Carp edema virus, an emerging threat to the carp (Cyprinus carpio) industry in China

Koi sleepy disease (KSD), also known as carp edema virus (CEV) disease, is an important, largely acute infection of common carp and ornamental koi (Cyprinus carpio). Since the first report of KSD in Japan in the 1970s, the disease has spread to many countries including England, France, the Netherlands, Germany, Italy, the Czech Republic, Austria, Poland, and India. In the spring of 2015, a koi farm in Hangzhou experienced an outbreak of an infectious disease. In this study we conducted clinical, histopathological, and molecular diagnostic analyses to determine the cause of the outbreak. CEV was found to be the causative agent of the disease. Moreover, PCR sequencing and phylogenetic analyses revealed that CEV was the extremely likely etiological agent of the mortality in this outbreak, which was further confirmed by sequence analysis of the PCR products obtained after nested PCR of the degenerate PCR. We also found that temperature is an important factor affecting carp morbidity and mortality in CEV infection. This is the first report of the detection of CEV in relation to KSD outbreaks in China. Our findings highlight the need for research on the CEV genome to better characterize the spread of CEV.