Abstract
Many commercial flocks in Egypt were infected with the low pathogenic avian influenza virus (LPAIV) subtype H9N2, resulting in severe economic losses, particularly when combined with other viral and bacterial diseases. The current study aimed to investigate the genetic evolution of LPAIV (H9N2) in broiler farms in Egypt during 2019 and evaluate its effect on the viral pathogenesis, cytokine response, innate immunity and oxidative state in specific pathogen free (SPF) chicks. Sixty tracheal swabs were collected from broilers and tested by RRT-PCR and the HA gene were sequenced for the genetic evolution. Then this isolated virus was experimentally studied in a total of 20 SPF chicks (21 days old) that were divided into two equal groups. Group-1(G1) was negative control and group-2 (G2) was experimentally infected with the isolated LPAIV (H9N2). The obtained results revealed that all isolated LPAIV (H9N2) in this study were belong to G1-like Eurasian sub-lineages (A/quail/Hong Kong/G1/97-like) and clustered with Israeli strains in group B with specific mutation specific for Egyptian AIV-H9N2 isolated in 2018-2019 forming new subgroup-I. The experimental infection with LPAIV (H9N2) (G2) in SPF chickens, induced mild clinical signs, moderate histopathological changes in lung and spleen, shedding of higher mean LPAIV (H9N2) titre in trachea, up regulation of both the pro-inflammatory (IL-6) and the anti-viral (IFN-γ) cytokines, significant decrease of the serum levels of lysozyme and disturbance of the oxidative state where the serum nitric oxide (NO) and the malondialdehyde (MDA) levels were significantly increased and the antioxidants glutathione (GSH) and superoxide dismutase (SOD) were significantly decreased. In conclusion, the LPAIV (H9N2) was continually circulated in broiler chicken farms and highly evolved. The induced oxidative stress in LPAIV (H9N2) infection could be contributed to the viral pathogenesis, up-regulation of pro-inflammatory and anti-viral cytokines and down-regulation of the innate immune response.

Key words: LPAIV (H9N2), molecular characterisation, pathogenesis, cytokines, immune response.