My wife stumbled across this by accident. She is so alergic to it, it's as if she see's it, she breaks out with it. Anyhow, a couple of years ago she had gotten some poison ivy and took a hot shower, just as hot as she could stand. The itching stopped and it quit spreading. When it started itching again, a couple of hours later, she ran hot water over the area again and the same thing happened. She kept doing this over a couple of days and it was virtually gone. She asked the Dr. about it and she said that the heat brings the histamines to the surface (that's what causes the itch) and the water washes it away. Soon there are no more histamines to cause the itch. I got some poison ivy on my left wrist the other day and have used the hot water tip and it absolutely works.
I had a nasty outbreak from poison ivy last year. Ended up taking prednisone with Benadryl for about a week! I used Weed B Gone this year on that plant and it seems to be working.