MICHIGAN, USA — Two cats that belonged to Michigan dairy workers died after being infected with bird flu. But it’s still not clear how the animals got sick or whether they spread the virus to people in the household, a new study shows.

Veterinary experts said the report, published Thursday by the U.S. Centers for Disease Control and Prevention, lacks detail that could confirm whether people can spread the virus to domestic cats — or vice versa.

“I don’t think there is any way of concluding that there was human-to-cat transmission based on the data that is presented,” said Dr. Diego Diel of the Cornell University College of Veterinary Medicine.

Cats in two separate households died in May after developing severe illnesses that were confirmed as the Type A H5N1 bird virus found in U.S. dairy herds, investigators reported. Both were indoor-only cats that had no exposure to infected cattle or birds and didn’t consume raw milk.

One cat that died belonged to a worker on a dairy farm in a county known to have bird flu-infected cattle. The worker reported symptoms of illness before the cat got sick. That cat fell ill, tested positive for H5N1 and had to be euthanized. A second cat in that household also got sick, but it recovered. A third cat in the household didn’t get sick and tested negative.

Meanwhile, an adolescent in the household got sick but tested negative for flu.

The other cat that died lived in a different household. That pet belonged to a dairy worker who transported raw milk and reported frequent splashes of milk on the face, eyes and clothing. That worker reported eye irritation, a possible symptom of bird flu, two days before the cat got sick.

The cat was known to roll in the worker’s dirty clothing and died within a day of developing signs of infection, the study authors wrote. Another cat in that household tested negative for the virus.

In both instances, the dairy workers declined to be tested for H5N1, the study said.

That means it’s impossible to know whether they directly transmitted the virus to their cats, said Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory. It’s more likely that the cats were infected by virus on the workers’ clothing, boots or hands, he said.

“We know that people at this point are not shedding large amounts of virus,” he said. “I don’t think it’s because this person coughed on their cat.”

The study concluded that reports of bird flu infections in indoor cats are rare, but “such cats might pose a risk for human infection.”

In the months since the cats died, there have been several reports of domestic cats becoming infected and dying after consuming unpasteurized milk or pet food contaminated with bird flu.

The new report underscores the need for more comprehensive testing for H5N1 in all arenas, Poulsen said.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.