A pathomorphological description of cross-stitch vertebrae in farmed atlantic salmon (Salmo salar L.)

Abstract 1This study provides a pathomorphological description of a novel vertebral deformity in farmed Atlantic salmon, provisionally named “cross-stitch” vertebrae due to its characteristic pathological appearance by radiography. In worst cases, this vertebrae deformation is associated with reduced fish welfare, visibly deformed fish and increased risk of downgrading at slaughter due to connective tissue formation and discoloration of surrounding skeletal musculature. To describe and illustrate the pathological details of the affected vertebrae, radiographic techniques, histological methods and scanning electron microscopy were used. Vertebral dislocations, reduced intervertebral spacing and inwardly bent distal endplates (abaxial lesions) of vertebrae most frequently located caudally to the dorsal fin, characterized the radiographic appearance. Symmetrical radially distributed axial lesions in the proximal part of endplates were observed by computer tomography, scanning electron microscopy and by histological methods. These lesions have to our knowledge not previously been described in other vertebral malformations in Atlantic salmon. In detail, abnormal cartilage deposition between the endplates of adjacent vertebral bodies and in the intertrabecular adipose tissue, could suggest an attempt to stabilize the affected vertebrae. Abnormal bone growth was visualized with Picro Sirius staining, showing disorganized structures of the collagen fibers in the compact bone of the vertebral endplates and in the vertebral growth zones, features that were not observed in vertebrae without radiographic signs of “cross-stitch”. Inter-trabecular inflammation containing a minor amount of myeloperoxidase (MPX+) cells replaced the adipose tissue usually found in trabecular bone of healthy vertebrae, but inflammatory cells were in general not a predominant finding in the investigated stage of “cross-stitch” lesions. With the pathomorphological basis in place, additional studies are needed in order to define the conditions resulting in cross-stitched vertebrae.