Deviations of the Vertebral Column
Among the deformations of the vertebral column in salmonids, are deviations of the axis. Axis deviations that can affect all regions of the vertebral column decrease the efficiency of swimming, the ability to acquire food and they can increase susceptibility to physiological imbalances.
Curvatures of the of the vertebral column are diagnosed as lordosis (ventral curvature), kyphosis (dorsal curvature) and scoliosis (lateral curvature). The three types of deviations can develop individually or together.
Although axis deviations are primarily diagnosed as bone pathologies, underlying causes are often unrelated to bone. Still, kyphosis and lordosis can lead to secondary bone alterations such as deformed vertebral bodies or a substitution of the notochord-derived intervertebral tissue by cartilaginous tissue. The latter process involves the transdifferentiation of bone forming cells (osteoblast) in to cartilage forming cells (chondroblasts) and can lead to fusion and compression of vertebral bodies. In severe cases, there is excessive proliferation of collagen that extends from the fused vertebral bodies into the surrounding musculature. There are also cases of axis deviations without secondary changes of the bone structures, particularly if axis deformations develop late in the production cycle.
The bone of the vertebral column of salmonids with axis deviations can have a decreased mineral content, 15% to 25% less compared to animals without alterations. This suggest that mineral deficiency or an imbalance between calcium and phosphorous intake could be a factor that is linked to the occurrence kyphosis, lordosis and scoliosis. On the other hand, experiments have shown that periods of severe mineral deficiency in Atlantic salmon do cause axis deviations. A undisputed primary cause for bending of the vertebral column is vitamin C deficiency. This deficiency not only affects the collagenous bone matrix but also other collagen type 1 based connective tissues. In most cases, however, spinal bending is not primarily related to bone defects, it is rather related to neural-muscular defects. Excess swimming activity is known to cause lordosis. This has been shown for farmed fish and experimentally for zebrafish. Likewise, axis deviations can be caused by a failure of swim bladder inflation.
The early occurrence of spine bending can be caused by defective notochord development, a malformation that can be triggered by elevated temperatures. The bone of early vertebral bodies forms around the notochord and will follow the curvature of a deformed notochord. Rearing density is another possible risk factor. Experiments with juvenile zebrafish show the absence of spine bending in animals raised at low densities.
Axis deviations may also occur late, a pathology that is not fully understood. A coincidence with inflammatory processes around the vertebral column has been proposed. On the other hand, experimentally induced inflammation around the vertebral column in Atlantic salmon did not lead to spine bending. Late lordosis, kyphosis and scoliosis that are observed in Pacific salmon may represent changes that relate to the onset of maturation and the animals semilparous* life cycle (*naturally, the animals spawn only once and become deformed prior to spawning).
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