Susceptibility of Representative Great Lakes Fish Species to the North Carolina Strain of Spring Viremia of Carp Virus (SVCV)

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) 2016/429): Spring Viraemia of Carp (SVC)

Spring Viraemia of Carp (SVC) was assessed according to the criteria of the Animal Health Law (AHL), in particular the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as in Article 9 and Article 8 for listing animal species related to SVC. The assessment was performed following the ad hoc method for data collection and assessment previously developed by the AHAW panel and already published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound ≥ 66%) or not (upper bound ≤ 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to the assessment performed here, it is uncertain whether SVC can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (45-90% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that SVC does not meet the criteria in Section 1 (Category A; 5-33% probability of meeting the criteria) and it is uncertain whether it meets the criteria in Sections 2, 3, 4 and 5 (Categories B, C, D and E; 33-66%, 10-66%, 45-90% and 45-90% probability of meeting the criteria, respectively). The animal species to be listed for SVC according to Article 8 criteria are provided.

First Isolation of a Herpesvirus (Family Alloherpesviridae) from Great Lakes Lake Sturgeon (Acipenser fulvescens)

The lake sturgeon (Acipenser fulvescens; LST) is the only native sturgeon species in the Great Lakes (GL), but due to multiple factors, their current populations are estimated to be <1% of historical abundances. Little is known about infectious diseases affecting GL-LST in hatchery and wild settings. Therefore, a two-year disease surveillance study was undertaken, resulting in the detection and first in vitro isolation of a herpesvirus from grossly apparent cutaneous lesions in wild adult LST inhabiting two GL watersheds (Erie and Huron). Histological and ultrastructural examination of lesions revealed proliferative epidermitis associated with herpesvirus-like virions. A virus with identical ultrastructural characteristics was recovered from cells inoculated with lesion tissues. Partial DNA polymerase gene sequencing placed the virus within the Family Alloherpesviridae, with high similarity to a lake sturgeon herpesvirus (LSHV) from Wisconsin, USA. Genomic comparisons revealed ~84% Average Nucleotide Identity between the two isolates, leading to the proposed classification of LSHV-1 (Wisconsin) and LSHV-2 (Michigan) for the two viruses. When naïve juvenile LST were immersion-exposed to LSHV-2, severe disease and ~33% mortality occurred, with virus re-isolated from representative skin lesions, fulfilling Rivers’ postulates. Results collectively show LSHV-2 is associated with epithelial changes in wild adult LST, disease and mortality in juvenile LST, and is a potential threat to GL-LST conservation.

Comparative virulence of spring viremia of carp virus (SVCV) genotypes in two koi varieties

Spring viremia of carp virus (SVCV), is a lethal freshwater pathogen of cyprinid fish, and Cyprinus carpio koi is a primary host species. The virus was initially described in the 1960s after outbreaks occurred in Europe, but a global expansion of SVCV has been ongoing since the late 1990s. Genetic typing of SVCV isolates separates them into 4 genotypes that are correlated with geographic origin: Ia (Asia), Ib and Ic (Eastern Europe), and Id (Central Europe). We compared infectivity and virulence of 8 SVCV strains, including 4 uncharacterized Chinese Ia isolates and representatives of genotypes Ia-d in 2 morphologically distinct varieties of koi: long-fin semi-scaled Beni Kikokuryu koi and short-fin fully scaled Sanke koi. Mortality ranged from 4 to 82% in the Beni Kikokuryu koi and 0 to 94% in the Sanke koi following immersion challenge. Genotype Ia isolates of Asian origin had a wide range in virulence (0-94%). Single isolates representing the European genotypes Ib and Ic were moderately virulent (38-56%). Each virus strain produced similar levels of mortality in both koi breeds, with the exception of the SVCV Id strain that appeared to have both moderate and high virulence phenotypes (60% in Beni Kikokuryu koi vs. 87% in Sanke koi). Overall SVCV strain virulence appeared to be a dominant factor in determining disease outcomes, whereas intraspecies variation, based on koi variety, had less of an impact. This study is the first side-by-side comparison of Chinese SVCV isolates and genotype Ia-d strain virulence in a highly susceptible host.

