the affected area.
I get it bad too, but am surrounded by the stuff all year so there's no getting away.
The Hot Water trick just before bed allows for sleep, and Benedryl helps a bit too.
I hate the bare vines in winter worst of all. In summer, the stuff can be dodged and worked around, it's just the secondary contact with equipment(Bruch chopper, tractor, rotovator, harvester etc.) that gets me.
Ivy shield dosn't work for beans...save your money.
Tecnu ain't bad for itch relief and Zanfel is OK too.
Bets thing I have found is plain old ETHER. If you have to work on something that has been exposed, hose it down and let it dry.
Get the crud on the skin and not near soap and lots of water, ETHER gets it off quick and minimizes the reaction.
Ether is NOT good for ya though..but it's a lesser of two evils for me.
Go easy on the Prednisone. That stuff is murder on your system and damages the liver like crazy. My Doc flat out told me that unless I'm gonna die from PI, he wont prescribe it again.
I'm fixing to NUKE(Paraquat) a clump of PI as soon as the field drys up a bit.
If it makes ya feel better, I'll post some pics of horrible suffering and pain inflicted on the enemy.
Stay safe!
Dingeryote