Cocirculation of Two Distinct Lineages of Equine Influenza Virus Subtype H3N8

Direct amplification and sequencing of the hemagglutinin (HA) genes of equine influenza virus subtype H3N8 was undertaken in order to characterize strains of this virus circulating in Sweden. The majority of viruses from outbreaks during 1997 analyzed belonged to the American lineage of H3 equine influenza, and one strain was shown to belong to the European lineage. Furthermore, it was shown that recent American-lineage strains are mutated at amino acid position 190 of the HA during serial passage in embryonated hens’ eggs. Host cell adaptation of these viruses thus takes place at antigenic region B of the HA.