Bird flu has been found in a pig for the first time in the U.S.

The farm was put under quarantine and all five pigs were euthanized so additional testing could be done. It's not a commercial farm, and U.S. agriculture officials said there is no concern about the safety of the nation's pork supply.

But finding bird flu in a pig raises worries that the virus may be hitting a stepping stone to becoming a bigger threat to people, said Jennifer Nuzzo, a Brown University pandemic researcher.

Pigs can be infected with multiple types of flu, and the animals can play a role in making bird viruses better adapted to humans, she explained. The 2009 H1N1 flu pandemic had swine origins, Nuzzo noted.

Bird flu spreads more easily from pigs to humans than birds to humans.