You might ask a vet, if you could find one that wouldn't charge you an office visit for the inquiry. That, or just do like you mentioned and take a pass on them. Around where I live an adult female rat of breeding age goes for $5.00 or so from a pet store and for a lot less from a private feeder/breeder. If $10.00 is the cost of some peace of mind, I'd take a pass on them rather than risk them actually having something that could injure or kill your snakes, remote as the chance may be.