Emergence and Evolution of Novel Reassortant Influenza A Viruses in Canines in Southern China.

The capacity of influenza A viruses (IAVs) to host jump from animal reservoir species to humans presents an ongoing pandemic threat. Birds and swine are considered major reservoirs of viral genetic diversity, whereas equines and canines have historically been restricted to one or two stable IAV lineages with no transmission to humans. Here, by sequencing the complete genomes of 16 IAVs obtained from canines in southern China (Guangxi Zhuang Autonomous Region [Guangxi]) in 2013 to 2015, we demonstrate that the evolution of canine influenza viruses (CIVs) in Asian dogs is increasingly complex, presenting a potential threat to humans. First, two reassortant H1N1 virus genotypes were introduced independently from swine into canines in Guangxi, including one genotype associated with a zoonotic infection. The genomes contain segments from three lineages that circulate in swine in China: North American triple reassortant H3N2, Eurasian avian-like H1N1, and pandemic H1N1. Furthermore, the swine-origin H1N1 viruses have transmitted onward in canines and reassorted with the CIV-H3N2 viruses that circulate endemically in Asian dogs, producing three novel reassortant CIV genotypes (H1N1r, H1N2r, and H3N2r [r stands for reassortant]). CIVs from this study were collected primarily from pet dogs presenting with respiratory symptoms at veterinary clinics, but dogs in Guangxi are also raised for meat, and street dogs roam freely, creating a more complex ecosystem for CIV transmission. Further surveillance is greatly needed to understand the full genetic diversity of CIV in southern China, the nature of viral emergence and persistence in the region's diverse canine populations, and the zoonotic risk as the viruses continue to evolve. Mammals have emerged as critically underrecognized sources of influenza virus diversity, including pigs that were the source of the 2009 pandemic and bats and bovines that harbor highly divergent viral lineages. Here, we identify two reassortant IAVs that recently host switched from swine to canines in southern China, including one virus with known zoonotic potential. Three additional genotypes were generated via reassortment events in canine hosts, demonstrating the capacity of dogs to serve as "mixing vessels." The continued expansion of IAV diversity in canines with high human contact rates requires enhanced surveillance and ongoing evaluation of emerging pandemic threats.