Characterization of H3N2 Swine Influenza Viruses in Iowa Swine
Since December 1998 when swine influenza virus (SIV) with H3N2 was first identified in Iowa swine, prospective and retrospective studies were conducted to monitor and evaluate H3N2 SIV infections in swine population in Iowa until February 2000. A serological survey revealed that H3 SIV had been widely spread in Iowa swine within first 6 months after initial identification and that both H1 and H3 SIV coexited. Many herds tested had the evidence that animals were exposed to both subtypes of SIV. In some cases, both subtypes were isolated from the same animal. All circumstantial evidences strongly suggested the emergence of new subtype due to reassortment of H1N1 and H3N2 strains; however, no evidence of new reassortant was yet found in Iowa during this study period. A one-way hemagglutination inhibition (HI) assay on banked field isolates of H3N2 SIV demonstrated that the majority of field isolates were antigenically conserve. These observations suggest that diagnostic assays using an initial Midwest H3N2 SIV isolate should be reliable for diagnosing infection of H3N2 SIV. This observation also suggests that a vaccine using an initial Midwest H3N2 SIV may provide reliable cross protection against a variety of H3N2 strains. However, a few isolates were found resistant to HI activity conferred by antiserum raised against a H3N2 Midwest isolate, warranting a continuous surveillance for antigenic drift among isolates.