Experimental infections of different carp strains with the carp edema virus (CEV) give insights into the infection biology of the virus and indicate possible solutions to problems caused by koi sleepy disease (KSD) in carp aquaculture

Abstract

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.

Carp edema virus from three genogroups is present in common carp in Hungary

“…In several cases, fish harboured a high virus load, with seven fish having more than 1 × 10 4 virus genome copies per 250 ng of DNA. A similar virus load was detected in previous infection experiments in koi or carp individuals affected by clinical KSD (Adamek, Oschilewski, et al., ). Hence, we consider a virus load in this order of magnitude as clinically relevant and the main cause of disease or mortality.…”

“…In previous investigations, virus variants from genogroup IIa were mainly found in koi (Adamek et al., ; Matras et al., ) and induced clinical KSD in these fish (Adamek, Oschilewski, et al., ). Carp from other breeding strains were infected by virus from this variant during cohabitation with koi suffering from clinical KSD, but harboured a low virus load in gill tissue and did not develop clinical KSD (Adamek, Oschilewski, et al., ). This suggests that carp infected with CEV from genogroup IIa might recently have had contact to koi suffering from clinical KSD.…”

Comparison of PCR methods for the detection of genetic variants of carp edema virus

“…In addition, nothing is known about the virulence of particular genetic variants or how likely they might be as a cause of mortality in the infected population. Recent observations in our laboratory during cohabitation experiments suggest differences in virulence between viruses from different genogroups (Adamek et al 2017). This underlines the need for a reliable detection method of CEV in carp populations at risk of infection.…”

“…For all qPCR assays the cycle threshold (C T ) values were read at a threshold of 0.05 of normalised fluorescence change (dRn). Information on the analytical sensitivity of the TiHo probe quantitative PCR and the TiHo SYBRGreen PCR is given in Adamek et al (2016Adamek et al ( , 2017. Currently, all quantitative PCRs (including CEFAS) are routinely run with supercoiled plasmid standards ranging from 10 7 to 10 1 copies of CEV p4a gene (see Fig.…”

Flavobacteria as secondary pathogens in carp suffering from koi sleepy disease

“…In cohabitation experiments, carp from SPF stocks (free of viruses and ectoparasites) were used. The propagation and cultivation of the carp stocks were previously described (Adamek, Oschilewski et al., ). The carp stocks were maintained in a recirculation system at the Fish Disease Research Unit, University of Veterinary Medicine, Hannover, Germany, which was free of CyHV‐3, CEV, spring viremia of carp virus (SVCV) and common carp paramyxovirus (CCPV) (Adamek, Oschilewski et al., ).…”