It is suspected the toxic agent may have come in with a load of grain from China. That's unknown.
Fortunately, very few animals have been affected (the Animal Medical Center, Cornell, the big national veterinary lists, etc. have been keeping, obviously, tight tabs on suspected cases).
Here's a brief summary from the vet lists:
A press conference has announced that the toxic agent involved in the pet food recall has been identified as aminopterin. Aminopterin is used as a rodenticide in some countries, it is illegal to use in the United States. Aminopterin was used as an anti-cancer drug in the US in the 1940's, but has since been replaced by other, less toxic, chemotherapeutics.
At toxic doses aminopterin causes nephrotoxicity - it affects the kidneys, at high doses forming crystals in the urine, and causing acute renal failure.
What owners should do: check the recall list again today - it has been updated -
www.fda.gov
See your veterinarian if you believe your animals have been exposed. If they have eaten the foods on the recall list within the last two months or so, their kidney function should be tested (screened) by your veterinarian.
Toxicity is treatable by aggressive fluid hydration, and diuresis to dissolve and remove the crystals from the kidneys. Treatment is similar to that used for any acute renal failure. At least partial renal recovery is anticipated in many cases. Toxicities are "dose-related", meaning animals that ingested more of the toxic substance are sicker than animals that have not. As aminopterin is a folic-acid inhibiting agent, and can suppress bone marrow function, animals should be monitored for anemia, etc. by blood tests. There are no current reports of animals suffering bone marrow suppression. Recovering animals should be monitored by your veterinarian on a regular basis.