There was an interesting recent article that looked at ingredients in pet foods and whether or not there were things found in the food that wasn't on the label.
"In order to see whether mislabeling occurred, the researchers tested 52 products by extracting DNA and testing it for the present of eight meat species: beef, goat, lamb, chicken, goose, turkey, pork and horse. In the end, the researchers found that chicken was the most common meat species found in the pet food products. Pork was the second most common, followed by beef, turkey and lamb. The least common was goose, and none of the products tested positive for horsemeat.
Of the mislabeled products, 13 were dog food and seven were cat food. Of these 20, 16 contained meat species that weren't included on the product label, with pork being the most common undeclared meat species."
This may seem a little shocking to some, but honestly it doesn't really concern me for most pets. Most of these ingredients are probably trace amounts, meaning that rather than a big chunk of chicken being thrown in there would be some residual bits on the equipment. So I wouldn't be concerned about this for my own dogs.
The concern is for pets that might have food allergies or other sensitivities to ingredients. This study is exactly why dermatologists don't consider any over-the-counter foods to be sufficient for a diet trial to rule out allergies. Let's say that a dog has a beef allergy. In order to prevent a reaction you pick a food that doesn't have beef on the ingredient list. So your dog shouldn't react, right? Well, maybe, except for the fact that there is a good chance that there are trace amounts of beef in the food anyway and even a small amount could trigger a reaction.
If your pet isn't allergic to food ingredients, this is a non-issue and really isn't news. But if your pet is known or suspected to have food allergies, please listen closely to your vet when they recommend a very specific diet. There is real science behind the recommendation.