It might surprise some dog owners to know that dog meat has been used in dog food. This practice was relatively common until the late 1990's. Even today there are no laws preventing man's best friend from ending up in dog food.
Please note the below information on dead dogs being used in dog food is true to the United States but may also occur in Canada and possibly other countries.
At some time or another pet food companies realized there was a cheap source of meat available... Dog Meat, and it could be purchased cheaply from animal shelters and the dog pound. These were places where dogs were euthanized weekly and there was always a need to dispose of the bodies. Nobody said anything and there were no laws preventing such from happening.
However what started to tip the scales was that the veterinarians who performed the euthanasias at the animal shelters started noticing it was taking more and more drugs to put these animals to sleep. They realized that somehow pets in animal shelters were building up an immunity to the medications used to end their lives. Typically veterinarians are called to shelters every week to euthanize between 10 and 200 animals (or more in larger centers) so this was where they saw the problems develop quite easily.
As such several investigations were launched to see how pets were building up immunities to the drugs. The FDA (Food and Drug Administration) launched an investigation. They found that sodium pentobarbital (the drug often used to euthanize pets) passed through the rendering process and ended up in pet food. They admitted finding the drug in some pet foods and stated “several retail feeds were confirmed for the presence of phentobarbital which could only have come from euthanized animals.” Most of these investigations were exposed by television news groups such as Seattle's King 5, and KMOV in St. Louis, Missouri.
What they exposed was something that many people already knew – dead dogs were being used to make dog food. Many animal shelters are government run. These poorly funded shelters found a way of generating extra income and disposing of the animal bodies all at once -Sell them cheaply to a rendering plant. You will note your local SPCA probably does not do this.
The Millstadt plant in Illinois was one that regularly purchased previously euthanized animals and would render them down to be used in pet foods. They would appear on the ingredient list of a pet food under the term Meat Meal, which can be any unidentified meat source. It is not only euthanized animals these rendering plants bought, but also road kill, and animals that died of disease.
When Hershel Pendell appeared in a Seattle King 5 television interview and was asked if pet food contained euthanized pets he said “If the ingredient list meat or bone meal, you don't know if it is cattle, or sheep, or horse... or fluffy.” Pendell was, at that time, president of AAFCO the board responsible in the USA for pet food regulations.
Following the leaks, and especially when a list of pet foods found to contain previously euthanized animals was made public, many food companies tightened their use of euthanized pets in pet food, however even today the practice is legal and continues.
You, the consumer should know that any food containing the mystery ingredient “Meatmeal” can be any dead animal, even cats, and dogs, although often it is horsemeat - typically foals from the PMU Industry. You will note meat meal is an unspecified source, as when compared to a proper ingredient such as “Chicken Meal, or Lamb Meal”.
Be aware that meat by-products can include things like collars, and cancerous tumors.
If the food does not say “Human Grade Ingredients” it can contain euthanized animals (although not necessarily dog).
Source Links (as well as my own knowledge on the subject - due to working in an animal shelter)