Pathogenesis and Transmission Assessment of 3 Swine-Origin Influenza A(H3N2) Viruses With Zoonotic Risk to Humans Isolated in the United States, 2017-2020.

The sporadic occurrence of human infections with swine-origin influenza A(H3N2) viruses and the continual emergence of novel A(H3N2) viruses in swine herds underscore the necessity for ongoing assessment of the pandemic risk posed by these viruses. Here, we selected 3 recent novel swine-origin A(H3N2) viruses isolated between 2017 to 2020, bearing hemagglutinins from the 1990.1, 2010.1, or 2010.2 clades, and evaluated their ability to cause disease and transmit in a ferret model. We conclude that despite considerable genetic variances, all 3 contemporary swine-origin A(H3N2) viruses displayed a capacity for robust replication in the ferret respiratory tract and were also capable of limited airborne transmission. These findings highlight the continued public health risk of swine-origin A(H3N2) strains, especially in human populations with low cross-reactive immunity.

Comparative Assessment of Severe Acute Respiratory Syndrome Coronavirus 2 Variants in the Ferret Model.

The continued spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in humans necessitates evaluation of variants for enhanced virulence and transmission. We used the ferret model to perform a comparative analysis of four SARS-CoV-2 strains, including an early pandemic isolate from the United States (WA1), and representatives of the Alpha, Beta, and Delta lineages. While Beta virus was not capable of pronounced replication in ferrets, WA1, Alpha, and Delta viruses productively replicated in the ferret upper respiratory tract, despite causing only mild disease with no overt histopathological changes. Strain-specific transmissibility was observed; WA1 and Delta viruses transmitted in a direct contact setting, whereas Delta virus was also capable of limited airborne transmission. Viral RNA was shed in exhaled air particles from all inoculated animals but was highest for Delta virus. Prior infection with SARS-CoV-2 offered varied protection against reinfection with either homologous or heterologous variants. Notable genomic variants in the spike protein were most frequently detected following WA1 and Delta virus infection. IMPORTANCE Continued surveillance and risk assessment of emerging SARS-CoV-2 variants are critical for pandemic response and preparedness. As such, in vivo evaluations are indispensable for early detection of variants with enhanced virulence and transmission. Here, we used the ferret model to compare the pathogenicity and transmissibility of an original SARS-CoV-2 isolate (USA-WA1/2020 [WA1]) to those of a panel of Alpha, Beta, and Delta variants, as well as to evaluate protection from homologous and heterologous reinfection. We observed strain-specific differences in replication kinetics in the ferret respiratory tract and virus load emitted into the air, revealing enhanced transmissibility of the Delta virus relative to previously detected strains. Prior infection with SARS-CoV-2 provided varied levels of protection from reinfection, with the Beta strain eliciting the lowest level of protection. Overall, we found that ferrets represent a useful model for comparative assessments of SARS-CoV-2 infection, transmission, and reinfection.

Risk Assessment of Fifth-Wave H7N9 Influenza A Viruses in Mammalian Models.

The fifth wave of the H7N9 influenza epidemic in China was distinguished by a sudden increase in human infections, an extended geographic distribution, and the emergence of highly pathogenic avian influenza (HPAI) viruses. Genetically, some H7N9 viruses from the fifth wave have acquired novel amino acid changes at positions involved in mammalian adaptation, antigenicity, and hemagglutinin cleavability. Here, several human low-pathogenic avian influenza (LPAI) and HPAI H7N9 virus isolates from the fifth epidemic wave were assessed for their pathogenicity and transmissibility in mammalian models, as well as their ability to replicate in human airway epithelial cells. We found that an LPAI virus exhibited a similar capacity to replicate and cause disease in two animal species as viruses from previous waves. In contrast, HPAI H7N9 viruses possessed enhanced virulence, causing greater lethargy and mortality, with an extended tropism for brain tissues in both ferret and mouse models. These HPAI viruses also showed signs of adaptation to mammalian hosts by acquiring the ability to fuse at a lower pH threshold than other H7N9 viruses. All of the fifth-wave H7N9 viruses were able to transmit among cohoused ferrets but exhibited a limited capacity to transmit by respiratory droplets, and deep sequencing analysis revealed that the H7N9 viruses sampled after transmission showed a reduced amount of minor variants. Taken together, we conclude that the fifth-wave HPAI H7N9 viruses have gained the ability to cause enhanced disease in mammalian models and with further adaptation may acquire the ability to cause an H7N9 pandemic.IMPORTANCE The potential pandemic risk posed by avian influenza H7N9 viruses was heightened during the fifth epidemic wave in China due to the sudden increase in the number of human infections and the emergence of antigenically distinct LPAI and HPAI H7N9 viruses. In this study, a group of fifth-wave HPAI and LPAI viruses was evaluated for its ability to infect, cause disease, and transmit in small-animal models. The ability of HPAI H7N9 viruses to cause more severe disease and to replicate in brain tissues in animal models as well as their ability to fuse at a lower pH threshold than LPAI H7N9 viruses suggests that the fifth-wave H7N9 viruses have evolved to acquire novel traits with the potential to pose a higher risk to humans. Although the fifth-wave H7N9 viruses have not yet gained the ability to transmit efficiently by air, continuous surveillance and risk assessment remain essential parts of our pandemic preparedness efforts.

Assessment of Molecular, Antigenic, and Pathological Features of Canine Influenza A(H3N2) Viruses That Emerged in the United States.

Background: A single subtype of canine influenza virus (CIV), A(H3N8), was circulating in the United States until a new subtype, A(H3N2), was detected in Illinois in spring 2015. Since then, this CIV has caused thousands of infections in dogs in multiple states. Methods: In this study, genetic and antigenic properties of the new CIV were evaluated. In addition, structural and glycan array binding features of the recombinant hemagglutinin were determined. Replication kinetics in human airway cells and pathogenesis and transmissibility in animal models were also assessed. Results: A(H3N2) CIVs maintained molecular and antigenic features related to low pathogenicity avian influenza A(H3N2) viruses and were distinct from A(H3N8) CIVs. The structural and glycan array binding profile confirmed these findings and revealed avian-like receptor-binding specificity. While replication kinetics in human airway epithelial cells was on par with that of seasonal influenza viruses, mild-to-moderate disease was observed in infected mice and ferrets, and the virus was inefficiently transmitted among cohoused ferrets. Conclusions: Further adaptation is needed for A(H3N2) CIVs to present a likely threat to humans. However, the potential for coinfection of dogs and possible reassortment of human and other animal influenza A viruses presents an ongoing risk to public health.