There are currently 16 identified H subtypes of influenza A; all 16 are known to infect avian species. In 2006 a novel influenza subtype (H2N3) was identified in a swine herd in Missouri. The hemagglutinin (HA) and neuraminidase (NA) genes were found to be avian in origin, raising concern regarding the potential for inter-species transmission of avian viruses to swine and humans. In early 2007, H2N3 influenza virus was isolated from a duck and a chicken from two separate backyard poultry flocks in Ohio. The nearly coincidental isolation of the same subtype in unnatural hosts in the Midwest United States raised the question about the origin of the viruses and the potential for these viruses to adapt to a new host and easily spread to nearby flocks or herds. Therefore, the viruses were further characterized by DNA sequencing and in vivo chicken pathogenicity testing. The virus sequences were compared to those from the 2006 swine isolates. In addition, the avian viruses were tested for cross-reactivity by virus neutralization (VN) and hemagglutination-inhibition (HI) using a panel of H2 reference sera including serum from the Missouri swine virus. Serologic survey on swine herds and poultry flocks in Ohio for H2 virus infection was also conducted to assess virus spreading.

Sequence comparisons for the H and N genes demonstrated that the avian viruses were similar, but not identical, to the swine viruses. Accordingly, the avian and swine isolates were also antigenically related as determined by HI and VN assays, suggesting that both avian and swine viruses originated from the same group of H2N3 avian influenza viruses. Although serological surveys using the HI assay on poultry flocks and swine herds in Ohio did not reveal further spreading of H2 virus from the index flocks, continuous surveillance will be necessary. Contemporary H2N3 avian influenza viruses appear to be easily adaptable to poultry and swine, raising concern regarding the potential for inter-species transmission of H2N3 avian virus to humans.