FAdV-4 can cause more noticeable clinical symptoms compared to FAdV-8b after infecting specific pathogen free chickens.

Chickens infected with FAdV-4 and FAdV-8b both exhibit hepatic lesions characterized by hemorrhagic necrosis and intranuclear inclusion body formation. However, only FAdV-4 induces pericardial effusion and acute mortality in chickens. To investigate the similarities and differences in the pathogenicity of HPS and IBH, this study intends to compare the infectivity and pathogenicity of FAdV-4 and FAdV-8b, 2 serotypes of fowl adenovirus isolated in our laboratory. The 2 viruses were respectively inoculated subcutaneously into SPF chicks at the neck. The clinical manifestations and pathological changes in these infected groups of chickens differed to some extent. Chickens infected with FAdV-4 exhibit evident depression and acute mortality, with a mortality rate of 60%; while those infected with FAdV-8b only display mild depression. Postmortem examination reveals serosanguinous effusion in the pericardial sac, spot-like hemorrhage, and focal necrosis in the liver of chickens infected with FAdV-4. Additionally, various degrees of edema are observed in organs such as the lungs, spleen, kidneys, and pancreas. In contrast, chickens infected with FAdV-8b exhibit spot-like hemorrhage and focal necrosis in the liver but do not display pericardial effusion or widespread organ edema. Histopathological examination demonstrates that both FAdV-4 and FAdV-8b can induce inflammatory reactions of varying degrees in the kidneys, pancreas, and duodenum of chickens, while reducing the necrosis of bursa of Fabricius, thymus, and spleen lymphocytes. Our data preliminarily reveal that both FAdV-4 and FAdV-8b can induce strong pathogenicity in chickens.