From the FDA:

Pet foods and treats contaminated with Salmonella and L. mono are of particular public health importance because they can affect both human and animal health. Pets can get sick from these pathogens and may also be carriers of the bacteria and pass it on to their human companions without appearing to be ill. People can get sick from handling contaminated pet foods and treats or touching surfaces that have had contact with the contaminated pet foods and treats. Additionally, if a person gets Salmonella or L. mono on their hands, they can spread the bacteria to other people, objects, and surfaces. The FDA is aware of recent cases in which humans and/or animals have gotten sick from exposure to Salmonella-contaminated pet foods (Salmonella-human cases, Salmonella-kitten, Salmonella-kitten and dog).  Although FDA is not aware of a documented case of a person acquiring L. mono infection from a pet food, once Salmonella or L. mono get established in the pet’s gastrointestinal tract, the animal can shed the bacteria in the feces when it has a bowel movement, and the contamination may continue to spread. Because animals can shed the bacteria in the feces when they have bowel movements, it’s particularly important to clean up the animal’s feces in yards or parks where people or other animals may become exposed, in addition to cleaning items in the home.

The Federal Food, Drug, and Cosmetic Act requires that all animal food, like human food, be safe to eat, produced under sanitary conditions, contain no harmful substances, and be truthfully labeled. Without an effective control for pathogens, such as cooking, animal food is more likely to contain pathogens such as Salmonella and L. mono. Refrigeration or freezing does not kill the bacteria.