Pathogenicity of a fowl adenovirus serotype 4 isolated from chickens associated with hydropericardium-hepatitis syndrome in China.

Hydropericardium-hepatitis syndrome (HHS) is characterized by pericardial effusion and hepatitis and causes huge economic losses in the poultry industry in China. In this study, a strain of fowl adenoviruses (FAdV-4) (GX-1) was isolated from liver samples of diseased chickens with HHS. Phylogenetic analysis based on complete genome gene revealed that GX-1 clustered with the C-type fowl adenovirus and was serotyped as FAdV-4. Pathogenicity testing showed that the GX-1 strain caused 100% mortality in 10-day-old specific pathogen-free chickens at a dose of 104 tissue culture infective doses (TCID50) within 3 d post-infection. A viral dose of 103 TCID50 resulted in a 16% survival rate before day 9 and at 102 TCID50 an 80% rate before day 6. At necropsy, livers from infected chickens were swollen and yellow brown with necrotic foci. The hearts were flabby with amber-colored and jelly-like fluid in the pericardial sacs. The kidneys were swollen and congested. Histologically eosinophilic intranuclear inclusion body could be seen in the hepatic cell. The result of histopathological examination also revealed that heart muscle fibers were fractured with extensive congestion and hemorrhaging. Other tissues like kidney, bursa of Fabricius, thymus, and spleen were observed degeneration and necrosis. Virus-specific antibodies appeared in serum beginning at day 14 and reached statistically significant levels at 21, 28, 35, and 42 dpi (P < 0.001). In conclusion, we identified a highly virulent FAdV-4 virus as causative agent of the HHS outbreak reported here. The FAdV-4 GX-1 strain will be valuable for vaccine evaluation and development to prevent and reduce the spread of HHS in the poultry industry.