"We've known certainly it's possible this could happen," said Centers for Disease Control and Prevention spokesman Tom Skinner. "This may be the first instance where we have documentation that transmission occurred involving cats or dogs."

The veterinarian who treated the cat, Dr. Brett Sponseller, said two of the three people in the cat's Iowa home had flulike symptoms before the cat became ill. The case was confirmed at both Iowa State and the U.S. Department of Agriculture.

Other influenza strains have been known to cross species, but Sponseller cautioned against drawing too many conclusions from the cat including whether other pets could also get the swine flu.

"It's well documented in influenza in general, but this is the first highly suspected case of H1N1 going from humans into a cat," he said.

The indoor cat was lethargic, had a loss of appetite and appeared to have trouble breathing after it became infected, Sponseller said. Its owners declined to comment.

Officials said pet owners should take the same precautions against spreading swine flu to pets as they would with humans.

Getting children vaccinated for swine flu can also help prevent the illness from spreading to pets. There is no swine flu vaccine for pets.