Increased public health threat of avian-origin H3N2 influenza virus during evolution in dogs

Influenza A viruses in animal reservoirs repeatedly cross species barrier to infect humans. Once an animal-borne virus with novel antigenicity acquired the efficient human to human transmissibility, it will become epidemic in the population. Dogs are the closest animal companions to humans and canine respiratory tract expresses both SAα2,3-(avian type) and α2,6-Gal (human type) receptors. However, the role of dogs in the ecology of influenza viruses is unclear. H3N2 avian influenza viruses transmitted to dogs around 2006 and have formed stable lineages. The long-term epidemic of avian-origin H3N2 virus in canine offers the best models to investigate the effect of dogs on the evolution of influenza viruses. Here, we carried out a systematic and comparative identification of the biological characteristics of H3N2 canine influenza viruses (CIVs) isolated in the worldwide over 10 years. We found that during the adaptation of H3N2 CIVs to dogs, H3N2 CIVs became to recognize the human-like SAα2,6-Gal receptor, gradually increased HA acid stability and replication ability in human airway epithelial cells, and acquired a 100% transmission rate via respiratory droplet in ferret model, which were essential hallmarks of being adapted to humans. We also identified that the frequency of substitutions related to human adaptation has gradually increased in H3N2 CIVs, and determined four cumulative molecular changes responsible for the increased airborne transmission ability in ferrets. Our results suggested that canine may serve as an intermediate for the adaptation of avian influenza virus to human. Continuous surveillance coordinated with risk assessment for CIVs is necessary.