Immersion immunization of common carp with bacterial ghost-based DNA vaccine inducing prophylactic protective immunity against spring viraemia of carp virus

The interactive applications of immunization route, vaccine type and delivery vectors are emerging as a key area of research within the field of mass immunization in fishery production. In an effort to improve DNA vaccine's immune efficiency in large-scale immunization, a promising bacterial ghost-loaded DNA vaccine was constructed based on Escherichia coli DH5α. In common carp was investigated the immune response to immersion immunization via related indicator analysis, and the challenge test of spring viraemia of carp virus (SVCV) was carried out. The result indicated that BG-loaded DNA vaccine induced higher serum antibody level than naked pEG-G. Simultaneously, the immunophysiological indicators and genes change at the more advanced levels in the BG/pEG-G immune group. At the treatment concentration of 20 mg/L of the BG/pEG-G group, IgM and IgZ expressions in vivo were markedly increased by 21.62 times and 6.91 times, respectively, and the relative percentage survival reached the peak of 59.57%. This study paves the way for future aquatic animal vaccine research, which aimed to develop the highly effective immersion vaccine system by delivery vectors, with the ultimate aim to prevent and restrict SVCV in actual production.

Preliminary screening and immunogenicity analysis of antigenic epitopes of spring viremia of carp virus

Glycoprotein (G) is the most common gene used in SVCV vaccine constructions. To identify the major immunogenicity determinant region of SVCV G gene, herein we truncated G gene to 4 parts (G-1, G-2, G-3 and G-4). Bioinformatics and the enzyme linked immunosorbent assay (ELISA) were used to identify the antigenicity of these 4 truncated G proteins. Immunological assays (serum antibody production, enzyme activity, immune genes expression and challenge test) were carried out to further identify the immunogenicity of the screened G protein in common carp. Moreover, to further verify the immune response of the screened G protein-based subunit vaccine, its protective effects on common carp against SVCV infection using single-walled carbon nanotubes (SWCNTs) as a carrier were evaluated. Results showed that G-3 protein could induce higher antibody titer than other truncated G proteins. Furthermore, carps vaccinated with G-3 and G (positive control) showed significant enhancement of immune response (serum antibody production, enzyme activity and immune related genes expression) when compared with control groups. Meanwhile, as a promising vaccine carrier, SWCNTs could significantly enhance the immune effect of naked subunit vaccine (G-3 and G). Notably, after SVCV challenge, there was no significant difference in immune protection between G-3 and G, nor between SWCNTs-G-3 and SWCNTs-G. These results so far suggest G-3 might be the potential antigen epitope of SVCV. This study lays a foundation for developing vaccine and immunodiagnostic techniques.

SVCV infection triggers fish IFN response through RLR signaling pathway

In mammals, virus infection of host cells triggers innate immune response, characterized by induction of interferon (IFN) and downstream IFN-stimulated genes (ISGs). The initiation of IFN antiviral response is dependent on host recognition of virus infection. In fish, similar IFN antiviral response is induced in response to RNA or DNA virus infection; however, the detailed mechanisms underlying recognition of a given virus and activation of downstream signaling remain largely unexplored. Using an infection model with Epithelioma papulosum cyprini (EPC) cells and spring viremia of carp virus (SVCV), a negative sense single-stranded RNA virus, we reported that fish RLR signaling pathway was involved in SVCV-triggered fish IFN response. IFN response was significantly initiated in EPC cells when infected with SVCV, as evidenced by activation of fish IFN promoters, upregulation of IFN and ISGs at mRNA and protein levels. However, function blockade of RIG-I and MDA5, two cytosolic receptors of fish RLR family, significantly attenuated the activation of fish IFN promoters and also the induction of fish IFN and ISGs by SVCV infection. Consistently, SVCV infection-triggered IFN response were blocked in EPC cells when transfected with the dominant negative mutants of pivotal RLR signaling factors, including MAVS, MITA, TBK1, IRF3 and IRF7. These results together shed light on the conservation of RLR-mediated IFN signaling that contributes to fish cells responding to RNA virus infection.

