Vets in the US state of Iowa said on Wednesday that a household cat had tested positive for swine flu - the first known case in the world of the new pandemic strain spreading to the feline population.
The domestic shorthair, a 13-year-old castrated male, apparently caught A(H1N1) off its owners - two of the three members of the family had previously contracted the deadly virus.
"This is the first cat that we are aware of with the pandemic H1N1," said Brett Sponseller, an assistant professor of veterinarian microbiology who helped treat the pet at Iowa State University's College of Veterinary Medicine.
After ruling out most common causes of respiratory diseases, a team of university and state vets looked at the family history of the owners and guessed swine flu might be the cause.
After five days of tests, their fears were confirmed.
"This was a big deal and everyone involved understood that," said Sponseller, adding that the owners had since recovered and that the cat had been given extra fluids and antibiotics and was "doing very well".
Cats have previously been diagnosed with H5N1, a virus commonly known as bird flu that spread from poultry to humans but this is thought to be a first for swine flu and has possible implications for the feared pandemic.
Sponseller said the danger of an infected cat spreading the A(H1N1) virus to humans or to other cats was unknown but didn't appear to be high.
The prevalence of the disease in the cat population was also unknown but the vet said he believed more cats would now be tested and confirmed to have the virus.
The cat owner, an employee at the Iowa vet school, insisted on anonymity and is not revealing the pet's name, Sponseller said.
"Indoor pets that live in close proximity to someone who has been sick are at risk," said Iowa state veterinarian David Schmitt.
"It is wise to monitor their health to ensure they aren't showing signs of illness," he said in a statement.
The Iowa Department of Public Health also reminded the public that people who are sick with H1N1 can spread the virus not only to humans but also to some animals.
The number of human swine flu deaths spiked by 700 in a week, the World Health Organisation said on Friday.
More than 5,700 people have died worldwide since the virus was first discovered in April, with most of the deaths - 4,175 - in the Americas region, it said.
More than 5,700 people have now died from the A(H1N1) since it broke out in April in Mexico and the United States in April, the WHO said.
Swine flu vaccine shortfalls have seen long lines forming outside health clinics in the US, with many people being turned away or going home without getting their swine flu shot as supplies ran dry.
