The following is quoted directly from the Association of American Feed Control Officials’ Questions and Answers pdf available from their website at http://www.aafco.org/Portals/0/Public/Q-AND-A-REGARDING-PETFOODREGS.PDF

4. Am I correct that may include materials from animals which died by means other than
slaughter are explicitly defined as adulterated unless the materials are rendered in compliance
with animal health and protein product regulations to destroy any potential microorganisms which may be in the products. The processes used are deemed to be adequate to control risk of
disease.
5. Beef spines and brains are allowed? Isn’t there a concern for BSE (or Feline spongiform encephalopathy in cats, for example?)
The current “Feed Ban”, federal regulation 21CFR589.2000 does not prohibit these materials
from render by-products for non-ruminant animal feeds and such materials may be used in animal feeds including livestock feeds and pet foods. The threat of BSE has been determined to be minimal from materials sourced within the US as evidenced by USDA sampling of animals at
slaughter.” http://www.aafco.org/Portals/0/Public/Q-AND-A-REGARDING-PETFOODREGS.PDF

Translation of answer 4: Yes, sick, dead and dying animal bodies are put into your pet’s food, if it’s food that you’re buying from the store.

Translation of answer 5: Yes, your pet is eating cow brains and spines and can therefore catch mad cow disease.” You could also catch mad cow disease, called Creutzfeldt-Jakob disease in humans, by eating your pet’s food. Although AAFCO says that the USDA says the exposure risk is “minimal”, keep in mind these two things:

  1. A classic sign of mad cow disease is the poor cow’s inability to stand.
  2. American meat slaughter houses were recently exposed by watchdog groups as shocking cattle who were too sick to stand so that they would get up and go be slaughtered. Some of us eat this. You may not after reading this: http://www.hfa.org/hot_topic/wash_post.html It’s accurately entitled “They Die Piece By Piece.” The animals are fully conscious.

For more horrifying reading about pet food, go to https://www.aafco.org/Publications.