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Outbreaks of koi sleepy disease (KSD) caused by carp edema virus (CEV) may seriously affect populations of farmed common carp, one of the most important fish species for global food production. The present study shows further evidence for the involvement of CEV in outbreaks of KSD among carp and koi populations: in a series of infection experiments, CEV from two different genogroups could be transmitted to several strains of naïve common carp via cohabitation with fish infected with CEV. In recipient fish, clinical signs of KSD were induced. The virus load and viral gene expression results confirm gills as the target organ for CEV replication. Gill explants also allowed for a limited virus replication in vitro. The in vivo infection experiments revealed differences in the virulence of the two CEV genogroups which were associated with infections in koi or in common carp, with higher virulence towards the same fish variety as the donor fish. When the susceptibility of different carp strains to a CEV infection and the development of KSD were experimentally investigated, Amur wild carp showed to be relatively more resistant to the infection and did not develop clinical signs for KSD. However, the resistance could not be related to a higher magnitude of type I IFN responses of affected tissues. Despite not having a mechanistic explanation for the resistance of Amur wild carp to KSD, we recommend using this carp strain in breeding programs to limit potential losses caused by CEV in aquaculture.Electronic supplementary materialThe online version of this article (doi:10.1186/s13567-017-0416-7) contains supplementary material, which is available to authorized users.

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Cyprinid herpesvirus 3 (CyHV-3), also known as koi herpesvirus (KHV), is the etiological agent of a virulent and lethal disease in common and koi carp. This study aimed to determine the genetic basis underlying the common carp immune response to the CyHV-3 virus. Two common carp lines (R3 and K) were infected with CyHV-3 by immersion. The R3 line presented a 20% higher survival rate compared to the K line and significantly lower viral loads as measured at day 3 post infection (p.i.). Microarray analysis using a common carp slides containing a number of 10,822 60-mer probes, revealed that 581 genes in line K (330 up-regulated, 251 down-regulated) and 107 genes in line R3 (77 up-regulated, 30 down-regulated), showed at least a 2-fold difference in expression at day 3 p.i. compared to day 0. Genes which showed at least a 4-fold difference in expression in both lines were selected as potential markers of a CyHV-3 infection in common carp. Additionally, 76 genes showed at least 2-fold differentially expression between K and R3 lines at day 3 p.i. Significantly higher expression of several immune-related genes including number of those which are involve in pathogen recognition, complement activation, MHC class I-restricted antigen presentation and development of adaptive mucosal immunity was noted in more resistant R3 line. Further real-time PCR based analysis provided evidence for higher activation of CD8(+) T cells in R3 line. This study uncovered wide array of immune-related genes involved into antiviral response of common carp toward CyHV-3. It is also demonstrated that the outcome of this severe disease in large extent could be controlled by genetic factors of the host.

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BackgroundInfections with carp edema virus, a pox virus, are known from Japanese koi populations since 1974. A characteristic clinical sign associated with this infection is lethargy and therefore the disease is called “koi sleepy disease”. Diseased koi also show swollen gills, enophthalmus, and skin lesions. Mortality rates up to 80 % are described. For a long period of time, disease outbreaks seemed to be restricted to Japan. However, during the last years clinical outbreaks of koi sleepy disease also occurred in the UK and in the Netherlands.Case presentationIn spring 2014 koi from different ponds showing lethargic behavior, skin ulcers, inflammation of the anus, enophthalmus, and gill necrosis were presented to the laboratory for diagnosis. In all cases, new koi had been purchased earlier that spring from the same retailer and introduced into existing populations. Eleven koi from six ponds were examined for ectoparasites and for bacterial and viral infections (cyprinid herpesviruses in general and especially koi herpesvirus (KHV) known formally as Cyprinid herpesvirus 3 (CyHV–3); and Carp Edema Virus). In most of the cases parasites were not detected from skin and gills. Only opportunistic freshwater bacteria were isolated from skin ulcers. In cell cultures no cytopathic effect was observed, and none of the samples gave positive results in PCR tests for cyprinid herpesviruses. By analyzing gill tissues for CEV in seven out of eleven samples by a nested PCR, PCR products of 547 bp and 180 bp (by using nested primers) could be amplified. An outbreak of Koi Sleepy Disease was confirmed by sequencing of the PCR products. These results confirm the presence of CEV in German koi populations.ConclusionA clinical outbreak of “koi sleepy disease” due to an infection with Carp Edema Virus was confirmed for the first time in Germany. To avoid transmission of CEV to common carp testing of CEV should become part of fish disease surveillance programs.Electronic supplementary materialThe online version of this article (doi:10.1186/s12917-015-0424-7) contains supplementary material, which is available to authorized users.

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