I hope the flies you are feeding are ones you've bought and hatched yourself. "Wild" flies from outdoors have been in every garbage can, dead animal remains, animal feces, etc. within a mile if not more. This is not a good clean fly. Grubco.com has good blue bottles for cheap and spiderpharm.com has houseflies(smaller) really cheap, too.
If you haven't bought fly larvae before, it's very easy to hatch your own and I'm happy to guide you through it. A lot of people raise their own but the pupae/larvae are very inexpensive and it's one less project.