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.

22Highlights 24 1. Differences in mortality rates during SVCV and CyHV-3 infections were recorded in carp 25 strains. 26 2. The higher resistance of the Rop strain was related to lower virus load and replication. 27 3. The magnitude of type I IFN response was not positively correlated with survival. 28 4. CyHV-3 has an ability to limit IFN response induced by sensing viral DNA by cells. 29 30 Abstract 34 Carp from breeding strains with different genetic background present diverse levels of 35 resistance to viral pathogens. Carp strains of Asian origin, currently being treated as Cyprinus 36 rubrofuscus L., especially Amur wild carp (AS), were proven to be more resistant to koi 37 herpesvirus disease (KHVD; caused by cyprinid herpesvirus 3, CyHV-3) than strains originating 38 from Europe and belonging to Cyprinus carpio L., like the Prerov scale carp (PS) or koi carp 39 from a breed in the Czech Republic. We hypothesised that it can be associated with a higher 40 magnitude of type I interferon (IFN) response as a first line of innate defence mechanisms against 41 viral infections. To evaluate this hypothesis, four strains of common carp (AS, Rop, PS and koi) 42were challenged using two viral infection models: Rhabdovirus SVCV (spring viremia of carp 43 virus) and alloherpesvirus CyHV-3. Infection experiments confirmed significant differences in 44 mortality between the analysed carp strains. The infection with SVCV induced a low mortality 45 rate and the most resistant was the Rop strain (no mortalities), whereas the PS strain was the most 46 susceptible (survival rate of 78%). During CyHV-3 infection, Rop and AS strains performed 47 better (survival rates of 78% and 53%, respectively) than PS and koi strains (survival rates of 48 35% and 10%, respectively). The evaluation of virus loads and virus replication showed 49 significant differences between the carp strains, which correlated with the mortality rate. The 50 evaluation of type I IFN responses showed that there were fundamental differences between the 51 virus infection models. While responses to the SVCV were high, the CyHV-3 generally induced 52 low responses. Furthermore, the results demonstrated that the magnitude of type I IFN responses 53 did not correlate with a higher resistance in infected carp. In the case of a CyHV-3 infection, 54 reduced type I IFN responses could be related to the potential ability of the virus to interfere with 55 cellular sensing of foreign nucleic acids. Taken together, the results broaden our understanding of 56 how common carp from different genetic lines interact with various viral pathogens. 57 monitoring the development of the infection in four tissues and subsequently measuring type I 113 IFN responses. Further in vitro studies were performed to better characterise anti-IFN actions of 114 CyHV-3. 115 116 117 For the infection experiments, the carp were transported to the National Veterinary 131Research Institute in Pulawy, Poland at a mean weight of 10.3 ± 8.2 g and placed at 18 °C in a 132 flow through system two week...