Yersiniosis

Physical Signs:

• Hemorrhages: Reddened areas on the skin, fins, gills, and internal organs.
• Exophthalmia: Protruding or bulging eyes.
• Darkened Skin: Affected fish may exhibit a darker coloration.
• Abdominal Swelling: Distended abdomen due to fluid accumulation.
• Ulcers: Open sores or lesions on the skin.
• Hemorrhagic Congestion: At the base of the pectoral and pelvic fins 4.
• Distended Vent: Especially in smaller fish 4.
• Pallor to Gills: Due to bacterial-induced anemia in small fish 4.

Behavioral Changes:

• Lethargy: Reduced activity and slow swimming.
• Loss of Appetite: Decreased feeding behavior and significant weight loss.
• Abnormal Swimming Patterns: Fish may swim erratically or in circles due to loss of equilibrium.
• Swimming Near the Surface: Indicative of respiratory distress 1.

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

Advanced Stages: More pronounced physical symptoms such as hemorrhages, exophthalmia, and ulcers, leading to high mortality if left unchecked.

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

• Direct Contact: Spread between infected and healthy fish through close proximity.
• 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.