As far as feeding a lice ridden rodent to your snake, it would not do any harm to the snake. Most parasites and diseases are pretty species specific and likely wouldn't infect your snake . However the question I would ask is why are you buying your feeders from someplace that takes such poor care of their animals? If they can't take the time to keep their feeders clean and parasite free, how many other corners are they cutting to produce such an inferior feeder animal? I'd find another source of feeders if I were you.