Effect of Foetal Bovine Serum in Oil‐Adjuvanted Vaccines on Spinal Deformities and Growth in Atlantic Salmon ( Salmo salar )

Numerous studies have shown that oil-adjuvanted vaccines (OAV) can induce a variety of different side effects in Atlantic salmon including spinal deformities (SD). Of these, a pathology first described in 2020 and named cross-stitch vertebrae has been associated with the use of inactivated salmonid alphavirus (SAV) vaccines. Historically, most viral vaccines for fish contain inactivated whole virus propagated in cell cultures. The inactivated viral component of the vaccine will, in addition to viral particles, also contain cell culture remnants such as cell debris, foetal bovine serum (FBS) and other nutrients. In the present study, five multivalent OAV formulations, with or without FBS and SAV, were tested in Atlantic salmon reared under controlled experimental conditions using different smoltification and temperature regimes. Fish vaccinated with formulations containing FBS showed significantly higher prevalence of gross pathology, cross-stitch vertebrae, and platyspondyly compared to those immunized with FBS-free vaccine formulations irrespective of whether SAV was included or not. The fish groups immunized with vaccines containing FBS also had significantly reduced growth rates compared to their unaffected counterpart groups, irrespective of the rearing strategy employed. The reduced growth rates were correlated with greater severity of cross-stitch pathology in individual fish having ≥ 10 consecutive vertebrae affected by this pathology. This is the first study to demonstrate the association between the presence of FBS in OAV with spinal deformities in Atlantic salmon.