A team of scientists uncovers avian genes in swine influenza virus strain.

Scientists in the United States collaborated in the discovery of a new strain of swine influenza virus (SIV), H2N3. The H2 viruses are unique to swine.

This new flu strain has a molecular twist: It is composed of avian and swine influenza virus strains.

USDA Agriculture Research Service (ARS) veterinarians Juergen Richt, Amy Vincent, Kelly Lager and Phillip Gauger conducted the breakthrough work with Iowa State University (ISU) visiting scientist Wenjun Ma, ISU veterinarian Bruce Janke and other colleagues at the University of Minnesota and St. Jude Children's Research Hospital, Memphis, TN. The ARS veterinarians work at the agency's National Animal Disease Center in Ames, IA.

The research team studied an influenza virus that was first identified by University of Minnesota veterinary diagnostician Marie Gramer. The virus infected two groups of pigs at separate production facilities in 2006, where both groups of pigs drank from ponds frequented by migrating waterfowl.

Molecular studies indicated the untypable virus was an H2N3 influenza virus that is closely related to an H2N3 strain found in mallard ducks. But this was the first time the strain had been detected in mammals.

Influenza viruses have eight gene segments, all of which are interchangeable between different virus strains. Two gene segments code for virus surface proteins that help decide whether an influenza virus can infect a specific host and start reproducing.

In the newly isolated H2N3 flu strain, researchers report that the avian H2 and N3 gene segments mixed with gene segments from common swine influenza viruses. This exchange and additional mutations gave the H2N3 viruses the ability to infect swine. Lab tests confirmed that this strain of H2N3 could also infect mice and ferrets.

Gramer clarifies the role of pigs in the identification of this new strain of SIV. “Pigs are suspected to be a mixing vessel for avian and human influenza viruses because cells in their airways and lungs carry receptors for both human and avian influenza viruses. Supporting this theory are findings of genetic reassortment between avian- and human-like influenza viruses in Italian pigs.”

Richard Webby, an international expert in human and animal influenza viruses at St. Jude Children's Research Hospital, worked closely with the many veterinarians involved with this research, and co-authored the scientific article describing the discovery.

In the article, Webby observes the role that pigs play in the transmission of influenza viruses: “Pigs have often been implicated in the emergence of human pandemic strains. More recent evidence has, however, shown that similar receptor expression is also available in the human and quail host, and the direct evidence that human pandemic viruses are generated in swine is ambiguous.”

“Nevertheless, our findings of H2N3 in swine provide further evidence for the potential of swine to promote reassortment between different influenza viruses,” the authors conclude.

The findings also support the need to continue monitoring swine and farm workers for H2 subtype viruses and other flu strains that might someday threaten swine and human health.

Gramer emphasizes that no illnesses were reported among farm workers in connection with the swine cases, and no workers were tested at the time. However, serologic testing of the workers is scheduled soon and will be conducted under a grant from the Centers for Disease Control and Prevention.

Does the newfound virus represent a threat to humans? “No more than any other influenza virus in the world,” Gramer says. “Transmission of flu from pigs to humans is likely rare. This flu, itself, is rare… we haven't found it again.”