Evaluation of the Virulence of Low Pathogenic H9N2 Avian Influenza Virus Strains in Broiler Chickens.

Simple Summary: The H9N2 subtype is widespread worldwide and is endemic among low-pathogenic avian influenza (LPAIV) viruses in the Middle East, North Africa and Asia. The virulence of H9N2 viruses of Middle Eastern and North African origin was investigated by different methods (necropsy, histopathology, immunohistochemistry and quantitative RT-PCR). Clear difference was demonstrated between H9N2 isolates belonging to the same lineage (G1): A/chicken/Morocco/2021/2016 strain proved to be significantly more virulent than those of the Egyptian and Saudi Arabian ones, which showed no remarkable difference. Our study aimed to investigate the virulence of three recent H9N2 LPAIV strains belonging to the G1 lineage, isolated from field infections in North Africa and the Middle East. Three-week-old commercial broiler chickens (in total 62) were included and randomly allocated into three infected test groups and one control group. Each test group was inoculated intranasally/intratracheally with one of the three H9N2 isolates at a dose of 10⁸ EID₅₀ virus. The control group received phosphate-buffered saline (PBS) via the same route of application. The pathogenicity was evaluated based on clinical signs and gross pathological and histopathological lesions, the viral antigen load was assessed through immunohistochemistry staining (IHC), and a semi-quantitative detection of the genetic material was conducted via a real-time PCR. Our findings confirmed the obvious respiratory tract tropism of the virus strains with variable renal tropism. In contrast to the highly pathogenic AIVs, the tested H9N2 strains did not show replication in the central nervous system. The virus presence and lesions, mainly in the respiratory tract, were predominant on dpi 5 and significantly reduced or disappeared by dpi 11. A clear difference was demonstrated among the three isolates: the A/chicken/Morocco/2021/2016 strain proved to be significantly more virulent than the Egyptian and Saudi Arabian ones, which showed no remarkable difference. [ABSTRACT FROM AUTHOR]