Comparison of the blood parameters and histopathology between grass carp infected with a virulent and avirulent isolates of genotype II grass carp reovirus

Grass carp hemorrhagic disease caused by grass carp reovirus (GCRV) is the most important disease for grass carp aquaculture. Its typical clinical symptom is haemorrhaging, although the mechanism was remained unclear. In this study, we investigated the differences in blood parameters and histopathological features between grass carp infected with a virulent and avirulent isolates of genotype II GCRV. Infection with the virulent isolate resulted in increases in 8 routine blood and 2 serum biochemical parameters (P < 0.05); while 9 routine blood and 5 biochemical parameters were significantly decreased (P < 0.05) compared with fish infected with the avirulent isolate. The majority of these alterations were related to hemorrhage, inflammatory reactions and organic damage. The histopathologic changes were primarily vasodilation and hyperaemia in multiple organs, lymphocyte and macrophage infiltration as well as severe vacuolar degeneration in spleen, kidney and liver. The histopathology changes in fish infected with the avirulent isolate were minimal. These results indicated that the pathogenicity of GCRV was primarily reflected in destruction of the blood circulatory system and parenchymatous organs. This study lays the foundation for further research on the pathogenesis of bleeding caused by GCRV infection and the use of blood parameters and histopathology as tools for disease diagnosis.