We just go on a case by case sort of thing. If the rat isn't well but hasn't yet lost a lot of condition/body weight, then we'll feed it off. If it's sickly, gone skinny, basically looks like crap we just euth and toss it. We go through about 100 live feed prey a month around here which really isn't that many and raise all our own feeders. I figure why go through all that work to end up feeding off junk animals when I've got perfectly good, chunky healthy feeder rats just sitting right there waiting to be ate.