Cyprinid herpesvirus 1 (CyHV-1) is a DNA virus which infects common carp and koi (Cyprinus carpio), resulting in high mortality in fish younger than 2 months of age. Cyprinid herpesvirus 2 (CyHV-2) infects goldfish (Carassius auratus L.) reared in aquaculture, causing high mortality in all size classes.

To determine the prevalence of these viruses in aquatic environments, we established a method for quantifying CyHV-1 and -2 DNA. Water samples were collected from 22 sites in Lake Biwa, Japan from 2008 to 2010 and were concentrated prior to analysis. CyHV-1 and -2 were detected in 5.2% and 4.9% of the samples and had a maximum concentration of 1.2 x 10³ and 7.4 x 10² copies DNA/L, respectively. CyHV-1 was detected with high frequency from February to June 2010 and from sampling points with water temperatures of 10 to 25^oC, which is the temperature range of viral replication. Moreover, compared to other zones of the lake, CyHV-1 showed higher concentrations in the reed zone. These results suggest that CyHV-1 is prevalent at breeding sites during the breeding season and may affect wild carp populations. On the other hand, CyHV-2 was detected with relatively higher frequency in 2009 but did not show any seasonal or spatial patterns. The presence of CyHV-2 suggests that the virus and/or goldfish are introduced to and present in Lake Biwa from aquaculture effluent.