Infections are caused by eating live dangerous bacteria (ex. Salmonella), fungi, or other infectious agents in food, leading to infection of the pet. Infections from pet food occur most commonly from raw diets and uncooked treats (ex. pig ears, bully sticks, freeze-dried treats)
Toxins can be naturally occurring such as bacterial (ex. botulism) or fungal toxins (ex. aflatoxin) that were missed by a manufacturer or can be a result of human error (ex. too much of a nutrient such as vitamin D is added to a food) or in very rare cases, intentional adulteration of the food. The recalls for melamine-contaminated pet foods in 2007 were due to adulteration – melamine was intentionally added to raw ingredients by one company as a form of food fraud.
Deficiencies are uncommon in good quality commercial complete & balanced pet foods; when they occur, they are typically due to quality control problems at specific manufacturers. As pet foods are fed as the sole or main source of nutrition to dogs and cats, too little of an essential nutrient in a specific food can lead to disease (ex. thiamine or taurine deficiency in a cat fed a deficient diet).
Sudden illness of multiple pets in the household who eat the same diet or treats
Sudden severe liver or kidney disease in previously healthy pets (this can be food-related but there are many infections and other non-food related issues that can also cause sudden liver and kidney issues!)
A pet who seems to be healthy and usually is eager to eat suddenly has less interest in their normal food or treats
A pet acts sick after eating from a newly opened bag or a can from a new case of cans of their usual diet and there are no other obvious causes like getting into the trash, etc.
A diet or treat you are feeding has been recalled for bacterial contamination or deficient or excessive amounts of nutrients or is being investigated by the US Food and Drug Administration (FDA).