In canon, after several failed attempts at using magical treatments, Arthur Weasely tried stitches on his Nagini bite, but the venom dissolved them. It wouldn't be surprising if the troll's bite, either naturally or due to the mastermind's buffings, would have similar properties interfering with muggle treatments. Actually, now that I think about it, that's not a bad explanation for why Harry's first aid attempt didn't work. Canon Voldemort does not like making a kill strike that can be easily treated (Rowling even intended for Arthur's bite to be fatal, according to interviews, but changed it while writing, presumably because Harry's link to Voldemort meant this would have broken him more than because it meant he could call for help in a timely fashion, not to mention the effects on his relationships with the Weaseleys.).