"That's a huge number, considering when you feed pet food no animal should die," says CBS News The Early Show veterinarian Dr. Debbye Turner.
A federal investigation is focusing on wheat gluten as the likely source of contamination that sparked a recall last Friday of 60 million cans and pouches of the suspect food, said Stephen F. Sundlof, the Food and Drug Administration's top veterinarian.
"I'm certain someone's going to figure this out because there are a lot of pet foods involved, a lot of pets involved and a lot of veterinarians who are upset," Dr. Ann Hohenhaus of New York's Animal Medical Center told CBS News.
Wheat gluten, a protein source, is commonly used as filler.
Agency investigators are looking at other ingredients as well. The wet-style pet food was made by Menu Foods, an Ontario, Canada-based company. The FDA on Monday had investigators at Menu plants in Emporia, Kan., and Pennsauken, N.J.
Menu Foods told the FDA it received the first complaints of kidney failure and deaths among cats and dogs from pet owners on Feb. 20. It began new tests on Feb. 27.
During those tests, the company fed its product to 40 to 50 dogs and cats and seven animals — the mix of species was not immediately known — died, Sundlof said. The contamination appeared more deadly to cats than to dogs, he said.
"Cats seem to be more susceptible to acute renal failure, what which is what this toxin is causing," Turner told Early Show co-anchor Harry Smith.
The recall now covers dog food sold throughout North America under, including Iams, Nutro and Eukanuba. The food was sold under both store and major brand labels at Wal-Mart, Kroger, Safeway and other large retailers.
The FDA has yet to tally how many reports it has received of cats and dogs suffering kidney failure or death. The company has reported just 10 deaths, of nine cats and a single dog.
"We are still trying to find out what the true picture is out there of animals. We're talking about 1 percent of the pet food (supply) and it's really just impossible to extrapolate at this point," Sundlof said.
Menu Foods spokeswoman Sarah Tuite told Associated Press Radio the company was "still trying to figure out the cause."
"We're testing and testing, but we can't identify the problem in the product," Tuite said.
Other companies — Nestle Purina PetCare Co., Procter & Gamble and Hill's Pet Nutrition Inc. — said that as a precaution they were voluntarily recalling some products made by Menu Foods.
A complete list of the recalled products along with product codes, descriptions and production dates was available from the Menu Foods Web site. The company also designated a phone number that pet owners could call for information — (866) 895-2708. The lines have been swamped by callers.
Tuite said the company has added more people and lines to cope with the calls. Callers who get a recording saying the line is out of order should try again, she added.
Concerned pet owners are calling veterinarians, too.
"All morning we've been getting phone calls," said one.
The company became aware of a potential problem after it received an undisclosed number of owner complaints that dogs and cats were vomiting and suffering kidney failure after eating its products.
Tuite told AP earlier the recalled products were made using wheat gluten purchased from a new supplier, which has since been dropped.
The FDA hasn't confirmed the identity of that company, but its Web site suggests it supplies only animal feed manufacturers, Sundlof said.
Wheat gluten itself wouldn't cause kidney failure, leading FDA investigators to suspect contamination by other substances, including heavy metals like cadmium and lead or fungal toxins. Aflatoxin, a corn fungus, sparked a 2005 dog food recall.
"In this case, we've just got renal failure and there are probably dozens of environmental contaminants, toxins, bacteria that might provoke an episode of renal failure," said Hohenhaus. "I don't think anyone has an idea right now."
The new recall covers the company's "cuts and gravy" style food, which consists of chunks of meat in gravy, sold in cans and small foil pouches from Dec. 3 to March 6.
The company said it makes pet food for 17 of the top 20 North American retailers. It is also a contract manufacturer for the top branded pet food companies.
Meanwhile, the inability of customers to get through to Menu Foods was proving frustrating on Monday.
Michael Ritter, 38, of Washington, Pa., met a busy signal countless times over the weekend after learning about the recall from local news reports. He wonders if pouches of Special Kitty-brand food from the local Wal-Mart were to blame for the kidney failure and death of his 3-year-old cat Cosmo.
Ritter's voice started to break as he described the loss of his cat a week before. "You get attached to them. You really do," Ritter said.
He said he was tired of trying to reach Menu Foods, and was going to contact someone else: a lawyer.
Laura Iskowitz, 33, of Monroe Township, N.J., was equally frustrated, saying she had called the information line "a hundred times" over the weekend.
She believed packages of Iams, Companion and Nutro-brand wet food were possibly to blame for the kidney-failure death of her 3-year-old dog Angel, a Labrador retriever mix.
"She truly was my best friend. And because of this food I don't have her anymore," Iskowitz, who has lived alone since Angel was euthanized on Jan. 30, told CBS News.