Abstract: In October 2017, an infectious disease broke out at a koi carp farm in Sichuan Province. It was characterized by drowsiness, abdominal swelling, gill swelling and eye depression. The cumulative mortality rate was 50%. In order to study its etiology and regularity of the disease, dissection, histopathology, electron microscopy and phylogenetic analysis were carried out. Principal symptoms included ocular opacity, abdominal swelling and gill necrosis. The gill, liver, kidney and brain tissues showed obvious degeneration, necrosis and massive inflammatory cell infiltration. Virus particles were found in kidney under transmission electron microscope. The specific amplification products of carp edema virus (CEV) were detected by nested-PCR. Based on CEV P4a gene, phylogenetic analysis showed that the homology between the isolated strain and British strain R083 was 90%. The result in this study was important for virus evolution, classification, viral diagonosis and disease control of carp edema virus.

Key words: carp edema virus disease(CEVD), carp edema virus(CEV), pathological injury, system evolution analysis