At least 471 cases of pet kidney failure have been reported in the 10 days since a nationwide recall of dog and cat food and about a fifth of those pets have died, a veterinarians' information service said Tuesday.

The maker of the recalled pet food has confirmed the deaths of only 16 pets.

Paul Pion, founder of the Veterinary Information Network, which counts 30,000 veterinarians and veterinary students as members, said Tuesday the number of reported kidney failure cases had already grown higher than the 471, but he said he wouldn't have an updated tally for a few days.

Complete list of recalled brands of pet food.

Of the reported cases, he said, 104 animals have died. The network's survey results were earlier reported by the Los Angeles Times.

Pion, a California veterinarian, said only 10 percent to 20 percent of the people who belong to his Web site had responded to a request for information.

"If we're only getting 10 percent of the veterinarians, you can do the math," he said.

Scientists at the New York State Food Laboratory last week identified the rodent poison aminopterin as the likely culprit in the scare that prompted the recall of 95 brands of "cuts and gravy" style dog and cat food by the maker, Menu Foods of Ontario, Canada.

Some pets that ate the recalled brands suffered kidney failure, and the company has confirmed the deaths of 15 cats and one dog. Aminopterin, a derivative of folic acid that was once used to induce abortions and is now banned as a rodent poison in the United States, can cause kidney damage in dogs and cats.

Scientists so far have offered no theories on how aminopterin got into the products of Menu Foods, which makes pet food for most of North America's top retailers.

Researchers at the New York food lab, Cornell University and other labs were still working Tuesday to pinpoint which individual ingredients were tainted with the poison, officials said. They also said there could still be undetected hazards in the food.

Sam Bornstein, a spokesman for Menu Foods, said the company has not yet updated the number of confirmed pet deaths. He said testing to determine the source of the aminopterin and how it got into the food "is continuing aggressively" but Menu did not have any new results.

The company on March 16 recalled products packaged from Dec. 3 to March 6 and advised retailers last week to remove all the products from their shelves to verify the dates they were packaged. Products not made between those dates can still be sold.

The recall covered products carrying names of major brands including Iams, Nutro and Eukanuba. The food was distributed throughout the United States, Canada and Mexico by major retailers such as Wal-Mart, Kroger and Safeway.

The U.S. Food and Drug Administration has said the investigation was focused on the ingredient wheat gluten.

Testing has not been easy.

Wheat gluten has "been a real difficult sample to work with," said Daniel Rice, director of the New York food lab. "It's real sticky, gummy. If you can imagine wet flour and you're trying to get that into solution and put that into an instrument."

Veterinary Information Network provides a secure site where veterinarians share clinical information and other resources such as conference schedules and continuing education courses.