Only the manufacturers of the pet food you use know where their ingredients come from or what contaminants might be in them. When a contract facility is producing it for them, they may not even know that.
Despite those eye pleasing ingredient photos on the bag labels and the wolves and lynx in their TV commercials, what actually goes into the bag or can doesn't look like that. At best, its the scraps, trimmings and offal left over from processing meat for human consumption. The big pet food conglomerates are international - their ingredients come from all over the world. Many contain meat from carcasses that were not up to snuff to meet government requirements for human consumption (Undenatured Inedible Meat Products = “4-D”). Unless the pet food container you buy says that the ingredients meet the requirements for human consumption, there is no guarantee whatsoever that they do. (ref) Other errors occur. (ref) The USDA considers meat unsuitable for human consumption as adequately "denatured" for dog and cat food once powdered charcoal or food dyes have been added to it. In some cases, only the containers need to note that what they contain is considered inedible for humans. (ref)
When the USDA classifies a products as being "inedible" they mean inedible for humans. They have no problem with that stuff being incorporated into your dog or cat's foods. (ref) Questionable meat and fish ingredients tend to be diverted to pet food channels. Tired old dairy cows, cattle and other livestock whose health is on the skids (the USDA Utility, Cutter, and Canner grades). Fish that sat too long on the dock. It’s a simple matter of economics. One cannot produce a mass-market pet food at a cost most pet owners would accept without making less scrupulous ingredient decisions than we make for ourselves.
Farm animals, approaching the end of their lives, are more likely to contain drug residues. They are also considerably more likely to end up in pet food than in your cuisine. Antibiotics and anti-arthritic medications (NSAIDs) are the most frequent medications that farm veterinarians administer. (ref1, ref2) When the cost of treatment is more than the animal’s value or the prognosis (outlook) for the animal is grave, its next stop is the auction barn. When those animals go to process, they form the lowest USDA beef grades – the ones destined for pet food. The “wash out time (time until all the drug is out of the body) for NSAIDs is considerable. Eight days after their last injection, 48-80% of these drugs can be still in their systems. (ref) It is not unusual for those residues to persist in meat. (ref1, ref2) The processing procedures used to create dog and cat food do not reach high enough temperatures to destroy them. (ref) NSAIDs can be hard on the kidneys. (ref1, ref2)
When Dick Van Patten owned Natural Balance Pet Foods, he advertised that the ingredients in his diets met human consumption grade and the facilities they were manufactured in were the same facilities producing packaged human food products. He is gone now and his company has passed on to other owners. Those claims are no longer made.
The bird in that photo above looks like a buzzard. Are you suggesting there is rotten meat in what I feed my cat and dog?
Its because the buzzards of India found out the hard way that eating tainted beef could be hazardous to their health. That is - when that meat contained NSAIDs. As of this writing, it is estimated that over 90% of them have died from kidney failure. (ref)
Some carnivores, cats and vultures included, have lost much of their ability to detoxify certain drugs. Scientists believe that that is due to the reduced abilities of some of their liver enzyme systems (P450 & glucuronidase enzyme systems) that detoxify certain drugs. (ref1, ref2, ref3)
That is the reason Tylenol is so toxic to cats. (ref) Its not just cats, some dogs share this defective detoxification system too. (ref) That is thought to be the reason why neither cats nor some dogs do as well as humans do (more likely to have side effects) when taking NSAIDs. (ref) One of the effects of NSAID exposure is to decrease the amount of blood that flows through the kidneys. That can be especially dangerous during times of dehydration. (ref)
When meat is analyzed by chemists at the USDA, FDA or another government entity for contaminants, it is generally examined for toxins, antibiotics, steroid (injected growth hormone), tranquilizers, bacteria and insecticide residues that are considered a threat. I believe that the meat used in pet foods is rarely if ever checked NSAID levels. These system are primarily in place to protect human health, not pet heath. (ref1, ref2)
I do not know of anyone who adequately monitors the drug content of the meat that goes into your dog or cat's food.
That is why I am more comfortable when your pets eat what you eat.