US authorities have yet to determine what exactly caused the deaths of more than 1,000 dogs that consumed jerky pet treats made in China, a Congressional panel heard.
Major pet supply retailers Petco and Petsmart have said they will phase out all China-made pet food in their stores over the coming months, amid growing consumer jitters about the safety of their ingredients.
Tracey Forfa of the Food and Drug Administration (FDA) told the Congressional-Executive Commission on China that more than 5,600 dogs in the United States are known to have fallen ill since 2007 due to jerky products imported from China.
"Unfortunately, to date, the FDA has not been able to identify a specific cause for the reported illnesses or deaths despite an intensive scientific investigation," said Forfa, deputy director of the FDA's Center for Veterinary Medicine.
Sixty percent of the sick dogs—of all sizes, ages and breeds—suffered gastrointestinal illness, while 30 percent exhibited kidney or urinary issues, including a rare kidney disease called Fanconi syndrome, she said.
"Without knowing what is causing the illnesses, and thus no means of screening products to ensure that they are safe, firms and authorities have limited options," added University of Minnesota professor Shaun Kennedy, an expert in food systems.
Worries about the quality of China-made pet food date back to 2007, when melamine, a chemical compound typically used for making plastics, was detected in some brands, prompting a sweeping recall.
Patty Lovera of Food and Water Watch, a non-profit advocacy group, said melamine is intentionally added to various food products in China to enhance their nitrogen content and thus pass protein tests.
Tuesday's congressional hearing was held amid wider questions over labelling food imports from China for human consumption.
Last year the Department of Agriculture gave a green light for China to export processed, cooked chicken to the United States, so long as the raw poultry originates from US slaughter houses.
"While no such chicken has entered our shores yet, it's possible that very soon this processed chicken could end up on our dinner tables and school lunchrooms," said Senator Sherrod Brown, one of the committee's co-chairmen.
"Americans want and require better answers, clearer labels and the peace of mind that the foods we import from China are safe," he said, urging Beijing to make "significant improvements" in its food safety system.