The small number of confirmed reports of pet deaths bolstered by a far larger number of unconfirmed anecdotal reports suggests cats were more susceptible to poisoning by the chemical melamine that tainted the now recalled pet food, officials with the Food and Drug Administration and American Society for the Prevention of Cruelty to Animals said Saturday.
"I am concerned we have a situation where we have a sensitive species and it is the cat," said Steven Hansen, a veterinary toxicologist and director of the ASPCA's Animal Poison Control center in Urbana, Ill.
Testing by the FDA and Cornell University has found melamine in samples of recalled pet food as well as in crystal form in the urine and kidney tissue of dead cats. They've also found the chemical, in apparently raw form in concentrations as high as 6.6 percent, in wheat gluten used as ingredient of the recalled cat and dog foods, said Stephen Sundlof, the FDA's chief veterinarian.
"There was a sizable amount of melamine. You could see crystals in the wheat gluten," Sundlof said.
"It has a very low toxicity, at least in rodents. The problem is, we don't have information in cats, and that seems to be the most susceptible species," Sundlof said of melamine. Sundlof also allowed that the tainted cat foods could have contained higher concentrations of melamine than did the dog foods.
Emergency vet Dr. Benjamin Davidson said finding melamine is not solid proof of what killed the pets.
"We know the compound is present, but there is no cause-and-effect relationship. We don't know that 'Yes, this is the compound that is definitely causing the renal disease,'" Davidson told CBS News correspondent Sharyn Alfonsi.
Nestle Purina PetCare Co. said Saturday it was recalling all sizes and varieties of its Alpo Prime Cuts in Gravy wet dog food with specific date codes. Purina said a limited amount of the food contained a contaminated wheat gluten from China.
Earlier this month, Menu Foods became the first pet food manufacturer to recall its products. It did so after cats began to fall sick and die during routine company taste tests of its wet-style pet foods, sold under nearly 100 store- and major-label brands across North America. Other than in the recalled products, melamine has not been found in other Menu Foods pet foods, the company said.
Melamine is used to make plastic kitchenware, glues, countertops, fabrics, fertilizers and flame retardants. It also is both a contaminant and byproduct of several pesticides, including cyromazine, according to the Environmental Protection Agency.
The United Nations Environment Program considers melamine of low potential risk, as does the EPA. The agency has sent FDA the database information it has on the chemical and will provide technical assistance as needed, EPA spokeswoman Enesta Jones said Saturday.
Sundlof said the FDA hadn't found any studies of melamine in cats, and the results of only a single 1945 study that tested it on dogs. That study suggested the chemical increased urine output when fed to dogs in large amounts.
"That was pretty much it," Sundlof said.
Still, it's well known that identical substances can have very different effects on cats and dogs. For example, the flea killer permethrin is OK to use on dogs but lethal to cats, Hansen said. The same could be the case with melamine.
"Cats are very sensitive to many different chemicals, whether drugs, pesticides or plants. We certainly know they have some unique physiological responses that make them susceptible in cases where we wouldn't expect it in other species," Hansen said.
The investigation has traced the melamine to wheat gluten that Menu Foods, Nestle Purina PetCare Co. and Hill's Pet Nutrition bought from an unnamed U.S. supplier. The latter two companies have recalled a limited number of products since Friday. The wheat gluten, a protein source, was imported from China.
Sundlof said the recall could expand further, depending whether other pet food manufacturers also bought wheat gluten from the same supplier.
"We're still in the process of tracing it at this point," Sundlof said. There is no indication the wheat gluten entered the human food supply, he added.