Yersiniosis

Physical Signs:
**Rainbow Trout — Classic/Acute Form - Enteric Red Mouth (ERM)
Most commonly associated with the "Hagerman" strain (Serotype O1, Biotype 1)

• Red/bleeding mouth — bleeding under the skin around the mouth, gums, and throat ScienceDirect
• Pop-eye (exophthalmia) — bulging eyes, often with blood spots inside the eye Carson, 2008
• Dark skin — the fish becomes noticeably darker in colour
• Bleeding fins — reddened, congested fins Fish Site, 2018
• Pale gills and swollen vent (anus)
• Internal bleeding — small haemorrhages on the liver, pancreas, swim bladder, and muscles ScienceDirect
• Enlarged, darkened spleen
• Inflamed intestine — reddened lower gut with a build-up of thick yellow fluid PubMed
• Swollen abdomen — the belly may swell and fill with colourless watery fluid; the intestines may turn yellow ResearchGate
• Watery blood — the blood may become watery due to a significant drop in red blood cell numbers ResearchGate

Atlantic Salmon and Chinook Salmon — Milder/Atypical Form
Associated with Serotype O1b / Norwegian virulent genetic variant

• The classic red mouth sign is often absent or subtle
• The most commonly detected sign is pop-eye and blood spots in the eye
• Most outbreaks in salmon occur during the juvenile (freshwater) phase or shortly after transfer to the sea Carson, 2008

Behavioral Changes:

• Changes in farmed salmon are often non-specific and typical of a general blood infection, including lethargy, difficulty breathing, and abnormal swimming behaviour Vetinst, 2025
• Lethargy: Reduced activity and slow swimming.
• Loss of Appetite: Decreased feeding behavior.
• Abnormal Swimming Patterns: Fish may swim erratically or in circles due to loss of equilibrium.
• Swimming Near the Surface: fish may swim near the surface or at the edges of the tank and stop eating 1.

Early Stages: Subtle signs such as reduced feed intake and slight lethargy, swimming near the surface, darkening of the skin and increase in mortality Fish Site, 2018

Advanced Stages: More pronounced physical symptoms such as hemorrhages, exophthalmia. Mortality starts low for ERM but then can increase rapidly Wrobel, 2019

Impact on Fish Health: Yersiniosis severely compromises immune function and overall vitality, making fish susceptible to secondary infections.

• Direct Contact: The main transmission, spreading between infected and healthy fish through close proximity Fish Site, 2018
• Waterborne: The bacterium can spread through water, especially under conditions of poor water quality and high organic load 134.

• Bacterial Culture: Isolation and identification of Yersinia ruckeri from tissue samples.
• PCR (Polymerase Chain Reaction): Detects bacterial DNA in tissue samples.
• Histopathology: Microscopic examination of tissue samples to assess the extent of damage and infection 14.

• Antibiotics: Administered through medicated feed or water to treat bacterial infections.
• Supportive Care: Improving water quality and reducing stress to support recovery.

• Implementing strict biosecurity measures to prevent the introduction and spread of Yersinia ruckeri.
• Regular monitoring of water quality and fish health to detect and address early signs of infection.

• Reducing stocking densities to decrease the risk of bacterial transmission.
• Using water treatment systems to maintain optimal water quality.
• Implementing fallowing periods and site rotations to break the infection cycle 14.

• United States (2007-2009): A series of outbreaks of Yersinia ruckeri occurred in farmed rainbow trout in the United States, particularly in the Midwest and Idaho regions, during 2007-2009. These outbreaks were associated with high mortality rates and were linked to the emergence of more virulent strains of Yersinia ruckeri. The outbreaks led to significant economic losses and prompted research into the development of more effective vaccines. 2.
• Norway (2019 - 2020): An outbreak of Yersinia ruckeri was reported in Norwegian salmon farms during 2019-2020, particularly affecting post-smolt salmon. This outbreak was significant due to its scale and the challenges it posed for the industry in controlling the spread of the disease. The outbreak led to increased mortality rates and highlighted the need for improved biosecurity measures and vaccination strategies in the region. 10.

Yersiniosis Incidence in Norway:

• The number of detections of Yersinia ruckeri, which causes yersiniosis, continued to increase in 2023 7.
• A high and increasing number of requested doses of injection vaccine against yersiniosis indicates significant problems with the disease 7.

Geographical Spread:

• Yersinia ruckeri clonal complex 1 (CC1) was found responsible for all major yersiniosis outbreaks diagnosed in Norwegian salmon farming since the late 1980s 6.
• Putatively avirulent Y. ruckeri strains were confirmed to be widespread in freshwater salmon hatcheries 6.

Economic Impact:

• Specific economic impact figures are not provided, but yersiniosis is mentioned as one of the significant health challenges in Norwegian salmon farming 7.

Treatment & Management:

• Intraperitoneal (ip) vaccination has been widely adopted, helping to control the disease 6.
• There is an increasing use of vaccination against yersiniosis, as indicated by vaccine statistics 79.
• Stressful management procedures may result in increased shedding of Y. ruckeri by sub-clinically infected fish.
• Thermal delousing procedures have been found to be effective for detection of Y. ruckeri in sub-clinically infected populations 6 10.

• In Australia, three forms of Y. ruckeri are known to occur: serotype O1b biotype 1, O1b biotype 2 and serotype O2.
• In New Zealand only serotype O1b is known to occur.