As mom used to say "look what the cat dragged in" and in this case it happened to be poison ivy. The urushiol oil will cling to cat fur which is one of the protectants for cats, dogs and other furry animals from the itch of poison ivy. I don't have much to offer though as to a solution. If the cats get out and you know they have gone wandering, you'll need to get the urushiol out of the fur. I'll have to leave this up to our other readers - perhaps one of them have a non-hosing down method.