First isolation of a rhabdovirus from perch Perca fluviatilis in Switzerland

Perca fluviatilis is a fish species of increasing interest to the Swiss fish farming industry. In recent years, recirculation systems have been specifically set up to increase production. In one of these farms, abnormal spiral swimming associated with elevated mortalities occurred in repeated batches of imported perch shortly after stocking on several occasions. No bacterial or parasitic etiology was detected, but a virus grown in bluegill fry (BF-2) cells was identified as perch rhabdovirus. Subsequent investigations of other samples suggested a viral tropism for the central nervous system (CNS). Phylogenetic analysis of the partial N and entire G gene sequences positioned this isolate in genogroup C of the species Perch rhabdovirus, with high nucleotide and amino acid (aa) sequence identities with the DK5533 strain isolated in Denmark in 1989. Comparative studies using other closely related isolates allowed the distinction of 2 serological Patterns among perch rhabdoviruses and the identification of a proline substitution by a serine in Position 147 of the glycoprotein potentially involved in antigenic differentiation. Even if perch imported onto the farm tested negative by virus isolation prior to transport, they may have been the origin of this outbreak since CNS tissue was not included in the samples that were analyzed. Another possibility might be a sub-clinical infection with a viral load in resident fish too low to be detected. This study reports the first isolation of a perch rhabdovirus in Switzerland, and emphasizes the necessity of optimizing diagnostic tools that facilitate better control of the risks associated with fish translocation

Validation of a model for ranking aquaculture facilities for risk-based disease surveillance.

A semi-quantitative model for risk ranking of aquaculture facilities in Switzerland with regard to the introduction and spread of Viral Haemorrhagic Septicaemia (VHS) and Infectious Haematopoietic Necrosis (IHN) was developed in a previous study (Diserens et al., 2013). The objective of the present study was to validate this model using data collected during field visits on aquaculture sites in four Swiss cantons compared to data collected through a questionnaire in the previous study. A discrepancy between the values obtained with the two different methods was found in 32.8% of the parameters, resulting in a significant difference (p<0.001) in the risk classification of the facilities. As data gathered exclusively by means of a questionnaire are not of sufficient quality to perform a risk-based surveillance of aquaculture facilities a combination of questionnaires and farm inspections is proposed. A web-based reporting system could be advantageous for the factors which were identified as being more likely to vary over time, in particular for factors considering fish movements, which showed a marginally significant difference in their risk scores (p≥0.1) within a six- month period. Nevertheless, the model proved to be stable over the considered period of time as no substantial fluctuations in the risk categorisation were observed (Kappa agreement of 0.77).Finally, the model proved to be suitable to deliver a reliable risk ranking of Swiss aquaculture facilities according to their risk of getting infected with or spreading of VHS and IHN, as the five facilities that tested positive for these diseases in the last ten years were ranked as medium or high risk. Moreover, because the seven fish farms that were infected with Infectious Pancreatic Necrosis (IPN) during the same period also belonged to the risk categories medium and high, the classification appeared to correlate with the occurrence of this third viral fish disease

Systemic infection in European perch with thermoadapted virulent Aeromonas salmonicida (Perca fluviatilis).

In non-salmonid fish, Aeromonas salmonicidacan cause local infections with severe skin ulcerations, known as atypical furunculosis. In this study, we present a systemic infection by a virulent A. salmonicidain European perch (Perca fluviatilis).This infection was diagnosed in a Swiss warm water recirculation aquaculture system. The isolate of A.  salmonicida encodes a type three secretion system (TTSS) most likely located on a plasmid similar to pAsa5/pASvirA, which is known to specify one of the main virulence attributes of the species A. salmonicida. However, the genes specifying the TTSS of the perch isolate show a higher temperature tolerance than strains isolated from cold-water fish. The function of the TTSS in virulence was verified in a cytotoxicity test using bluegill fry and epithelioma papulosum cyprinid cells

Antigens of the type-three secretion system of Aeromonas salmonicida subsp. salmonicida prevent protective immunity in rainbow trout

Aeromonas salmonicida subsp. salmonicida is the etiologic agent of furunculosis, a frequent and significant disease of fisheries worldwide. The disease is largely controlled by commercial oil adjuvanted vaccines containing bacterins. However, the mechanisms leading to a protective immune response remain poorly understood. The type-three secretion system (T3SS) plays a central role in virulence of A. salmonicida subsp. salmonicida and thus may have an influence on the immune response of the host. The aim of this study was to evaluate the role of the T3SS antigens in mounting a protective immune response against furunculosis. Rainbow trout were intraperitoneally vaccinated in two independent experiments with bacterins prepared from a wild-type A. salmonicida strain and an isogenic strain carrying a deletion in the T3SS (ΔascV). Fish were challenged with the wt strain eight weeks after vaccination. In both trials, the survival rate of trout vaccinated with the ΔascV strain was significantly higher (23-28%) in comparison to the group vaccinated with the wt strain. High-throughput proteomics analysis of whole bacteria showed the ascV deletion in the mutant strain resulted in lower expression of all the components of the T3SS, several of which have a potential immunosuppressive activity. In a third experiment, fish were vaccinated with recombinant AcrV (homologous to the protective antigen LcrV of Yersinia) or S-layer protein VapA (control). AcrV vaccinated fish were not protected against a challenge while fish vaccinated with VapA were partially protected. The presence of T3SS proteins in the vaccine preparations decreased the level of protection against A. salmonicida infection and that AcrV was not a protective antigen. These results challenge the hypothesis that mounting specific antibodies against T3SS proteins should bring better protection to fish and demonstrate that further investigations are needed to better understand the mechanisms underlying effective immune responses against A. salmonicida infection