First detection of carp edema virus in association with cyprinid herpesvirus 3 in cultured ornamental koi, Cyprinus carpio L., in China

An outbreak of a carp disease occurred in a koi pond in early October 2016, Chengdu, southwest of China. Characteristic signs of infected fish include erratic swimming of moribund fish at the water surface, respiratory distress, presence of thick and foggy mucus covering the body and gills. Skin ulcers were also noticed in a few cases. Microbiological, parasitological and virological investigations were carried out with fish samples collected from various localities. Histologically, the gills, kidney and liver showed discernible changes. By analyzing gill tissues in six samples by PCR and nested-PCR for detection of cyprinid herpesvirus 3 (CyHV-3) and carp edema virus (CEV) respectively, PCR products of 292 bp and 478 bp (by using nested primers) could be amplified. The outbreak of carp disease caused by CEV and CyHV-3 mixed infection was confirmed by sequence analysis of the PCR products. The viral load in the gills and kidney were quantified by real-time TaqMan PCR, which in turn would aid the evaluation of the significance of the virus to the etiology of the disease. The result demonstrated that CEV plays the major role to the death of fish. Phylogenetic analyses revealed that the CEV isolate (GenBank accession no. MG189371) showed closest identity to the CEV isolated from UK (KX254027) with 99% homology. To our knowledge, this is the first report a carp disease caused by CEV and CyHV-3 mixed infection in China.