Isolation and genetic characterization of H3N8 equine influenza virus from donkeys in China

During the 2007 outbreak of equine influenza (EIV) in China, an influenza virus designated A/donkey/Xinjiang/5/2007 (donkey/Xinjiang/2007) was isolated from a symptomatic donkey in Xinjiang Uygur Autonomous Region, China. To analyze the genetic evolution of the new isolate, the hemagglutinin (HA) gene of donkey/Xinjiang/2007 was amplified and sequenced. Sequence alignment, prediction of glycosylation sites and phylogenetic analysis of the HA1 protein of donkey/Xinjiang/2007 showed most similarity to the Florida sublineage clade 2 of the American lineage of equine influenza viruses. The HA1 sequence of donkey/Xinjiang/2007 had high identity with the Chinese EIV strains isolated in recent years and had low identity with A/swine/Anhui/1/2006, a horse-derived influenza virus isolated from pigs in China. Compared with vaccine strains, the HA1 amino acid sequence of donkey/Xinjiang/2007 varied in the five putative antigenic sites to different degrees and distributed in a different branch of the HA1 phylogenetic tree. Our findings provide further evidence of an American H3N8 equine influenza virus in China in addition to the European H3N8 equine influenza virus. Accordingly, a vaccine against equine influenza in China should contain components of both strains of viruses to prevent widespread infection.