The pathogen is similar to the one that caused the 1957 flu pandemic, says the USDA Agricutural Research Service.

Reporting the findings in the May June issue of Agricultural Research magazine, it says the virus was first identified by a University of Minnesota veterinary diagnostician - and it proved to have a startling new twist because it contained genes of both swine and avian influenza viruses.

Molecular studies indicate that the mystery pig pathogen is actually an H2N3 influenza virus closely related to an H2N3 strain found in mallard ducks—being seen for the first time in mammals.

Both swine facilities used pond water frequented by migrating waterfowl.

In the newly isolated swine H2N3, the avian H2 and N3 gene segments mixed with gene segments from common swine influenza viruses, giving it the ability to infect swine—as well as mice and ferrets

. This suggests the need for continued monitoring of both swine and livestock workers for H2-subtype viruses and other influenza strains.