Ferrets are a valuable model for influenza pathogenesis, virus transmission and antiviral therapy studies. In our analysis of the current literature related to the use of the ferret model shortly after the emergence of the 2009 H1N1 pandemic (H1N1pdm) influenza virus, we noted substantial variation in clinical disease outcomes for ferrets that were administered similar H1N1pdm viruses intranasally at a dose of 106 TCID50. Interestingly, comparison of these ferret studies revealed that, of the animals in the studies, both volume of inoculum administered and animal age at the time of infection. We therefore began to assess the role of inoculum volume and animal age on the severity of clinical disease ferrets experimentally infected with influenza virus. We administered the H1N1pdm (A/California/07/2009) and the H3N2-variant (H3N2v) (A/Minnesota/11/2010) influenza viruses to young (8 week old) and old (6 month old) ferrets intranasally in a range of inoculum volumes (0.2, 0.5, or 1.0 ml), and followed viral replication, clinical disease and pathology over 6 days. In addition to clinical disease we examined the ferret's immune response following infection and we evaluated the anatomy of the ferret's respiratory tract and their contributions to disease severity. We found that clinical illness and respiratory tract pathology were most severe and most consistent in older (6 month old) ferrets and when the viruses were administered in a volume of 1.0 ml. Previous studies in our lab showed similar clinical disease outcomes in young and old ferrets infected with highly pathogenic avian influenza (HPAI) virus (A/Vietnam/1203/2004; H5N1). Using a modified micro-CT imaging method, we found that the right main stem bronchus was consistently larger in diameter than the left main stem bronchus though the latter was longer and straighter. Based on our findings, these anatomic features likely influence the distribution of inoculum in the lower respiratory tract. A 1.0 ml volume of inoculum is optimal for delivery of virus to the lower respiratory tract of ferrets, particularly when evaluation of clinical disease is desired. Furthermore, we highlight important anatomical features of the ferret lung that influence the kinetics of viral replication, clinical disease severity and lung pathology.
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