Carp edema virus, a great threat for carp and koi farming in Henan, China

A carp disease with clinical signs of acute gill rot (AGR) has frequently occurred in Henan province of China since 2013, leading to substantial economic losses. However, the causative agent of virus-related carp AGR (CAGR) disease has been controversial. Our study showed that carp edema virus (CEV), specifically the sub-genogroup IIa, might be the primary agent causing virus-related CAGR that is endemic to Henan, China. This conclusion is based on a cohabitation experiment using isolates from an outbreak in 2019 and a retrospective analysis of CAGR samples collected between 2013 and 2015 in Henan, China. In the cohabitation experiment, the clinical development and pathological changes of CEV disease were observed. The pathological alterations in the fish brain were first noticed to be strongly associated with clinical signs of being sleepy, suggesting that pathological alterations in the fish brain may be a critical differential characteristic of CEV. Another important finding in our study was that the mortality in common carp and koi carp was strongly associated by a temperature change and less correlated to the viral loads in organs. Considering the frequent outbreaks of virus-related CAGR disease in Henan (China), effective prevention, good environmental stress management, and efficient diagnosis are needed. Based on comparing the qPCR and the nested PCR designed at the CEFAS Weymouth Laboratory, we recommend using the qPCR method on gills to identify this pathogen because of the assay's sensitivity and the viral load in gill tissues.