Viral replication kinetics and pathogenesis of pandemic H1N1 and H7N9 influenza virus infection in isogenic guinea pigs

Isogenic guinea pigs uniquely combine the virtues of the mouse and ferret models for influenza A virus infection. Like mice, they are inbred and therefore suitable for studies on cellular immunity and, like ferrets, they effectively transmit the virus. In order to pave the way for new lines of research into viral transmission and cellular immunity, a baseline study was performed to assess replication kinetics and the pathogenesis of pandemic H1N1 (pH1N1) and H7N9 virus infection in this particular strain of guinea pigs. Forty female isogenic guinea pigs 12-16 weeks were kept under biosafety level 3 conditions and inoculated with pH1N1 virus intranasally (IN n=12) or intratracheally (IT n=10) and sacrificed on day 1, 2, 3, 4 or 7. Remaining animals were inoculated IT with either H7N9 virus (n=12) or PBS (n=6) and sacrificed on day 2 and 7. Swabs were taken daily and full necropsies were performed. Infectious virus titres in tissue samples and swabs were determined using standard methods and immunohistochemical (IHC) and histopathological changes were evaluated by light microscopy. Virus inoculated animals showed peak average viral titres in nasal secretions at day 2 post-inoculation (pi) and by day 7 pi infectious virus was no longer detectable for pH1N1 and low for H7N9 virus inoculated animals. Intranasal inoculation with pH1N1 resulted in higher peak viral excretion via the nose than IT inoculation. pH1N1 IN inoculated guinea pigs showed viral titres in only nasal epithelium at day 4. For pH1N1 IT inoculated animals, infectious virus was recovered up to day 4 from nasal epithelium, trachea, lung and cerebral tissue. For H7N9 IT inoculated animals, virus was recovered from nasal epithelium, lung, cerebrum and trachea on day 2 as well as nasal epithelium on day 7. Histological evaluation revealed more widespread and severe lesions after IT than IN inoculation for pH1N1. Overall, H7N9 inoculated animals showed the most severe lesions. Immunohistochemically, viral antigen positive cells were restricted to the nasal cavity for pH1N1 IN inoculation. For IT inoculation of both pH1N1 and H7N9, viral antigen was more diffusely dispersed in epithelial cells of the respiratory tract and alveolar macrophages around day 2 and still present in the lung and nasal epithelium respectively by day 7