This article is from the Pet Owner's Guide to Common Small Animal Poisons, by Julie Dahlke, DVM, a graduate of the University of Minnesota, College of Veterinary Medicine.
This is primarily a problem of dogs and cats that roam freely around
the farm or neighborhood with easy access to "road kill," garbage
cans, etc. However, any animal that eats decaying, rotten carcasses or
other food material (i.e., left-over hamburger) that has been
contaminated by bacteria and bacteria-produced toxins is susceptible
to this poisoning. The toxicity of the rotten food lies largely in
toxins produced by bacteria in the food material which are then
delivered in the meal to the dog or cat and cause severe
gastrointestinal upset. Clinical signs can include vomiting, diarrhea
(which may be bloody), fever, abdominal pain, and weakness. Severely
affected animals can go into shock and even die as a result of the
absorbed bacterial toxins.
For those animals who are not restricted in their activity it is
impossible to prevent possible garbage poisoning (as well as the all
too common "hit-by-car" injuries which are a much more common and
deadly risk for free-roaming animals). However, if your animal has
"escaped" and you suspect he or she has gotten into something very
unappetizing (frequently the odor of the meal is obvious even before
the pet throws it up!) be aware that this type of poisoning can be
quite serious and follow up with your veterinarian if you see any
signs of illness (repeated vomiting, lethargy, depression).