Susceptibility and Pathology in Juvenile Atlantic Cod Gadus morhua to a Marine Viral Haemorrhagic Septicaemia Virus Isolated from Diseased Rainbow Trout Oncorhynchus mykiss

The first known outbreak caused by a viral haemorrhagic septicaemia virus (VHSV) strain of genotype III in rainbow trout occurred in 2007 at a marine farm in Storfjorden, Norway. The source of the virus is unknown, and cod and other marine fish around the farms are suspected as a possible reservoir. The main objective of this study was to test the susceptibility of juvenile Atlantic cod to the VHSV isolate from Storfjorden. As the pathology of VHS in cod is sparsely described, an additional aim of the study was to give a histopathological description of the disease. Two separate challenge experiments were carried out, using both intra peritoneal (ip) injection and cohabitation as challenge methods. Mortality in the ip injection experiment leveled at approximately 50% three weeks post challenge. Both immunohistochemical and rRT-PCR analysis of organs sampled from diseased and surviving fish confirmed VHSV infection. No VHSV was detected in the cohabitants. The results indicate that Atlantic cod has a low natural susceptibility to this VHSV genotype III strain. One of the most extensive pathological changes was degeneration of cardiac myocytes. Immunohistochemistry confirmed that the lesions were related to VHSV. In some fish, the hematopoietic tissue of spleen and kidney showed degeneration and immunostaining, classical signs of VHS, as described in rainbow trout. Positive immunostaining of the capillaries of the gills, suggests this organ as a useful alternative when screening for VHSV.

Re-Emergence and Spread of Haemorrhagic Septicaemia in Germany: The Wolf as a Vector?

Since 2010, outbreaks of haemorrhagic septicaemia (HS) caused by Pasteurella (P.) multocida capsular type B (PmB) emerged in Germany. In 2017, we noticed a close spatiotemporal relationship between HS outbreak sites and wolf (Canis lupus) territories. Thus, the main objectives of our study were to investigate the molecular epidemiology of German PmB-HS-isolates and to assess the role of wolves as putative vectors of this pathogen. We collected 83 PmB isolates from HS outbreaks that occurred between 2010 and 2019 and sampled 150 wolves, which were found dead in the years 2017 to 2019, revealing another three PmB isolates. A maximum-likelihood-based phylogeny of the core genomes of 65 PmB-HS-isolates and the three PmB-wolf-isolates showed high relatedness. Furthermore, all belonged to capsular:LPS:MLST genotype B:L2:ST122RIRDC and showed highly similar virulence gene profiles, but clustered separately from 35 global ST122RIRDC strains. Our data revealed that German HS outbreaks were caused by a distinct genomic lineage of PmB-ST122 strains, hinting towards an independent, ongoing epidemiologic event. We demonstrated for the first time, that carnivores, i.e., wolves, might harbour PmB as a part of their oropharyngeal microbiota. Furthermore, the results of our study imply that wolves can carry the pathogen over long distances, indicating a major role of that animal species in the ongoing epidemiological event of HS in Germany.

Evidence of Transcriptional Shutoff by Pathogenic Viral Haemorrhagic Septicaemia Virus in Rainbow Trout

The basis of pathogenicity of viral haemorrhagic septicaemia virus (VHSV) was analysed in the transcriptome of a rainbow trout cell line inoculated with pathogenic and non-pathogenic VHSV isolates. Although both VHSV isolates showed similar viral replication patterns, the number of differentially expressed genes was 42-fold higher in cells inoculated with the non-pathogenic VHSV at 3 h post inoculation (hpi). Infection with the non-pathogenic isolate resulted in Gene Ontologies (GO) enrichment of terms such as immune response, cytokine-mediated signalling pathway, regulation of translational initiation, unfolded protein binding, and protein folding, and induced an over-representation of the p53, PPAR, and TGF-β signalling pathways. Inoculation with the pathogenic isolate resulted in the GO enrichment of terms related to lipid metabolism and the salmonella infection KEGG pathway involved in the rearrangement of the cytoskeleton. Antiviral response was evident at 12hpi in cells infected with the pathogenic isolate. Overall, the data showed a delay in the response of genes involved in immune responses and viral sensing in cells inoculated with the pathogenic isolate and suggest transcriptional shutoff and immune avoidance as a critical mechanism of pathogenicity in VHSV. These pathways offer opportunities to further understand and manage VHSV pathogenicity in rainbow trout.