TL;DR for the title:
Employees [from this investigation] can be seen removing the intestines of dead, disease-infected piglets and mixing them with piglet feces in a blender — a mixture to be fed to the adult breeding pigs — causing one worker to gag.
The practice, called “feedback,” is common in the pork business (or “controlled oral exposure” in industry jargon).
The article itself goes into more depth about all the horrific things in the pork industry such as these
The pork industry has pushed pigs to their biological limits, leading to many bizarre practices beyond feedback, many of which are inhumane. To name one example recently in the news: There are horse farms that impregnate horses, extract their blood for a serum, abort their pregnancies, and then sell the serum to pig farms to induce puberty in young female pigs and produce larger litters. Holden Farms, like most pig breeding farms, confine pregnant pigs in gestation crates, cages so small they can’t turn around for practically their entire lives.
Wasn't this how we got mad cow disease?
Feeding dead cows back to cows is indeed the main way that mad cow disease shows up. Though there's also atypical mad cow disease where it just randomly shows up too
being genetically identical can also contribute to prion disease.