X-ray-based morphometrics : an approach to diagnose vertebral abnormalities in undermineralized vertebrae of juvenile triploid all-female rainbow trout (Oncorhynchus mykiss) fed with a phosphorus deficient diet

Extended phosphorus (P) deficiency has been linked to an increase of skeletal deformities in farmed salmonids. To date, X-ray examination still is the most practical way to detect abnormalities that are not externally visible. The aim of this study was to develop a standardized method based on empirical measurements that provides an early radiograph-based detection of deformities. Triploid all-female rainbow trout (60.8 +/- 1.6 g) were fed with phosphorus (P)-deficient and P-sufficient diets (total P = 0.5% and 0.92%, respectively). Experiments were carried out in 12 circular re-circulating tanks (n = 140/tank; 8.5-52.7 kg m(-3); 12 +/- 0.3 degrees C; 18L: 6D). All fish were x-rayed at week 5, 15 and 24 to monitor vertebral abnormalities. Fish were assigned to a vertebral phenotype (normal, biconcave or compressed) when all vertebrae of the caudal region (V31-44) showed the same type of abnormalities. At week 9, 18 and 27, ten fish per tank were randomly sampled according to their vertebral phenotype. Vertebrae V38-V39 were collected and a second X-ray was performed to confirm the above-mentioned phenotypes. Morphometrics (vertebral length and height; intervertebral distances and vertebral body angles) were measured directly on radiographs. For all parameters measured, comparisons were made between normal P-sufficient fish and different scenarios of vertebral development (normal, restored, stable and aggravated) in P-deficient fish. The two parameters showing the most sensitivity were the median intervertebral distance and dorsal/ventral vertebral endplate angles. When analysed together, these two parameters allow a quantitative distinction of the different vertebral phenotypes. Measuring dorsal/ventral vertebral endplate angles seems a promising approach to detect aberrant vertebral development at early stages as slight changes were observed in P-deficient vertebrae that were assessed as normal by visual examination. Further studies including histological examinations and comparison with other radiological imaging techniques are required to confirm these results