You might have to deworm them periodically. Not sure what you could do about making sure the 'kill' hasn't been poisoned. That would be my main concern.
They won't eat voles. Don't' think they eat moles either but can't remember.
Seger vom Sitz von der Hose
Forever loved and missed.....Banshee
~The strength of one's opinion should not exceed their knowledge on the matter~