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ABSTRACT-Three regions of koi herpesvirus (KHV) genomic DNA were compared for 34 samples from Japan, six from Indonesia, two from Taiwan, one from the Philippines, 13 from the Netherlands, one from the UK, one from the USA and one from Israel. The analyzed genomic regions included known PCRdetection targets (SphI-5, 9/5 and the thymidine kinase gene). The KHVs from Asian countries were very homogeneous, although two variants were noted based on a single nucleotide polymorphism. In contrast, seven variants were found in KHVs from outside of Asia, and although closely related to one another, they were clearly distinct from those from Asian. The results suggest that a clear genetic distinction exists between Asian and European (including each single isolate from the USA and Israel) types of KHV, and that unique types of KHV were independently introduced or emerged in the respective geographic locations.

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Cyprinid herpesvirus 3 (CyHV-3), which causes a lethal disease in common carp, Cyprinus carpio L., and koi, C. carpio koi, first occurred in Lake Biwa, Japan in 2004. To elucidate distribution of CyHV-3 in a wild common carp population, we conducted a PCR survey of CyHV-3 among such fish in Lake Biwa in 2006. Only 6% (1/18) of the common carp smaller than 300 mm were positive with PCR, whereas 31% (18/58) of fish larger than 300 mm were positive. To evaluate their past exposure to CyHV-3 infection based on the presence of antibodies, we also measured the levels of serum anti-CyHV-3 antibodies in the carp, using an enzyme-linked immunosorbent assay. None (0/26) of the fish smaller than 300 mm was positive for the antibodies, whereas 54% (33/61) of fish larger than 300 mm were positive. Of the antibody-positive individuals, 44% (14/32) were also positive by PCR strongly suggesting that wild common carp that survived infection become CyHV-3 carriers. Five individuals were positive by PCR but negative for antibodies indicating that their infection with CyHV-3 had occurred recently. These results suggest that transmission of CyHV-3 from carriers to naïve common carp is still occurring in Lake Biwa.