Influenza A viruses are responsible for large losses of human lives around the world and continue to present a great challenge to both domestic and global public health. As a zoonotic pathogen, influenza A viruses have a wide range of natural host reservoirs, such as avian, swine, equine, and sea mammals. All four known pandemic strains are of animal origin. Human infection of animal origin influenza A viruses by contacting wild or farmed animals (e.g. birds or swine) has caused serious public concerns. Recent emergence and epidemics of canine influenza viruses have broadened this list and further complicated influenza ecology. With the pet dog as its natural host, canine influenza virus minimizes the distance between animals and humans more than ever before. Compared to contacts with dogs, human contacts with those wild or farmed animals are minimal. Because these emerging canine influenza viruses are antigenically different from contemporary seasonal influenza viruses, indicating a lack of current human herd immunity to these canine influenza viruses, canine influenza viruses present a potentially significant threat to public health. This study intends to explore the molecular mechanisms of canine influenza infection, especially to characterize the host receptors that determine canine influenza host tropisms. This study is based on the hypotheses that canines would have a unique set of glycan receptors that determine canine influenza host tropisms and that influenza A virus would mutate to bind to these canine specific receptors when jumping from other hosts to the canine. This project has the following two specific aims: (1) Identify the viral mutation(s) in influenza hemagglutinin protein that determine binding specificity between canine influenza virus and host receptor; (2) Characterize types and distribution of glycans in canine airway epithelium cells. Successful completion of the proposed studies will help elucidate both the mutations in influenza hemagglutinin and host receptors required for canine influenza infection and thus potentially improve anti-influenza drug delivery strategies, especially for those drugs targeting glycan receptors. The omics and integrative systems biology approaches developed in this study are novel and could be adapted to study the host tropisms of influenza A viruses in human and other hosts.