Bacterial Kidney Disease (BKD)

Physical Signs:

• Swollen Kidney: Enlarged, pale, and granular kidneys upon internal examination 1 2.
• Exophthalmia: Protruding or bulging eyes 1 2.
• Skin Lesions: Ulcers, boils or open sores on the skin 2 3.
• Hemorrhages: Reddened areas (petechiae) on the skin, fins, and internal organs 1 2.
• Abdominal Swelling: Distended abdomen due to fluid accumulation and enlarged organs 1 2.

Behavioral Changes:

• Lethargy: Reduced activity and slow swimming 23.
• Loss of Appetite: Decreased feeding behavior and significant weight loss 23.
• Abnormal Swimming Patterns: Fish may swim erratically or in circles due to distress and weakness 2 3.

Early Stages: Subtle signs such as reduced feed intake and slight lethargy 2 3.

Advanced Stages: Pronounced physical symptoms, including swollen kidneys, soft and hard nodules on the kidneys and other organs, skin lesions, and hemorrhages, leading to high mortality 1 2.

Impact on Fish Health: BKD severely compromises immune function and overall vitality, making fish susceptible to secondary infections and reduced growth rates 1 2.

Causative Agent: Renibacterium salmoninarum bacterium 1 2.

Transmission Methods:

• Direct Contact: Spread between infected & healthy fish through close proximity 1 2.
• Waterborne: The bacterium can spread through water, especially under conditions of poor water quality and high organic load 1 2.
• Vertical Transmission: Infected broodstock can transmit the bacterium to their offspring through eggs 1 2.

Clinical Examination: Prior to pronounced presentation of internal nodular growths on the kidneys and other organs, BKD is difficult to positively identify. Individuals can be behaviorally assessed to some success, but microbiological and histopathological investigation is necessary for a concrete diagnosis 8.

Laboratory Tests:

• Bacterial Culture: Isolation and identification of Renibacterium salmoninarum (R. salmoninarum) from tissue samples, most effectively lesions or soft nodules. In culture, R. salmoninarum is slow-growing (up to 16 weeks at 15° C), but is easily distinguished from other species and must be cultured on specific media 5 7 8.
• PCR (Polymerase Chain Reaction): Detects bacterial DNA in tissue samples, frequently used for screening and thus in conjunction with more sensitive methods 5 7 8.
• Histopathology: Microscopic examination of kidney and other tissues to assess the extent of damage and infection, namely necrosis and granuloma-associated inflammation 7 8.
• Fluorescent Antibody Test (FAT): Used for confirmatory identification, and has with limited success identified R. salmoninarum in asymptomatic individuals 5 7.
• Enzyme-Linked Immunosorbent Assay (ELISA): Quantifies the level of soluble antigen (p57) in kidney tissue 4 7.

Current Treatments: No effective treatment currently exists for BKD. Erythromycin can be used to control R. salmoninarum, but can not reliably cure a population and thus eradicate BKD from dense aquaculture settings 8 9.

Experimental Treatments: A field study of 4 vaccines was conducted on Atlantic salmon (off-label) for efficacy in prevention of BKD-related mortality following a natural outbreak, with limited success; development of various treatments is ongoing, but the live Arthrobacter vaccine Renogen is still today the only somewhat effective prophylaxis 10 11 .

Biosecurity Protocols:

• Implementing strict biosecurity measures is crucial to prevent the introduction and spread of any pathogen, but R. salmoninarum is particularly difficult to eradicate 5 8 12.
• Regular disinfection of equipment and facilities helps to minimize the risk of contamination 6.

Farm Management Practices:

• Reducing stocking densities to decrease the risk of bacterial transmission 4 12.
• Using water treatment systems to track and optimize water quality 4 12.
• Regular health monitoring and early detection programs to identify and manage outbreaks promptly 5.
• Culling of infected broodstock and/or total hatchery depopulation followed by disinfection 5.

Notable Outbreaks:

• Scotland (1933): First reported in wild Atlantic salmon populations in the rivers Spey and Dee 7 13.
• Japan (1970s): BKD is believed to have been introduced with imports of Coho salmon eggs certified as infection-free 5.
• Norway (2022 - 2024): BKD was positively diagnosed in farmed Atlantic salmon and rainbow trout from at least 21 marine grow-out and broodstock sites, as well as a land-based facility. A 2024 survey of wild salmon returned negative results 8.

Response Strategies and Outcomes:

• Scotland: In 1997, when BKD was first diagnosed in a rainbow trout farm, the infected farm and a downstream farm were emptied, cleaned, disinfected, and restocked after a 60-day fallowing period. However, the disease reappeared the following year 5.

Lessons Learned: The importance of early detection, robust biosecurity, and coordinated response efforts in controlling BKD outbreaks.

BKD Incidence in Scotland:

• The prevalence of BKD in trout and salmon farms is low 7.
• Between 1990 and 2002, BKD was not recorded in wild salmonids from Scotland, with the last recorded case in wild fish occurring in the 1960s 4.

Treatment Regimes:

• All fish farms holding salmon or trout are inspected at least once a year for BKD 7.
• Farms confirmed as infected with BKD are controlled by movement restrictions and must enter into an eradication program 7.

• BKD has been identified both on land and at sea 7 8.
• Aquaculture surveillance programs between 2005 - 2021 have previously returned negative results for BKD 8.
• BKD has been detected as recently as 2021 in wild brooding anadromous salmonids during mandatory screening of fish aimed for use in restocking initiatives 8.