Retail Baitfish in Michigan Harbor Serious Fish Viral Pathogens

Indigenous small cyprinid fish species play an important role in Great Lakes ecosystems and also comprise the backbone of a multimillion-dollar baitfish industry. Due to their widespread use in sport fisheries of the Laurentian Great Lakes, there are increasing concerns that baitfish may introduce or disseminate fish pathogens. In this study, we evaluated whether baitfish purchased from 78 randomly selected retail bait dealers in Michigan harbored fish viruses. Between September 2015 and June 2016, 5,400 baitfish divided into 90 lots of 60 fish were purchased. Fish were tested for the presence of viral hemorrhagic septicemia virus (VHSV), spring viremia of carp virus (SVCV), golden shiner reovirus (GSRV), fathead minnow nidovirus (FHMNV), fathead minnow picornavirus (FHMPV), and white sucker bunyavirus (WSBV). Using the epithelioma papulosum cyprini cell line and molecular confirmation, we demonstrated the presence of viruses in 18 of the 90 fish lots (20.0%) analyzed. The most prevalent virus was FHMNV, being detected in 6 of 30 lots of Fathead Minnow Pimephales promelas and 3 of 42 lots of Emerald Shiners Notropis atherinoides. We also confirmed GSRV in two fish species: the Golden Shiner Notemigonus crysoleucas (5 of 11 lots) and Fathead Minnow (3 of 30 lots). Two VHSV (genotype IVb) isolates were recovered from a single lot of Emerald Shiners. No SVCV, FHMPV, or WSBV was detected in any of the fish examined. Some of the infected fish exhibited clinical signs and histopathological alterations. This study demonstrates that live baitfish are a potential vector for the spread of viral pathogens and underscores the importance of fish health certifications for the Great Lakes baitfish industry.

Analysis of complete genome and pathogenicity studies of the spring viremia of carp virus isolated from common carp (Cyprinus carpio carpio) and largemouth bass (Micropterus salmoides): An indication of SVC disease threat in Korea

A batch of wild common carp and largemouth bass died in Andong, Gyeongsangbuk-do province, South Korea, in 2016. Moribund fish showed typical signs of spring viremia of carp (SVC) disease, which causes acute hemorrhage in the skin and ascites. Thus far, SVC disease has been detected in several regions of the world but never in South Korea. Suspecting the infectious agent to be the SCV virus (SVCV), the moribund fish were sampled and screened. The isolated virus developed a cytopathic effect in EPC cells. Both viral isolates from the common carp (ADC-SVC2016-1) and largemouth bass (ADC-SVC2016-3) were identical in terms of their genome sequence, which were 11,034 bp nucleotides in length. Genome comparison exhibited greater sequence similarity with the Asian SVCV sequences available at NCBI. Phylogenetic analysis revealed that the Korean SVCV isolates were clustered within the Asian clade. More specifically, evolutionary analysis by using the P gene sequences showed that the Korean isolates were sub-cladded within the Iai genogroup but diverged from Chinese strains of SH150514 and SH160901. The Korean isolates shared more than 98% sequence similarity with the two Chinese SVCV isolates, suggesting that the spread of SVCV originated from China. The isolated virus had cytopathic effects on EPC cells. Virus transmission studies showed that the virus exhibited the highest virulence at 15 °C, which was also dependent on the method used, with the injection method being better than the immersion and cohabitation methods. This is the first study to document that Korean SVCV isolates may be epizootic in wild common carp and other susceptible animal populations in South Korea.