Outbreak of carp edema virus disease in cultured ornamental koi in a lower temperature in China

Abstract

Carp edema virus (CEV) is the pathogen of carp edema virus disease (CEVD), an emerging infectious disease with high morbidity and mortality worldwide. In the winter of 2016, a disease outbreak occurred in a koi pond in Chengdu, located in the southwest of China. The water temperature at that time was 11–13 °C. The clinical signs of the diseased koi were lethargy, unresponsiveness, enophthalmos, swollen gills, and hemorrhagic skin. Histopathological changes were observed in moribund fish, such as significant cell hyperplasia in the gill lamellae, and swelling and vacuolar degeneration in epithelial cells of the renal tubules and hepatocytes. Electron microscopy of renal tissues showed virions of a pox-like virus (200–235 nm diameter). The presence of specific 528 bp and 478 bp fragments was detected by nested PCR. An experimental infection demonstrated cumulative mortality of 15% (3/20) at a water temperature of 13 ± 2 °C. Furthermore, sequence analysis of the partial 4a genes showed that the CEV isolate (MG189372) in this study had 100% homology with a UK strain (KX254027). The evidence showed that the pathogen involved in this outbreak was CEV. To the best of our knowledge, this is the first report of carp disease caused by CEV at low temperature in China, demonstrating that CEV disease should be monitored in the cold season and at low temperatures. The findings are of profound significance to reducing the risk of CEVD outbreaks in carp aquaculture.