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In the past three years, outbreaks of bird flu, formally called highly pathogenic avian influenza, in wild animals spilled over to dairy cows and poultry, infecting several dozen humans and even killing one American.

"There is not necessarily grounds to fear that the viruses in the frozen environment are more pathogenic."

A new strain of a highly pathogenic bird flu known as H7N9 has surfaced at a poultry farm in Mississippi where chickens are raised for breeding.

“That is the mutation found in the first human case, which was extremely pathogenic in ferrets,” said Yoshihiro Kawaoka, an infectious disease expert at the University of Wisconsin, Madison and the University of Tokyo.

The agency “continues to prioritize the response to highly pathogenic avian influenza,” it told the news agency.