Pasteurellose hos norsk laks

The current project was initiated due to an increasing prevalence of pasteurellosis, caused by a previously undescribed Pasteurella species, among farmed salmon in production areas 2–5, after many years of absence of the disease. The overarching goal of the project was to generate knowledge about the current situation so that the disease could be effectively prevented, controlled, and managed in the best possible way. Results generated during the project, which were quickly disseminated to the industry via the project's website, podcast, webinar, workshops, and relevant industry channels, have undoubtedly contributed to increased focus on general biosecurity regarding pasteurellosis, particularly related to fish transport and delousing treatments.

The main conclusion from the project must be that the results collectively revealed a convincing link between non-medical delousing (IMM) and clinical outbreaks of pasteurellosis.

Results supporting this conclusion are several:
1. Pasteurella atlantica, the bacterium involved in the current epizootic, was shown in controlled infection trials to be poorly transmissable from fish to fish but demonstrated the ability to kill fish after injection, i.e., the bacterium is pathogenic once it bypasses the skin barrier, the salmon's first-line of defence;
2. Water samples taken during IMM treatment contained visible amounts of salmon blood, confirming that the salmon are injured during treatment and that the lesions represent a breach in the skin barrier, potentially allowing pathogenic bacteria to enter;
3. Infected fish, both visibly sick and apparently healthy carriers, shed Pasteurella bacteria under stressful IMM treatment. At least in thermal delousers, where large amounts of fish are treated in a limited volume of heated water, this results in increased infection pressure for both already infected and non-infected fish; and finally,
4. A statistically significant association was found between both thermal and brush/vortex-based delousing and subsequent outbreaks of pasteurellosis in the month following delousing.

Interestingly, no such association was found after freshwater-based delousing, and laboratory studies showed that P. atlantica, which is fastidious in terms of nutrient content in growth media and has a limited ability to survive in natural seawater, was completely inactivated after 1 hour of exposure to pure freshwater. This suggests that the use of freshwater during stressful handling, such as transport, pumping, or delousing of at risk fish populations, should be further investigated.

P. atlantica was found to be sensitive to UV radiation, with a >3 log10 inactivation at UV-doses of 10 mJ/cm². Exposure to formic acid at pH 3.0 inactivated the bacteria completely within 4 hours. Sodium hypochlorite at concentrations from 50–200 mg/l were found to inactivate Pasteurella within both 10 and 30 minutes exposure at 4 °C.

Whole-genome sequencing (complete sequencing of the bacterial DNA) of both new and historical isolates, followed by bioinformatics analysis, showed that the current epidemic in PO2–5 is caused by a single clonal complex (i.e., nearly identical isolates) of P. atlantica, indicating that the epizootic began as a single point outbreak with subsequent spread to other locations. The same technique was used to map the relationship of isolates from a new pasteurellosis outbreak on the Helgeland coast (PO10) in late 2024. This analysis shows that the strains involved belong to the same clonal complex as in PO2–5 and are not closely related to isolates from previous outbreaks in Northern Norway. This is a strong signal/indication of a serious biosecurity failure involving transmission from south to north via boat, fish, or other human-related cause.

Environmental DNA analysis, which used both existing PCRs and PCRs developed during the project, performed on a relatively broad selection of farms, showed that such analyses represent an effective way to detect Pasteurella bacteria in infected fish populations before a potential clinical outbreak, thus identifying a risk of future outbreak. The results also indicated that Pasteurella infection is somewhat more widespread than disease diagnostic data suggests and that the infection does not necessarily result in outbreak of disease. This suggests that pasteurellosis outbreaks can be avoided following early environmental DNA-based detection, if less stressful production conditions are introduced thereafter.

Other findings indicated that genetic and phenotypic studies of pathogenic Pasteurella bacteria isolated from sick salmon in Scotland and Norway from current and historical outbreaks showed such close genetic relatedness to a bacterium type previously associated exclusively with whales, dolphins, and porpoises, that the project group has proposed officially moving these fish pathogenic strains from the genus Pasteurella to the genus Phocoenobacter. It was not possible to detect these fish pathogenic bacteria types from the few stranded marine mammals the project has investigated, so a reservoir in marine mammals cannot be confirmed, but the relatively high cultivation temperature optimum of the fish pathogenic isolates and detection of a closely related type in pilot whales on the Faroe Islands supports the hypothesis that cetaceans may be a likely reservoir.