Differences in the susceptibility of Japanese indigenous and domesticated Eurasian common carp (Cyprinus carpio), identified by mitochondrial DNA typing, to cyprinid herpesvirus 3 (CyHV-3).

In 2004, a massive mortality of wild common carp (Cyprinus carpio) due to CyHV-3 infection occurred in Lake Biwa. Although common carp of two different mitochondrial types (Japanese indigenous and domesticated Eurasian) occur in the lake, the majority of the dead fish seemed to be the indigenous type. The apparent high mortality in the indigenous type implies a higher susceptibility of this type to CyHV-3. To test the hypothesis that the susceptibility of indigenous and Eurasian types differ, we performed experimental infections with CyHV-3 among 2 groups of the indigenous type, and for the Eurasian type 4 groups of domesticated common carp and 4 groups of koi carp. Fish were immersed in CyHV-3 isolate and kept at 24°C. Both groups of the indigenous type died more rapidly compared with the 8 groups of the Eurasian type. Cumulative mortality in both indigenous groups reached 95-100%, whereas the cumulative mortalities of domesticated common carp (30-95%) and koi carp (35-100%) were more varied. CyHV-3 genome in the organs of the indigenous type increased more rapidly after the viral exposure and reached higher peak levels than those of the domesticated strain. These findings revealed that susceptibility of the indigenous type of carp to CyHV-3 can be considered especially high.