Researchers ID swine flu with avian influenza genes

Ames, Iowa — Scientists have discovered a new subtype of influenza virus in swine that has not previously been recovered from mammals.

The virus subtype, H2N3, is normally found in wild waterfowl, in which it does not cause disease.

A H2N3 subtype virus was responsible for the human influenza pandemic of 1957, but the H2 protein in this recently discovered virus is not closely related, researchers say. Experts have silenced any additional alarm by noting that the strain is not related to the H5 subtype that caused Asia's influenza outbreak.

Researchers say the discovery initially made by Dr. Marie Gramer, a University of Minnesota diagnostic pathologist, is a result of the fact that pigs can serve as a mixing vessel for swine, avian and human influenza viruses.

The findings, published in the Dec. 26 Proceedings of the National Academy of Sciences of the United States of America, reveal that infected swine from two production facilities drank water drawn from ponds frequented by wild waterfowl known to carry the H2N3 virus subtype.

Agriculture Research Service veterinarians Drs. Juergen Richt, Amy Vincent, Phillip Gauger and Kelly Lager as well as Dr. Bruce Janke, an Iowa State University pathologist, and other researchers helped Gramer identify the isolates from samples she received.

Call for surveillance

Janke characterizes the strain's infection as "fairly typical" compared to other swine flu viruses. He notes that, until 1998, only one subtype of swine-flu virus was prevalent — classical H1N1. That changed when swine started contracting H3N2, a triple-reassortment virus with additional genes from human and avian viruses. For reasons not yet clear, this subtype H3N2's hybrid structure apparently has made its external proteins more favorable to change and likely ushered in other flu variations, Janke explains.

For that reason, the study suggests "... It would be prudent to establish surveillance in pigs and in workers who have occupational exposure."

Still, H2N3 represents no extraordinary risk, Janke contends.

"The virus acted like all other flu problems for pigs. It seems to have disappeared, not spreading beyond those two farms. There was nothing particularly unusual about it. It doesn't appear to be highly infectious."