Identification and rapid diagnosis of the pathogen responsible for haemorrhagic disease of the gill of Allogynogenetic crucian carp

In recent years, an epizootic causing severe mortality among Allogynogenetic crucian carp (ACC), designated as haemorrhagic disease of ACC gill, occurred in Yancheng city of Jiangsu province of China. Obvious haemorrhage in the gills of moribund fish and a mortality rate of 100% were observed when ACCs were artificially infected with liver homogenate from diseased fish. A herpes-like virus, with enveloped virions ranged from 170 to 220 nm in diameter, could be observed in the tissues of challenged ACCs by examination with electron microscopy. Specific products representing the polymerase and helicase genes of Cyprinid herpesvirus-2 (CyHV-2) could be amplified from the challenged fish, suggesting that the haemorrhagic disease of ACC gill was caused by infection with CyHV-2. To rapidly diagnose CyHV-2-infected fish, an easy and effective detection assay with loop-mediated isothermal amplification (LAMP) was established. The LAMP assay was more sensitive than conventional PCR and the limit of detection was approximately 100 copies of target DNA. With this LAMP assay, CyHV-2 could be detected in some asymptomatic ACCs from the epidemic area and in eggs from the diseased ACCs, suggesting that CyHV-2 could infect ACCs latently and that the virus may be passed onto offspring by vertical transmission.