Pasteurellosis

Physical Signs:

• Septicemia: Systemic bacterial infection affecting multiple organs. 6.
• Hemorrhage and inflammation: Ulcers in skeletal and cardiac muscle, hemorrhaging in internal organs, and inflammation around the pericardium, abdominal wall, pseudobranchs, and the base of the pectoral fins 6.
• Exophthalmia:  Severe inflammation of the eye and eye socket in some cases 6.
• Histopathology: Abundant inflammatory cells, tissue necrosis, and short rod-shaped bacteria in affected organs 6.

Behavioral Changes:

• Lethargy: Reduced activity and sluggish swimming 5.
• Loss of Appetite: Decreased feeding behavior 5.
• Abnormal Swimming Patterns: Fish may swim sluggishly and in random patterns 5.

Early Stages: Subtle signs such as reduced feed intake and slight lethargy.

Advanced Stages: While the disease can become clinically obvious late in the production cycle, outbreaks have also occurred in newly transferred salmon, and disease progression may lead to severe pathology and/or early harvest decisions because of poor welfare 6, 7.

Impact on Fish Health: Pasteurellosis severely compromises overall health and often leads to significant mortality 5, 8. Research has shown that mortalities resulting directly from outbreaks ranges from low to moderate 7, 9.

• Septicemia: Systemic bacterial infection affecting multiple organs 3, 8, 11.
• Hemorrhages: Reddened areas around the jaw and base of fins 3, 8, 11.
• Histopathology: Systemic infection with bacterial aggregates observed in tissue sections of internal organs 3, 6.

Acute: Skin lesions, haemorrhage at fin bases 3, 11.

Chronic: Characteristic white spots on skin/around eyes, frayed fins, redness around mouth/fin bases 3, 11.

Impact on Fish Health: Up to 100% mortality observed in infected lumpfish, with survivors potentially acting as asymptomatic carriers of the bacteria that can lead to re-emergence of disease if triggered by a stress event 3. Compared to salmon, lumpfish seem highly susceptible and may develop marked pathology even when salmon exposed to similar isolates show few or no clinical signs 8.

Causative Agent: Bacteria in the genus Phocoenobacter, namely the species skyensis, and atlanticus which includes the subspecies atlanticus and cyclopteri 1, 3.

Transmission Methods:

• Horizontal transmission: While exact transmission routes and survival strategies in seawater are still unknown for the bacteria, outbreaks recorded in nearby locations within the same time period can indicate horizontal transmission occurs between farms 10. Under laboratory conditions, transmission of disease from lumpfish to Atlantic salmon in a cohabitation environment was unsuccessful, however lumpfish are highly susceptible to disease caused by both subspecies 8.
• Pasteurellosis occurs at all lumpfish life stages, and the bacteria have been detected in eggs and milt, suggesting possible vertical transmission 3.

Clinical Examination: Observation of physical and behavioral symptoms 5, 6.

Laboratory Tests:

• Bacterial Culture: Isolation and identification of Phocoenobacter atlanticus on blood agar with 2% NaCl. Grows as small grey colonies within 2-4 days, with slight alpha hemolysis 6.
• MALDI-TOF MS: Mass spectrometry for rapid identification 6.
• Whole Genome Sequencing: For precise identification and discrimination of Phocoenobacter atlanticus isolates 1, 7.

Biosecurity Protocols:

• Implementing strict biosecurity measures on farms and well boats to prevent the introduction and spread of disease.
• Proper cleaning and disinfection of transport vessels and well boats between treatments and localities 16.
• Regular monitoring through the use of eDNA [12](Strand et al., 2026, and rapid diagnosis of suspected cases 5.

Farm Management Practices:

• Careful consideration of cleaner fish use and potential disease transmission 8.
• The association with thermal/mechanical delousing suggests that shifting toward freshwater delousing could reduce salmon pasteurellosis risk 9.
• Pasteurellosis has been recorded soon after transfer of lumpfish to salmon cages and following stress events, emphasizing stress minimization 3.

Notable Outbreaks:

• Norway (2012): First documentation of pasteurellosis in lumpfish, caused by Ph. atlanticus subsp. cyclopteri 2.
• Norway (2018): First documentation of concerning number of outbreaks of pasteurellosis in Atlantic salmon caused by Ph. atlanticus subsp. atlanticus
• Norway (2020-2022): Approximately 50 diagnosed cases annually in south-western Norway 1. Also detected as far north as PO10 in 2024 and PO7 in 2025 10, 14.
• Scotland: Outbreaks typically associated with Phocoenobacter skyensis serotype O2 5.

Response Strategies and Outcomes:

• Norway: Increased focus on rapid diagnosis and identification of Ph. atlanticus isolates 1, 5.
• Norway: The Food Safety Authority is considering listing pasteurellosis in Atlantic salmon a notifiable disease 17.
• Scotland: Development of serotyping methods for Ph. skyensis 5.

Lessons Learned: The importance of routine bacteriology, rapid diagnosis, stress reduction, and proper biosecurity measures in controlling pasteurellosis outbreaks 1, 5, 16.

Pasteurellosis Incidence in Norway:

• Pasteurellosis was first diagnosed in Atlantic salmon in Norway in 1989 18.
• It has been a rapidly increasing problem in Atlantic salmon in Western Norway since 2018
• Approximately 50 diagnosed cases were registered annually in farms in south-western Norway between 2020 and 2022 1.

Geographical Spread:

• Recent outbreaks have been primarily associated with Western Norway, with new outbreaks being recorded further north (PO10 and PO7) in 2024 and 2025 1, 10, 14.

Economic Impact:

• While specific economic figures are not available, the increasing frequency of outbreaks suggests a growing economic concern for the salmon farming industry in Norway 1.

Treatment & Management:

• There is currently no commercially available vaccine against pasteurellosis in lumpsfish or Atlantic salmon, however autogenous vaccines are available for Atlantic salmon 10, 14.
• Research is ongoing to establish factors leading to disease outbreaks and how to best experimentally challenge fish with this emerging pathogen, as well as research into vaccine development 8, 13, 19.

Pasteurellosis incidence in Scotland:

• Ph. skyensis was first discovered in Scotland at three separate sites over a four-year period 4.
• Recent outbreaks in Scotland have been associated with serotype O2 of Ph. skyensis 20.

Geographical spread:

• Published literature lists outbreaks occurring in localities on the Isle of Skye and in the Western Highlands and Outer Hebrides of Scotland 20.

Economic impact:

• While specific economic impact data for Scotland is available, one case study mentions 500 tonnes of Atlantic salmon with an average weight of 4kg were lost during a single outbreak at a seawater site on the Isle of Lewis 20.

Treatment & management:

• Similar to Norway, there is no mention of a commercially available vaccine against pasteurellosis in Scotland.