Emergence of fatal European genotype CyHV-3/KHV in mainland China

Cyprinid herpesvirus 3 (CyHV-3), also known as koi herpesvirus (KHV), is a highly infectious causative agent to common carp and koi worldwide. The virus is mainly consisted of European and Asian genotype isolates. To date, no European genotype CyHV-3 has been found emerging in the East and Southeast Asian regions. In late March 2011, an outbreak of CyHV-3 disease occurred in Guangzhou City, Guangdong Province, China, resulting in the deaths of approximately 200 large-sized adult koi within four weeks. One moribund koi was sampled for CyHV-3 isolation. Thus, a CyHV-3 was isolated in KCF-1 cells and designated as KHV-GZ11. Abundant mature or immature virions in infected KCF-1 cells were observed under a transmission electron micrograph. In addition, intra-nuclear inclusion body-like structures with masses of virions were also observed. Based on the TK and ORF136H genes, the sequence analyses revealed that KHV-GZ11 is a distinct European genotype of CyHV-3. Moreover, the infectivity experiment showed that KHV-GZ11 was highly virulent to koi. In summary, we are the first to confirm the emergence of fatal European genotype CyHV-3/KHV in East and Southeast Asia. Our study will provide new insight to explore the virus origin and epidemiology, as well as its pathogenicity.