First Detection of Koi Herpesvirus in Cultured Common Carp Cyprinus carpio in Japan

Mass mortality among common carp Cyprinus carpio, especially in large-sized fish cultured in Lake Kasumigaura, has occurred since early October 2003. The affected fish swam lethargically near the surface. A histopathological examination revealed hyperplasia and necrosis of the epithelial cells in the gills. DNA of the koi herpesvirus (KHV) was detected in the gills and kidneys of moribund fish by polymerase chain reaction (PCR) assay. The virus could not be isolated from the tissues, but the inoculation of gill homogenate from affected fish into healthy carp reproduced the disease conditions and killed the fish. KHV was detected from the gills of the experimentally infected fish by PCR, and the virus was isolated from those fish with KF-1 cells.