A rat poison illegal in the United States was the source of the Menu Foods pet food contamination, according to ABC News:
A source close to the investigation tells ABC News that the rodenticide, which the source says is illegal to use in the United States, was on wheat that was imported from China and used by Menu Foods in nearly 100 brands of dog and cat food.
There is some good news according to the source. Knowing the chemical should aid veterinarians who are treating animals that have been sickened by the pet food.
Aminopterin is used in the United States in, of all things, a cancer drug, according to the source.
If your dog or cat is sick, tell your vet! —MEGHANN MARCO
Rat Poison to Blame for Pet Food Contamination [ABC News] (Thanks, Kevin!)