Seriously gross. What's causing this? (Graphic picture)

[Tiny Boidae]

I threw out all the ones that exploded and fed off the good ones. This is really the only one of two local suppliers that I have (the other being PetSmart) and it's been hit or miss. They often run out of certain sizes so when I get the mice, I've been handed bags with bloody mice covered with feces before. It's just frustrating really.