Hello,
I am new to this board but
here is an article about puppy mills from PETA
It is about an investigator that went undercover to work at a puppy. The investigator found things like "the collar on a Labrador retriever had not been adjusted as the dog grew and had become embedded in his flesh. Even though the gangrenous skin fell away as the collar was removed, it was treated with nothing but a worm-repellant spray." It seems there is also a video.
This is a factsheet from PETA and has some further possible source of research cited at the bottom.
Hope this helps
I can do some research if it is helpful, let me know