| Summary: | Vertebral column malformations in Atlantic salmon occur under farming conditions and in wild specimens and are commonly diagnosed by X-ray. The literature uses varied terms to describe malformations, and often, different terms refer to similar phenomena or similar terms describe unrelated pathologies. A coherent classification of spine defects is lacking. This shortcoming complicates the comparison of studies, makes it difficult to link particular malformations to particular causes and hampers the assessment of the prevalence of spinal deformities. Expanding available systems to categorise skeletal malformations in different teleost species, we propose 20 types of salmon vertebral column malformations that are repetitively observed under farming conditions. As vertebral column deformities are usually diagnosed by X-ray, the classification of deformity types is based on radiological observations that have been obtained over the past 10 years from more than 5000 specimens. The proposed deformity types should be identifiable independent from the type (analogue, digital) and the quality of radiographs. We propose nine types of compression and fusion-related deformities, four types of abnormal radiotranslucent or radiopaque vertebral bodies, three types of spine curvatures, three types of symmetry deviations and displacement of vertebral bodies and one type of severe multiple malformations. Based on the suggested categories we discuss the relationships between types of vertebral body malformations. Several types of malformations involve the development of heterotopic cartilage, a pathology that is possibly linked to mechanical overload and accelerated growth. Refined categories for vertebral body malformations should help link particular types of malformations to particular causes. It should also help find out which deformities represent final stages and which deformities represent transitory stages.
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