The mesquite tree thorns, catus, prickly pear, thorn bushes, stininging nettles, cedar shrubs, rattlesnakes, wild boar, deer, rabbits, armillos, scorpions, chiggars, mosquitos, lizards, horned frogs, (and now fire ants!) tend to keep the poison ivy away from the Hill Country.
Then again, who would notice a little poison ivy or poison oak?