Over 3,600 dogs and cats have fallen sick from certain pet jerky treats since 2007, in an outbreak that veterinarians at the Food and Drug Administration (FDA) have called “one of the most elusive and mysterious” they have come across. But those leading the investigation of the contaminated jerky treats manufactured in China still don’t have the answer to what has caused the illnesses.

The FDA today proposed a rule to improve animal food safety, with the aim of preventing foodborne illness among both pets and their owners. The rule, under the FDA Food Safety Modernization Act, would require animal food manufacturers to have a formal plan and procedures to prevent foodborne illness. For the first time, this proposition would make animal food facilities address sanitation and other solid manufacturing practices.

“Today’s announcement addresses a critical part of the food system, and we will continue to work with our national and international industry, consumer and government partners as we work to prevent foodborne illness,” FDA Commissioner Margaret A. Hamburg said in the release.

Shortly after eating the treats in question, pets susceptible to the disease experienced vomiting, diarrhea, decreased activity or an increase in drinking water and urination. Some severe cases have afflicted the animals, including kidney failure, stomach bleeding, and a rare kidney disorder.

The FDA’s Center for Veterinary Medicine (CVM) conducted over 1,200 tests in their investigation, visiting the manufacturers of the jerky pet treats which were located in China, according to a report released this week. Investigators also met and discussed the issue with experts in academia, industry, state labs and foreign governments, but to no avail: the reason behind the sickened pets remains unknown. The FDA is asking pet owners to notify them if their pets fall ill. Veterinarians may require samples of blood, urine or tissue from sickened pets for FDA analysis.

“Our beloved four-legged companions deserve our best effort, and we are giving it,” CVM Director Bernadette Dunham said in the FDA report.