Nephrocalcinosis

Physical Signs:

• No obvious external signs: Severe cases may be visible on dissection, but most cases are mild and can only be detected through histology 7, 6
  • Kidney Lesions: pronounced kidney lesions with a marbling and whitish mottling appearance, which are largely calcium deposits: confirmed histologically by von Kossa staining 4.
  • Calcified deposits may appear as white longitudinal stripes in the kidney, or as mineralised material in the pseudobranch (the fish's reduced first gill arch) and stomach wall 8.
• Renal casts: Plugs of material visible, with the naked eye, within the tubles or ducts 4.

Behavioral Changes:

• Lethargy: Reduced activity and lethargy 4.
• Loss of Appetite: Decreased feeding behavior and slower growth 6
• Abnormal Swimming Patterns: Fish may exhibit erratic or sluggish swimming due to discomfort 4.

Contributing Factors: 

• Elevated CO₂ (Primary Risk Factor): Monitor CO₂ continuously. Keep levels below 13 mg/L where possible, and investigate immediately if levels exceed 20 mg/L 7, 5. Risk can increase during cold water temperature periods potentially due to decrease CO₂ stripping efficiency 9. Links have also been found with use of continuous light, but that could be elevated CO₂ due to more activity and feeding than use of winter light signal 11

• Hyperoxia — Elevated Dissolved Oxygen: A combination of hypercapnia and hyperoxia drives hypoventilation in fish, leading to respiratory acidosis and the internal acid-base disturbance that triggers mineral precipitation. Avoid sustained oxygen supersaturation, particularly when CO₂ levels are also elevated. The combined effect is greater than either factor alone 4, 10

• Premature seawater exposure: before smoltification can cause osmotic stress and result in unbalanced levels of ions in fish blood, increasing the risk for nephrocalcinosis. Align salinity introduction with photoperiod signals and verified smolt status 9.

• RAS (Recirculating Aquaculture Systems): carries a higher inherent risk than flow through systems, as CO₂ and oxygen management is more complex 6

• Increased salt loads through the feed and water may increase prevalence 10

• Diet-induced nephrocalcinosis has only been found for aquarium reared spotted wolffish (Anarhichas minor) 12. However ongoing research is looking at dietray factors, amongst others 13

• Histopathology: Microscopic examination of kidney tissues to identify calcium deposits and assess the extent of damage, the most precise method for assessing tissue damage and progression 6b

• X-ray / Radiography: Non invasive imaging of calcium deposits in the kidneys 5, 6b. It is cost-effective, does not require euthanasia, and enables monitoring of disease progression in individual fish 6b However, radiography has limited ability to assess soft tissue lesions and cannot reliably distinguish between pathologies such as nephrocalcinosis and chronic granulomatous inflammation with secondary calcification 6b.

• Ultrasound: Uses sound waves to detect different stages of nephrocalcinosis, potential to be used at same time as fish vaccination 15, 16

• Blood analysis: Key indicators include hypercalcaemia, elevated plasma magnesium, glucose, and AST, alongside raised plasma pH and reduced plasma chloride levels 6 7. Note that the use of anaesthesia during blood sampling can significantly influence plasma values and should be accounted for when planning and interpreting results.7.

• Urocystolithiasis, which is the formation of stones of plugs in the urinary bladder. This can occur from nephrocalcinosis but also from other reasons. In severe cases it can lead to complete blockage of the urinary tract 14.
• Hemorrhagic Smolt Syndrome (HSS)/Hemorrhagic Diathesis(HD): can often occur simutaneously, though looking at time series, nephrocalcinosis was mainly found earlier 7
• BKD: Kidney damage caused by nephrocalcinosis can, in some cases, resemble visible findings associated with the notifiable (in Norway) disease bacterial kidney disease (BKD) 8.
• Chronic granulomatous inflammation with secondary calcification (= an enclosing of a problem the fish can’t remove, which then becomes mineralized with calcium) can appear the same as nephrocalcinosis when using x-ray radiography 6b

Current Treatments:

• Currently no licensed pharmaceutical treatment or medical intervention that removes or dissolves existing mineral deposits in fish kidneys.

Supportive Care:

• Water Quality Management: Reducing and correct CO₂ and O₂ water levels 9.
• Changing the Source of Alkalinity in RAS: Shown for Nile Tilapia (Oreochromis niloticus) that changing from calcium carbonate (CaCO₃) to sodium bicarbonate (NaHCO₃), prevalence of nephrocalcinosis was reduced 2

Biosecurity Protocols:

• It is not an infectious disease.

Farm Management Practices:

• Regular samples for histopathology, or x-ray radiography to detect early signs, as it can heal in milder cases. Particularly in the 6–8 weeks before planned sea transfer, as it can also indicate post-transfer survival 8, FHF, 2022.
• Gross examination of kidneys and urinary tract for white mineral build up, on dead fish 8
• Strict water quality management practices to maintain optimal conditions, treating CO₂ stripping and oxygen dosing as critical control points 9.
• Water Quality Testing: Regular assessment of water parameters such as temperature, CO₂, O₂, pH, salinity and photoperiod 9.

• Norway (>2020): Nephrocalcinosis has been increasingly observed in Norwegian salmon farming, particularly in recirculating aquaculture systems (RAS). Research conducted by the Norwegian Veterinary Institute identified a significant rise in cases of nephrocalcinosis, prompting the industry to implement stricter monitoring and management practices to control CO₂ levels and improve water quality 8

Nephrocalcinosis Incidence in Norway:

• The Norwegian Veterinary Institute 2024 fish health report stated nephrocalcinosis as a common and significant diagnosis affecting health for especially trout and to a lesser extent salmon 8

Geographical Spread:

• In ongrowing facilities, nephrocalcinosis is typically detected within the first few months after seawater transfer, likely representing pre-existing damage from the hatchery stage 8
• In the Veterinary Institute’s records, nephrocalcinosis was detected in 54 submissions from 43 commercial salmonid farms, it was also found in turbot, pike, and in some samples from wild salmonids 8
• It is particularly challenging to monitor due to the lack of visible external symptoms in affected fish 6b.

Economic Impact:

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Treatment & Management:

• Current control measures include improved water quality management 6b
• Recent advancements in diagnostic techniques, such as adapted x-ray imaging, have been developed to screen for nephrocalcinosis in live fish in the field 5, 6b.
• Collaboration between researchers and fish farmers is essential to improve understanding and management of the disease 5