Investigation of Koi Herpesvirus Latency in Koi

Abstract

Koi herpesvirus (KHV) has recently been classified as a member of the family of Alloherpesviridae within the order of Herpesvirales. One of the unique features of Herpesviridae is latent infection following a primary infection. However, KHV latency has not been recognized. To determine if latency occurs in clinically normal fish from facilities with a history of KHV infection or exposure, the presence of the KHV genome was investigated in healthy koi by PCR and Southern blotting. KHV DNA, but not infectious virus or mRNAs from lytic infection, was detected in white blood cells from investigated koi. Virus shedding was examined via tissue culture and reverse transcription-PCR (RT-PCR) testing of gill mucus and feces from six koi every other day for 1 month. No infectious virus or KHV DNA was detected in fecal secretion or gill swabs, suggesting that neither acute nor persistent infection was present. To determine if KHV latent infections can be reactivated, six koi were subjected to a temperature stress regime. KHV DNA and infectious virus were detected in both gill and fecal swabs by day 8 following temperature stress. KHV DNA was also detectable in brain, spleen, gills, heart, eye, intestine, kidney, liver, and pancreas in euthanized koi 1 month post-temperature stress. Our study suggests that KHV may become latent in leukocytes and other tissues, that it can be reactivated from latency by temperature stress, and that it may be more widespread in the koi population than previously suspected.

Virus genomes and virus-host interactions in aquaculture animals

“…Through use of bioluminescence imaging, the skin covering a fish's fins and body has been shown to be the major port of entry for cyprinid herpesviruses [131]. Tissue culture and RT-PCR testing results indicate that herpesviruses may become latent in leukocytes and other tissues following a primary infection, and that they can be reactivated from latency by temperature stress [132]. Recently, B cells have been identified as a major site where CyHV-3 can become latent [133].…”

Koi Herpes Virus: A Review and Risk Assessment of Indian Aquaculture

“…In wild populations, KHV can remain as carrier in asymptomatic fish and act as reservoir of infection [83]. Several months after initial exposure to the virus, temperature-dependent reactivation of KHV infection can occur in exposed fish [19,78]. This latency feature of KHV probably contributes in the spread of this pathogen to new geographic locations.…”

Herpesviruses of Fish, Amphibians and Invertebrates

“…Latent infections are characterized by the absence of infectious virus particles and regular viral transcription and replication, but the presence of intact viral genomic DNA and transcription of latency associated transcripts. Accordingly, the presence of viral DNA (and not infectious virus) has been demonstrated in fish surviving infections with IcHV-1 and CyHV-3 (Eide et al, 2011a;Gray et al, 1999), whereas viral replication was absent in such www.intechopen.com fish (Eide et al, 2011b;Stingley et al, 2003). It has been shown in vitro that CyHV-3 is able to persist in cultured cells at a nonpermissive temperature, with viral propagation and viral gene transcription being turned off, and reactivated upon return to the permissive temperature (Dishon et al, 2007).…”

“…It has been shown in vitro that CyHV-3 is able to persist in cultured cells at a nonpermissive temperature, with viral propagation and viral gene transcription being turned off, and reactivated upon return to the permissive temperature (Dishon et al, 2007). Reactivation of fish herpesviruses in vivo has been demonstrated for AngHV-1 following dexamethasone treatment (van Nieuwstadt et al, 2001), and for CyHV-3 following temperature stress (Eide et al, 2011b;St-Hilaire et al, 2005).…”