PROGRESS IN X-RAY-BASED DIAGNOSIS OF SKELETAL MALFORMATION IN FARMED FISH

Introduction: X-rays, methode of choice to diagnose skeletal malformations

For an informed analysis of vertebral column malformations in farmed fish, x-rays remain the method of choice. Today, the skeleton can be also analyzed by micro-ct and nano-ct techniques (Kague et al. 2024), but only x-rays facilitate the examination of large numbers of individuals.

New insights from life x-rays

Initially used to monitor animals after grading or at harvest, technology now also promotes life x-rays. In combination with electronic pit tagging, repetitive x-rays from the same animals let to a new understanding of several type of vertebral column malformations. From repetitive life x-rays we have learned to distinguish malformations that will increase, from those that will not further progress, and from those that can recover. Indeed, not all diagnosed malformations must have a bad prognosis (Witten et al. 2006; Drabikova et al. 2022). Further, we obtain a better understanding about the causes of vertebral column malformations by combining single high quality X-rays with histopathology and other analytical procedures. The combination of analytical procedures helps to determine the severity of malformations and to determine if bone or cartilage is affected. Also, specific types of bone alterations, visible on x-rays, provide information about the health of intervertebral disks. Eventually, for several types of malformations an improved diagnose is possible based on single X-rays only (Fig. 1).

Figure 1. Standard X-rays, Atlantic salmon showing three conditions of phosphorus deficiency. Each condition has a different prognosis. A. Negative prognosis because this condition can develop into a large center of vertebral fusion. B. Hyperdense vertebrae can be completely restored under good husbandry conditions. C. Intervertebral spaces appear too broad. An x-ray artefact related to the presence of large amount s of non-mineralized bone. An immediate increase of dietary phosphorous supply can restore the vertebral bodies.

Repair, containment and aggravation

In Atlantic salmon, hyperdense vertebrae (hyper radiodense) are a condition that can be reverted during the production cycle. The same applies to other phosphorous deficiency related bone malformations, detected on x-rays (Fig. 1). Fused vertebral bodies can be remodeled into one single, functional vertebral body. A condition without problems for animal welfare or product quality (Drabikova et al. 2022; Witten et al. 2006, 2019). In the zebrafish model lordosis can be reverted (Printzi et al. 2022). On the other side, specific alterations of intervertebral disks, ribs, haemal and neural foreshadow severe malformations and some malformations only surface late in the production cycle.

Better x-ray quality equals more data

A key for x-ray based diagnosis of skeletal malformation is image quality. Standard x-rays are sufficient to analyze harvest size animals but skeletal malformations that surface late in the production cycle may well start earlier in life. To this end, ultra-high-resolution x-technology can be used to diagnose early life stages in high numbers. This makes it possible to determine period within the production cycle at which skeletal malformations initiate. Ultra-high-resolution x-rays also provide information about internal bone structures, the bone microanatomy, important data (Fig.2).

Figure 2. High resolution x-rays can be used to diagnose skeletal malformations in juvenile animals, early life stages that are too small for standard x-rays. Also, high resolution x-rays display internal bone structures which is of high diagnostic value.

Conclusions

High quality rays, repetitive x-rays from the same individual and the combination of x-rays with histopathology and other analytical techniques have increased the understanding vertebral column malformations. This helps to provide better diagnosis and prognosis based on radiographs.

References

Drábiková L, Fjelldal PG, De Clercq A, Naveed Y, Morken T, McGurk C, Witten PE (2022) What will happen to my smolt at harvest? Individually tagged Atlantic salmon help to understand possible progression and regression of vertebral deformities. Aquaculture 559, 738430 https://doi.org/10.1016/j.aquaculture.2022.738430

Kague E, Kwon RY, Busse B, Witten PE, Karasik D (2024) Standardization of bone morphometry and mineral density assessments in zebrafish and other small laboratory fishes using x-ray radiography and micro–computed tomography. Journal of Bone and Mineral Research, 39(12): 1695–1710, https://doi.org/10.1093/jbmr/zjae171

Printzi A, Mazurais D, Witten PE, Madec L, Zambonino-Infante J-L, Koumoundouros G (2022) Juvenile zebrafish (Danio rerio) are able to recover from lordosis. Scientific Reports 12:21533, https://doi.org/10.1038/s41598-022-26112-2

Witten PE, Fjelldal PG, Huysseune A, McGurk C, Obach A, Owen MAG (2019) Bone without minerals and its secondary mineralization in Atlantic salmon (Salmo salar): The recovery from phosphorus deficiency. Journal of Experimental Biology 222, jeb188763. https://doi.org/10.1242/jeb.188763

Witten PE, Obach A, Huysseune A , Baeverfjord G (2006) Vertebrae fusion in Atlantic salmon (Salmo salar): Development, aggravation and pathways of containment. Aquaculture 258:164-172. https://doi.org/10.1016/j.aquaculture.2006.05.005