Distribution of the introduced cyprinid herpesvirus 3 in a wild population of common carp, Cyprinus carpio L.

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Abstract
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.