The Food and Drug Administration is still struggling to pin down exactly how thousands of pets got sick after eating flavored jerky treats manufactured in China.
Nearly 600 dogs and cats have died and more than 3,600 have been sickened after eating chicken, duck and sweet potato flavored jerky treats since 2007.
The FDA said on Tuesday that they have not been able to track down a cause of the illnesses and are appealing to pet owners and veterinarians to report any more cases of sickness or death that could be linked to the treats.
"This is one of the most elusive and mysterious outbreaks we've encountered," said Bernadette Dunham, director of the FDA's Center for Veterinary Medicine.
"Our beloved four-legged companions deserve our best effort, and we are giving it."
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FDA officials asked veterinarians in a letter to send in samples of any suspicious treats to be tested for contamination.
"The agency urges pet owners to be cautious about providing jerky treats," an FDA bulletin warned.
"If you do provide them and your pet becomes sick, stop the treats immediately, consider seeing your veterinarian, and save any remaining treats and the packaging for possible testing."
The two of the largest sellers of jerky treats — Nestle Purina PetCare Co. and Milo's Kitchen — recalled some of their products from store shelves nationwide in January in response to officials in New York finding traces of poultry antibiotics.
The FDA said the antibiotics are likely not the cause of the deaths but that exposure over a long period of time may trigger some sickness.
Kendal Harr, a veterinary clinical pathologist, told NBC News that the specific compound responsible for the illnesses continues to elude experts.
"I think that what it tells us is that the intoxicant is something that we're not used to dealing with as a toxin in North America," she said.
Pets who ate the tainted treats may experience decreased appetite, vomiting and diarrhea.
Severe cases have involved kidney failure, gastrointestinal bleeding, and a rare kidney disorder, according to the FDA.