A mysterious and ongoing outbreak of illness among pets has been apparently caused by jerky treats from China and has affected more than 4,000 animals, NBC News reported.

Nearly 600 pets have been killed from the illness and at least 3,600 have been sickened. But the Federal Drug Administration (FDA) is scrambling to find the answers as to why, asking veterinarians and pet owners for help.

Last counted in January, 500 deaths and 3,200 sicknesses had been reported. Although those figures increased, the rate at which they did so has slowed significantly.

"To date, testing for contaminants in jerky treats has not revealed a cause for the illnesses," Martine Hartogensis, a deputy director for the FDA's Center for Veterinary Medicine, said in a new report. "Despite these warnings, we have continued to receive reports of illnesses in both cats and dogs."

The jerky treats, including chicken, duck and sweet potato flavors, have caused mostly gastrointestinal and kidney problems in pets. The outbreak has affected mostly dogs - of all sizes, breeds and ages - but has also affected at least ten cats.

"We still are extensively testing treats for a number of things," Hartogensis told NBC News. "We do seem to be getting some leads, but we still have a little bit of a ways to go."

The FDA has submitted an open letter to veterinarians asking them to submit detailed information and analysis of any related cases. The agency has requested urine samples of affected pets and blood test results as well.

"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," said Kendal Harr, a veterinary clinical pathologist who has been tracking instances of the outbreak.

Hartogensis said the FDA could not perform a specific recall because they do not know the specific compound causing the problem. They also do not have enough evidence to start a blanket recall of all the products causing the problem.

"If we do find an adulterated product, we will recall them," Hartogensis said. "In terms of doing a blanket recall, at this point we don't have enough evidence to do a blanket recall within the authority that we have."