COMPARISON OF THREE METHODS FOR NEPHROCALCINOSIS DIAGNOSTICS IN FARMED RAINBOW TROUT Oncorhyncus mykiss
Introduction
Nephrocalcinosis in Rainbow trout is a well-known problem in Norwegian fish farming1 and considered the most important problems in the freshwater stage2. With increasing production and use of new production systems, the need for new, non-invasive or non-lethal diagnostic methods such as radiography3 and ultrasound is emerging, to give consistent evaluations and diagnostics. Now a variety of scoring systems and methods are used for nephrocalcinosis assessment, and their respective strengths and limitations are not fully understood.
Material and methods
A total of 777 Rainbow trout were examined using radiography and macro scoring to assess nephrocalcinosis severity. A subset of 24 fish were selected for histological examination, widely regarded as the reference method for assessing renal pathology in nephrocalcinosis. The three methods evaluated were: radiography (detection of mineral deposits throughout the kidney), macro scoring (gross pathological changes on the kidney surface), and histology (cellular-level characterisation of renal pathology).
Results and discussion
Agreement between methods was highest in moderate to severe cases of nephrocalcinosis. In low-grade cases, histology revealed structural changes that were not consistently detected by radiography or macro scoring. Notably, some of the histologically most severe cases showed low radiographic and macro scores. These kidneys were characterised macroscopically by fibrosis and altered texture and colour, and histologically by extensive tissue damage, including loss of interstitial tissue with key roles in haematopoiesis, immune function and hormone production.
These findings indicate that the different diagnostic methods capture distinct aspects of nephrocalcinosis, with radiography reflecting mineral burden, macro scoring reflecting superficial structures and histology revealing structural tissue damage. The results further suggest that current macro scoring systems may benefit from including additional descriptors such as texture, contour and colour in order to better reflect chronic pathological changes.
Figure 1: Radiograph of Rainbow trout kidney with nephrocalcinosis score 3.
Acknowledgment
This project was funded by FHF (Fiskeri- og havbruksnæringens forskningsfinansiering, project number 901874).
References
Smart, G.R., Knox, D., Harrison, J.G., Ralph, J.A., Richards, R.H., Cowey, C.B (1979). Nephrocalcinosis in rainbow trout Salmo girdneri Richardson; the effect of exposure to elevated CO2 concentrations. J. Fish. Diseases, 279-289.
Moldal T, Wiik-Nielsen J, Oliveira VHS, Svendsen JC og Sommerset I. Fiskehelserapporten 2025
Klykken C, Dalum AS, Reed AK, Attramadal K, Olsen RE, Boissonnot L. Radiological detection of nephrocalcinosis in farmed Atlantic salmon Salmo salar L. J Fish Dis. 2022 Dec;45(12):1883-1888.