Equine and Canine Influenza H3N8 Viruses Show Minimal Biological Differences Despite Phylogenetic Divergence
Feng KH, Gonzalez G, Deng L, Yu H, Tse VL, Huang L, Huang K, Wasik BR, Zhou B, Wentworth DE, Holmes EC, Chen X, Varki A, Murcia PR, Parrish CR
The A/H3N8 canine influenza virus (CIV) emerged from A/H3N8 equine influenza virus (EIV) around the year 2000 through the transfer of a single virus from horses to dogs. We defined and compared the biological properties of EIV and CIV by examining their genetic variation, infection, and growth in different cell cultures, receptor specificity, hemagglutinin (HA) cleavage, and infection and growth in horse and dog tracheal explant cultures. Comparison of sequences of viruses from horses and dogs revealed mutations that may be linked to host adaptation and tropism. We prepared infectious clones of representative EIV and CIV strains that were similar to the consensus sequences of viruses from each host. The rescued viruses, including HA and neuraminidase (NA) double reassortants, exhibited similar degrees of long-term growth in MDCK cells. Different host cells showed various levels of susceptibility to infection, but no differences in infectivity were seen when comparing viruses. All viruses preferred α2-3- over α2-6-linked sialic acids for infections, and glycan microarray analysis showed that EIV and CIV HA-Fc fusion proteins bound only to α2-3-linked sialic acids. Cleavage assays showed that EIV and CIV HA proteins required trypsin for efficient cleavage, and no differences in cleavage efficiency were seen. Inoculation of the viruses into tracheal explants revealed similar levels of infection and replication by each virus in dog trachea, although EIV was more infectious in horse trachea than CIV.