Investigation of Koi Herpesvirus Releasing and Latent Sites in Persistently Infected Koi

Koi herpesvirus (KHV) is an enveloped virus with an icosahedral electron-dense core of 100-110 nm surrounded by tegument protein. The genome of the KHV comprises linear double-stranded DNA (dsDNA) of 295 kbp and its genomic sequence was similar with Cyprinid herpesvirus 1 and 2, so was named as Cyprinid herpesvirus 3. In 2009, KHV was classified as Alloherpesviridae. The first outbreak of this disease was reported in 1997 in Germany, then USA and Israel in 1998. Rapid spread of KHV occurred because of the trade of koi, and in December 2002, KHV has been identified in Taiwan. How and where KHV preserve in the environment and in carrier fish is important, but still unknown. Many researchers proposed that the virus may persist in fish droppings, sediments, and survived fish after infection, or preserve in other fish species and animals, then when the water temperature is to a permissive temperature, virus will be released and infect carp again.
Generally herpesviruses including fish herpesvirus show characters about latent infection and reactivation. In spring and autumn, many koi farms which had been diagnosed KHV positive, are often with fatal reports due to the recurrence of KHV in Taiwan. The aim of this study is to investigate whether KHV survived fish will become carriers, and which organs or tissues will KHV be latent, and whether the virus be released from KHV carrier, if the water temperature was proper. In this study, we changed water temperature to induce KHV carriers and these carrier would cohabitate with naïve fish, and then, locate the KHV by a more sensitive and specific diagnostic method, nested polymerase chain reaction (nested-PCR). Our results indicated that KHV carrier can transmit KHV to naïve fish no matter in 23oC or 32oC and that KHV TK gene can be detected by nested PCR in all organs (gill, liver, spleen, kidney, head kidney, brain, intestine and gonad) in carrier fish and cohabitated fish. Thus, our results suggest that KHV exist with very low level in internal organs without favorite during persistent infection. We also suggest that PCR is not the accurate method to screening carrier, and need the more sensitive method to detect the carrier fish.