Viral haemorrhagic septicaemia (VHS) is a serious disease in several fish species. VHS is caused by the rhabdovirus viral haemorrhagic septicaemia virus (VHSV). To prevent spreading of the pathogen, it is important to use a fast, robust, sensitive and specific diagnostic tool to identify the infected fish. Traditional diagnosis based on isolation in cell culture followed by identification using, for example, ELISA is sensitive and specific but slow. By switching to RT-PCR for surveillance and diagnosis of VHS the time needed before a correct diagnosis can be given will be considerably shortened and the need for maintaining expensive cell culture facilities reduced. Here we present the validation, according to OIE guidelines, of a sensitive and specific Taqman-based real-time RT-PCR. The assay detects all isolates in a panel of 79 VHSV isolates covering all known genotypes and subtypes, with amplification efficiencies of approximately 100%. The analytical and diagnostic specificity of the real-time RT-PCR is close to 1, and the analytical and diagnostic sensitivity is comparable with traditional cell-based methods. In conclusion, the presented real-time RT-PCR assay has the necessary qualities to be used as a VHSV surveillance tool on par with cell culture assays.

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ABSTRACT. It is generally recommended that a high level of asepsis be maintained during surgical implantation ofelectronic tags into fish. However, documentation of a positive effect of asepsis in fish surgery is lacking. Tocompare the effects of surgical implantation performed under different sanitary conditions, 100 hatcherysalmon smolts (Salmo salar) were surgically implanted with tags with and without trailing antenna andwere kept in a hatchery facility. After 34 days, the surviving smolts were euthanized and survival, growthand healing were compared between fish tagged under aseptic conditions and fish tagged without regardto aseptic technique. The results demonstrated that there was no detectable difference in survival, growthand healing between the treatments. Thus, this study could not provide evidence supporting the generalrecommendation of achieving a high level of asepsis during fish surgery.

In two separate trials pigs were experimentally infected with Lawsonia intracellularis at 5-6 weeks of age followed by antibiotic treatment and resolution of the primary infection and then re-inoculated at 12-13 weeks of age. A treatment-control group of pigs received the primary infection and antibiotic treatment only, and served as control for the antibiotic treatment of the primary infection. A challenge-control group of pigs received the second inoculation dose only at 12-13 weeks of age to control infectivity of the challenge-dose and susceptibility of pigs to L. intracellularis at this age. Pigs were monitored for shedding of L. intracellularis in faeces by PCR, and for the development of antibodies and responses of acute phase proteins in serum. The presence of L. intracellularis antigen in the intestinal mucosa was examined in post mortem samples by immunohistochemistry. In both trials primary infected pigs were protected from infection after challenge inoculation as evidenced by absence of faecal shedding of L. intracellularis, lack of changes in acute phase protein concentrations after challenge and with low levels of bacterial antigen in the intestinal mucosa of re-inoculated pigs comparable to that of the treatment-control pigs. In contrast, challenge-control pigs shed L. intracellularis in faeces, had L. intracellularis antigen extensively present within all layers of the intestinal mucosa and developed a significant acute phase protein response in serum after the experimental infection. The acute phase protein response to L. intracellularis infection was detected as an increased rise in the serum concentrations of C-reactive protein and haptoglobin from day-6 post infection, and increased serum concentrations of haptoglobin were generally seen 2-3 weeks after inoculation both at 5-6 and 12-13 weeks of age. In conclusion substantial protection against L. intracellularis infection was found in the re-inoculated pigs in contrast to the development of infection in age-matched control pigs. The acute phase protein responses reflected both the observed protection against L. intracellularis infection upon secondary challenge and that increased resistance to the infection develops with age.

Ichthyotoxic algal blooms are normally considered a threat to maricultured fish only when blooms reach lethal cell concentrations. The degree to which sublethal algal concentrations challenge the health of the fish during blooms is practically unknown. In this study, we analysed whether sublethal concentrations of the ichthyotoxic alga Prymnesium parvum affect the susceptibility of rainbow trout Oncorhynchus mykiss to viral haemorrhagic septicaemia virus (VHSV). During exposure to sublethal algal concentrations, the fish increased production of mucus on their gills. When fish were exposed to the algae for 12 h prior to the addition of virus, a marginal decrease in the susceptibility to VHSV was observed compared to fish exposed to VHSV without algae. If virus and algae were added simultaneously, inclusion of the algae increased mortality by 50% compared to fish exposed to virus only, depending on the experimental setup. We concluded that depending on the local exposure conditions, sublethal concentrations of P. parvum could affect susceptibility of fish to infectious agents such as VHSV.