Isolation of a highly pathogenic spring viraemia of carp virus strain from grass carp (Ctenopharyngodon idella) in late summer, China, 2016.

Spring viraemia of carp virus (SVCV) is an OIE-listed notifiable pathogen, which has brought huge economic loss to the aquaculture industry. Outbreaks of SVC mostly occur in spring with water temperature 11-17°C. Presently, there is an increase in detection during import quarantine testing and associated with outbreaks of SVCV outside of China, yet China is regarded as the origin of SVCV Asian clade. However, recent isolates from the Shanghai area all showed to be low pathogenic to their original hosts. In this study, we isolated a new SVCV strain (nominated as SH160901) from grass carp in late summer in Shanghai, 2016. Phylogenetic analysis showed this strain formed a distinct new lineage in the Asian clade along with our isolate SH150514 in 2015, and was divergent from all other identified Asian isolates. Cell infection test demonstrated that this strain replicated most efficiently at 25°C and 28°C, and could induce obvious cytopathic effect in infected cells. In vivo infection test revealed this strain could cause severe symptoms in experimentally infected fish at 16-20°C. Inoculated fish died at 100% in grass carp and common carp (Cyprinus carpio carpio) within 13days, and at 100% in goldfish (Carassius auratus) and 90% in koi (Cyprinus carpio koi) within 40days. Experimental infections at 26°C also induced moderate mortalities in grass carp (25%) and common carp (20%). The biological changes characterized for SVCV isolate SH160901 warrant changes to surveillance plans, specifically there is a need to broaden the testing parameters previously associated with SVCV.