Concentrations of a Koi herpesvirus (KHV) in tissues of experimentally-infected Cyprinus carpio koi as assessed by real-time TaqMan PCR

Abstract

The Koi herpesvirus (KHV) is a herpes-like virus now recognized as a worldwide cause of mortality among populations of koi Cyprinus carpio koi and common carp Cyprinus carpio carpio. Temperature is a key factor influencing virus replication both in cell culture and in the tissues of experimentally infected fish. Genomic DNA sequences were used to optimize a rapid real-time TaqMan PCR assay to detect and quantify KHV DNA as found in the tissues of virus-exposed fish. The assay allowed analytical enumeration of target KHV genome copies ranging from 10(1) to 10(7) molecules as present in infected cell lines or fish tissues. The new assay was specific for KHV and did not detect DNA from 3 related herpes-like viruses found in fish, the Cyprinid herpesvirus 1 (CyHV-1), Cyprinid herpesvirus 2 (CyHV-2), Ictalurid herpesvirus 1 (IcHV-1) or the KF-1 cell line used for virus growth. Concentrations of KHV DNA were evaluated in 7 different tissues of replicate groups of virus-exposed koi held at water temperatures of 13, 18, 23 and 28 degrees C. Viral DNA was detected among virus-exposed koi at all 4 water temperatures but mortality was only observed among fish at 18, 23, and 28 degrees C. Time and temperature and the interactions between them affected concentrations of viral DNA detected in tissues of koi exposed to KHV. Although there were no recognized patterns to viral DNA concentrations as found in different tissues over time, KHV genome copies for all tissues increased with time post virus exposure and with water temperature. The remarkably rapid and systemic spread of the virus was demonstrated by the presence of viral DNA in multiple tissues 1 d post virus exposure. The greatest DNA concentrations found were in the gill, kidney and spleen, with virus genome equivalents consistently from 10(8) to 10(9) per 10(6) host cells. High levels of KHV DNA were also found in the mucus, liver, gut, and brain. Koi surviving infection at 62 to 64 d post virus exposure contained lower KHV genome copies (up to 1.99 x 10(2) per 10(6) host cells) as present in gill, kidney or brain tissues.