Experimental infection of pigs and ferrets with "pre-pandemic," human-adapted, and swine-adapted variants of the H1N1pdm09 Influenza A virus reveals significant differences in viral dynamics and pathological manifestations

Influenza A viruses are RNA viruses that cause epidemics in humans and are enzootic in the pig population globally. In 2009, pig-to-human transmission of a reassortant H1N1 virus (H1N1pdm09) caused the first influenza pandemic of the 21st century. This study investigated the infection dynamics, pathogenesis, and lesions in pigs and ferrets inoculated with natural isolates of swine-adapted, human-adapted, and "pre-pandemic" H1N1pdm09 viruses. Additionally, the direct-contact and aerosol transmission properties of the three H1N1pdm09 isolates were assessed in ferrets. In pigs, inoculated ferrets, and ferrets infected by direct contact with inoculated ferrets, the pre-pandemic H1N1pdm09 virus induced an intermediary viral load, caused the most severe lesions, and had the highest clinical impact. The swine-adapted H1N1pdm09 virus induced the highest viral load, caused intermediary lesions, and had the least clinical impact in pigs. The human-adapted H1N1pdm09 virus induced the highest viral load, caused the mildest lesions, and had the least clinical impact in ferrets infected by direct contact. The discrepancy between viral load and clinical impact presumably reflects the importance of viral host adaptation. Interestingly, the swine-adapted H1N1pdm09 virus was transmitted by aerosols to two-thirds of the ferrets. Further work is needed to assess the risk of human-to-human aerosol transmission of swine-adapted H1N1pdm09 viruses. Influenza A viruses (IAVs) have a high evolutionary rate and evolve through genetic drift and reassortment events, elevating their pandemic threat. The last pandemic caused by IAV was a consequence of a spillover of IAV from pigs to humans in 2009 (referred to as H1N1pdm09). The viral and host markers important for host adaptation of IAVs are poorly defined, which makes it difficult to evaluate the pandemic potential of novel swine IAV strains. A method of predicting the risk of human-to-human transmission of IAVs is by using the ferret as an animal model. In this study, we assessed the degree of host adaptation to pigs and ferrets of three different H1N1pdm09 viruses by virological, clinical, and pathological measures. We found that a swine-adapted H1N1pdm09 had the highest viral fitness but the lowest virulence in pigs and that a human-adapted H1N1pdm09 had the highest viral fitness but the lowest virulence in ferrets. In both animal models, we found that an H1N1pdm09 virus resembling a pre-pandemic variant had an intermediary viral fitness but the highest virulence. Our findings indicate that host adaptation is important for the viral fitness and virulence and that an increased viral fitness is not always related with a higher virulence.