So after the first vet visit did you unplug the UTH or did it continue to run unregulated?
BTW a lamp dimmer does the same job as a rheostat, this one is from Lowes but Home Depot should also carry them, so you could have had a fix for your UTH a week ago for $15. They are a short-term solution until you get a thermostat. http://www.lowes.com/pd_70717-539-TT...ductId=3189447