Drugs used to euthanize pets have been found in some pet foods, leading to rumors the product may include rendered dead cats and dogs.
Definitive evidence proving the claim of rendered pets in pet food (such as DNA testing) is lacking.
Urban legends about pets are some of the most tenacious and reaction-provoking, prominent among them a long-held belief that pet food (particularly the cheapest brands) is made, in part, from the rendered euthanized remains of cats and dogs themselves:
I have heard from a friend of mine (who heard it from her professors in college) that Old Roy can or once did pose a danger to dogs.
According to what she head, Old Roy is made partially from Old Roy himself. That is, the dog food contains parts of euthanized dogs. Euthanasia drugs, of course, do not disperse or go inert, which is why when you have pets put down, you must have them cremated (at least that is what I am told). According to her source, several dogs had died from ingesting the drug in pet food.
The origins of that belief are difficult to pin down. Like many rumors it tends to spike in popularity at seemingly random intervals, but one of its earliest and more prominent versions appeared around 1997 on a web page that has since vanished (although its text remains the basis for many versions of the rumor):
In 1981 while Martin Zucker and I wrote the first of my two books, How to Have a Healthier Dog, we discovered the full extent of the negative effects of commercial pet foods of that time. Much more recently, San Francisco Chronicle staff writer John Eckhouse went even further with a two-part exposé entitled "Pet-Food Labels Baffle Consumers," and (a good candidate for a horror movie title) "How Dogs and Cats Get Recycled Into Pet Food."
In the second article, published on February 19, 1990, Mr. Eckhouse, an investigative reporter, writes: "Each year, millions of dead American dogs and cats are processed along with billions of pounds of other animal materials by companies known as renderers. The finished products — tallow and meat meals — serve as raw materials for thousands of items that include cosmetics and pet food." There were the usual denials by pet food executives. Yet federal and state agencies, including the Food and Drug Administration and medical groups such as the American Veterinary Medical Association and the California Veterinary Medical Association, confirm that pets, on a routine basis, are rendered after they die in animal shelters or are disposed of by health authorities, and the end product frequently finds its way into pet food ... Now that the rendering companies are entities unto their own they can service many slaughterhouses, plus process any other animal remains that can be rendered. But first, to prevent the condemned meat from being rerouted and used for human consumption, government regulations require that the meat must be "denatured" before it is removed from the slaughterhouse. The denatured carcasses and other waste can then be transported to the rendering facility.
Slate referenced the claim in a 2013& article, suggesting it could possibly be true due to a lack of stringent oversight of pet food manufacturers:
There is essentially no federal enforcement of standards for the contents of pet food. FDA technically has authority, but the agency has passed this off to a set of partnerships and nongovernmental organizations that encourage mostly voluntary compliance with the few federal standards. The Association of American Feed Control Officials takes the lead in setting and maintaining standards, but it conducts no testing of food and has no enforcement authority ... [E]ven California allows rendered pets to be processed and sold out of state for pet food as meat and bone meal. The city of Los Angeles alone sends about 200 tons of dead pets to a rendering plant each month. There is no inspection of pet food or meat and bone meal shipped in from other states.
Many pet food manufacturers, including [a] site run by a pet food industry group, say that they are not using rendered pets to make a cannibal of your dog. But how would they really know? There is no simple way to look at a shipment of meat and bone meal and tell exactly what species are in the mix. The protein percentage of a load of cats and dogs looks basically the same as a shipment of carcasses from a poultry farm. The rendering industry gets very vague about what is in meat and bone meal, even in otherwise highly technical documents.
Much of the evidence offered to corroborate the presence of dead pets in pet food can be traced back to a photograph of indeterminate origin (which is graphic) and an undated video news report from Seattle television station KING. In that video, former Association of American Feed Control Officials (AAFCO) president Hersh Pendell also states that it's impossible to tell exactly what's in rendered meat (but doesn't say it necessarily includes dog or cat carcasses):
Plants that collect their raw materials from a variety of offsite sources are called independent rendering plants. Independent plants obtain animal by-product materials, including grease, blood, feathers, offal, and entire animal carcasses, from the following sources: butcher shops, supermarkets, restaurants, fast-food chains, poultry processors, slaughterhouses, farms, ranches, feedlots, and animal shelters.
