Swine flu has been transmitted to elephant seals living off the coast of California.
The H1N1 strain is the same one that caused a worldwide pandemic in 2009. It is the first time the flu strain has been confirmed in a marine mammal.
"We thought we might find influenza viruses, which have been found before in marine mammals, but we did not expect to find pandemic H1N1," said Dr Tracey Goldstein, who led the US team that made the discovery. "This shows influenza viruses can move among species."
Nasal swabs identified the flu infection in two elephant seals living off the Californian coast. An immune reaction to the virus in another 28 elephant seals indicated more widespread exposure.