Not toxic to livestock, according to the site linked below. Let's hope it won't affect your kitty cat either, but please give the vet a call asap.

Here is a link that might be useful: miner's lettuce

I doubt you will have a problem with your cats eating miners lettuce in small quantities. I know the cats we had at the second ranch I grew up on had free range of the place, miners lettuce rgew everywhere and I have no doubts the cats sometimes ate the stuff. The cats never had any ill effects from it....I love the stuff, great with green goddess salad dressing, hmmmmmmmm