The mention of the term "animal shelters" as one source of material for rendering plants again suggests the possibility that cats and dogs are being rendered, but many shelters also take in a variety of other species (including ones more typically consumed by humans and their pets), such as chickens, ducks, geese, rabbits, goats, and various farm animals.
Some versions of the rumor stem from legitimate instances in which pet food has been recalled over the presence of pentobarbital (a drug whose uses include the euthanization of companion animals), a situation which occurred in February 2017. However, that particular mishap (which resulted in the death of at least one dog) was attributed to contaminated beef and not to the presence of rendered dogs or cats in pet food.
A risk assessment issued by the Food and Drug Administration (FDA) on 28 February 2002 addressed reports that pentobarbital might be losing its effectiveness as an anesthetizing agent for dogs and other animals because they were being exposed to it through its presence in pet foods:
The low levels of exposure to sodium pentobarbital (pentobarbital) that dogs might receive through food is unlikely to cause them any adverse health effects, Food and Drug Administration scientists concluded after conducting a risk assessment.
During the 1990s, FDA’s Center for Veterinary Medicine (CVM) received reports from veterinarians that pentobarbital, an anesthetizing agent used for dogs and other animals, seemed to be losing its effectiveness in dogs. Based on these reports, CVM officials decided to investigate a plausible theory that the dogs were exposed to pentobarbital through dog food, and that this exposure was making them less responsive to pentobarbital when it was used as a drug.
The investigation consisted of two parts. First, CVM had to determine if dog food could contain residues of the drug. Second, if residues were found, the Center had to determine what risk, if any, the residues posed to dogs.
In conjunction with this investigation, the Center wanted to determine if pet food contained rendered remains of dogs and cats.
The FDA assessment noted that "pentobarbital is routinely used to euthanize animals [so] the most likely way it could get into dog food would be in rendered animal products" and that "[pentobarbital] seems to be able to survive the rendering process," leading that agency to posit that euthanized companion animals could be posing a contamination risk if they were being rendered into pet foods. However, although the FDA in their investigation of pet foods "found [that] some samples contained pentobarbital," they found no evidence of cat or dog DNA in those samples and suggested the more likely source of pentobarbital in pet foods was rendered cattle or horses:
Because pentobarbital is used to euthanize dogs and cats at animal shelters, finding pentobarbital in rendered feed ingredients could suggest that the pets were rendered and used in pet food.
[S]cientists, as part of their investigation, developed a test to detect dog and cat DNA in the protein of the dog food. All samples from the most recent dog food survey (2000) that tested positive for pentobarbital, as well as a subset of samples that tested negative, were examined for the presence of remains derived from dogs or cats. The results demonstrated a complete absence of material that would have been derived from euthanized dogs or cats. The sensitivity of this method is 0.005% on a weight/weight basis; that is, the method can detect a minimum of 5 pounds of rendered remains in 50 tons of finished feed. Presently, it is assumed that the pentobarbital residues are entering pet foods from euthanized, rendered cattle or even horses.
In February 2018, the J.M. Smucker Co. withdrew shipments of several brands of dog foods amid reports that the product was tainted with pentobarbital. How the drug might have entered the pet food supply chain was unknown, but the company said they were "focusing on a single supplier of a minor ingredient used at one manufacturing facility."
Although advances in DNA testing have made it much easier to detect the possible presence of material derived from euthanized dogs or cats in pet foods, we have turned no reports documenting anyone's finding that to be the case. Despite two decades of sustained interest in this rumor, as far as we know the most affirmative conclusion reached by those who have investigated this claim is that "we can't prove this isn't happening."