Assessment of parthenogenesis and tissue tropism of three H9N2 avian influenza virus isolates after experimental infection in Specific-Pathogen-Free chickens

Avian influenza virus (AIV) subtype H9N2 is one of the most important AIV in poultry around the world. An experiment was carried out for studying the pathogenicity of three H9N2 isolates. Eighty four one-day-old SPF chickens were divided randomly into “four groups (21 chicks per group)” in separate isolators with positive Air pressure. At the age of 30 days-old chickens in group 1,2, and 3 were inoculated with 106EID50 of three H9N2 isolates by oculo-nasal route, 4th group was kept as control and inoculated with mock allantoic fluid. Samples including oropharyngeal and cloacal swabs and various tissues were collected at 2, 4, 6, 8, 10 and12 days post-inoculation (PI). The Real-time PCR assay was used for detection of the virus swab and tissue samples. In experiment groups, H9N2 AIV was detected from all examined tissues in 2, 4 and 6 dpi, exception was liver and thymus. Maximum histopathological lesions were seen from 2 till 8 dpi in trachea, lung, liver, kidney, spleen, cecal tonsil and thymus eventually. Lesions in other tissues including cecal tonsils, and cloaca was not significant. In immunohistochemical investigation, the presence of antigen in the epithelial cells of lung, kidney and intestinal tissues, was observed on 4 and 6 days post inoculation. Considering the fact that H9N2 viruses are of low pathogenicity but they show various clinical signs in commercial flocks. The results of this study showed that there was some pathogenic differences between three isolates but the differences were not significant.