Evaluation of correlation of EIV(H3N8) replication and  pathological changes in lungs of BALB/c mice

Equine influenza (EI) is a highly contagious viral respiratory disease caused by the equine influenza virus (EIV). Frequent antigenic changes of EIV warrants thorough pathological evaluation and in-vivo replication of the circulating viruses in an experimental animal model for selection of vaccine strain. In the present study, EIV (H3N8) was inoculated into BALB/c mice and subsequently, gross, histopathological and immunohistochemical examination of the lungs was performed to evaluate the correlation of virus replication with corresponding pathological changes. Microscopically, at 1dpi, lungs from mice infected with EIV revealed degeneration of epithelial tissues, whereas indirect immunoperoxidase staining (IIPT) results showed high EIV antigen positivity in the bronchial and bronchiolar epithelium and mild positivity in the infiltrating cells. Subsequently from 1 dpi to 3 dpi, the virus replicated and infected other parenchymal cells, which led to recruitment of inflammatory cells. Microscopically moderate peribronchial, peribronchiolar and perivascular cuffing of neutrophils and macrophages were observed, whereas IIPT exhibited, scanty and moderate EIV antigen positive interstitial infiltrating macrophages. At 5 and 7 dpi, severe, diffuse interstitial thickening, bronchial, bronchiolar and alveolar epithelial necrosis and denudation, perivascular and peribronchial mononuclear cells cuffing were observed, which could be correlated with IIPT as high EIV antigen positivity of infiltrating macrophages and type II pneumocytes. However, from 11 dpi onwards, microscopically, moderate interstitium thickening and perivascular and peribronchial cuffing were observed whereas, tissues were negative for EIV antigen in IIPT. Mild, focal inflammation could be observed at 14 dpi. Present findings signify the correlation of virus replication with corresponding histopathological changes, which will be useful for assessing the efficacy of anti-influenza drug molecules and EIV vaccines.