Contaminated pet food may have killed more than 300 cats and dogs earlier this year.
There were no accurate counts earlier on how many pets had died from eating contaminated food, although estimates had run from a few dozen to several thousand.
The Michigan State University study showed the cause of death may have been related to melamine and cyanuric acid, two food contaminants that turned deadly when pet food manufacturers combined them.
"When combined, they form crystals which can block the kidneys," said Wilson Rumbeiha, an associate professor in Michigan State's Diagnostic Center for Population and Animal Health.
"Unfortunately, these crystals don't dissolve easily. They go away slowly, if at all, so there is the potential for chronic toxicity," Rumbeiha said.
The contaminated pet food was imported from China and sold in North America . After pets started dying, companies recalled thousands of varieties of pet food and treats sold under more than 100 brands.
Rumbeiha found that 347 cases met the criteria for what he called "pet food-induced nephrotoxicity." The cases involved 235 cats and 112 dogs.
A request for comment was left Thursday with the U.S. Food and Drug Administration, which oversees pet food safety among its other duties.
Michigan State based its findings on data collected from veterinarians, veterinary technicians and pathologists from April 5 through June 6.
The survey was commissioned by the American Association of Veterinary Laboratory Diagnosticians.
The Michigan State study found that more cats and smaller dogs got sick than larger dogs, and that the most cases of animals sickened by the contaminated food occurred in Texas, Illinois and Michigan .
About a quarter of the affected animals already had a condition that made them more susceptible, such as kidney or cardiovascular disease.
"The good news is we are not seeing any new cases," Rumbeiha said.
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