Horticopia gives Poison Ivy (
Toxicodendron radicans ) a range from Canada to Guatemala, zone 5A to 9A, which gets you well into Mexico.
The best product we've found to remove and soothe is Ivy Off,
details here. It is even effective after exposure. We give every new hire a bottle, and keep one in the first aid kit for on-site cleanup. The faster you scrub off the oil, the better.
There is a very good chance your help is being re-exposed by their boots, the truck steering wheel, shovel handles...any surface that could have come in contact with the plant could have and retain the oil for weeks. P-train, you'll need to wash down all your equipment that was used at the suspected site to prevent continued exposure. Your help should scrub their boots and belts, toss their leather gloves, etcetera.
The only problem with the "leaves of three, let it be" maxim is that loads of plants have three leaflets, like Boxelder, Bindweed, and Clematis.
Oddly enough, I used to take, but haven't for years. I still manage to give it to my woman though, a one way ticket to a week on the couch!