Merck Animal Health Study Identifies Fetal Bovine Serum in Vaccines as Key Risk Factor for Spinal Deformities in Farmed Atlantic Salmon

New peer-reviewed research demonstrates a strong association between residual fetal bovine serum in oil-adjuvanted vaccines and the prevalence of “cross-stitch” vertebrae, a condition linked to significant growth reduction

RAHWAY, N.J., November 1, 2026 – Merck Animal Health, a division of Merck & Co., Inc., Rahway, N.J., USA, today announced the publication of a new study identifying fetal bovine serum (FBS), a cell culture remnant in some vaccine manufacturing processes, as a primary contributing factor associated with the development of cross-stitch vertebrae (CSV), a specific spinal pathology linked to reduced growth and compromised welfare in farmed Atlantic salmon (Salmo salar). The study published in Journal of Fish Diseases and titled “Effect of Foetal Bovine Serum in Oil-Adjuvanted Vaccines on Spinal Deformities and Growth in Atlantic Salmon (Salmo salar)” demonstrates a strong association between FBS and the development of cross-stitch vertebrae (CSV), a spinal pathology associated with reduced growth, impaired fillet quality, and compromised welfare in Atlantic salmon. These findings provide important new insights into a condition of significant relevance to the aquaculture industry.

“At Merck Animal Health, our commitment to innovation drives us to solve complex health and welfare challenges facing the aquaculture industry,” said Stephanie Steeves, global lead, Aqua, Merck Animal Health. “This research is a pivotal step forward in understanding vaccine-associated side effects in salmon. By pinpointing the specific role of FBS, we can better refine vaccine production to enhance fish welfare, improve production outcomes, and provide our customers with safer, more effective health solutions.”

The study investigated the effects of five different oil-adjuvanted vaccine (OAV) formulations, with and without an inactivated salmonid alphavirus (SAV) antigen and/or FBS, in Atlantic salmon. While previous industry observations had associated these deformities with certain SAV vaccines, this study was designed to isolate the causative component.

Key Study Findings:
-High Prevalence of Deformities with FBS: Fish receiving vaccines containing FBS showed a significantly higher prevalence of cross-stitch vertebrae (up to 51.7%) and gross spinal pathology (up to 39.2%).
-In contrast, fish immunized with FBS-free vaccines, including those containing the SAV antigen, had a prevalence of cross-stitch vertebrae between 0% and 1.0%.
-Significant Growth Reduction: The study established a direct link between the severity of cross-stitch vertebrae and reduced growth. Fish with 10 or more consecutively affected vertebrae had significantly lower slaughter weights.
-Economic and Welfare Implications: These findings confirm that cross-stitch vertebrae pathology impairs growth performance, leading to potential economic losses and compromised fish welfare.

“This study provides the aquaculture industry with a clear, data-driven explanation for a costly and challenging fish health issue,” said Ragnar Thorarinsson, lead author and senior research scientist, Merck Animal Health. “Our findings unequivocally show that removing FBS from the vaccine formulation virtually eliminates the risk of these specific deformities. This allows for the development of improved vaccines that protect against critical viral diseases such as Pancreas Disease when inactivated whole virus components are used without compromising spinal health or growth performance.”

Merck Animal Health recently launched AQUAVAC S7. While traditional viral vaccines use Fetal Bovine Serum (FBS) during factory cell growth, AQUAVAC S7 is built completely without serum components. This product is a heptavalent (seven-component),serum-free vaccine specifically developed for farmed Atlantic salmon. This enhances fish safety, reduces the footprint of animal-derived items, and maintains premium immunity levels.

About the Study Design
The study was conducted under controlled experimental conditions, where Atlantic salmon were assigned to different smoltification and temperature regimes to mimic real-world production cycles. The fish were vaccinated with one of five multivalent OAV formulations and monitored for approximately 10 months in a sea cage. At the end of the study, researchers performed comprehensive evaluations for vaccine-induced side effects, spinal deformities via radiology, and growth performance.

About Merck Animal Health  
Merck Animal Health, a division of Merck & Co., Inc., Rahway, N.J., USA, is a global animal health business committed to The Science of Healthier Animals™. For more than 130 years, we have pioneered groundbreaking science. Today, we are driven by continuous innovation to develop breakthrough medicines, vaccines and technology. Rooted in direct experience on the farm and in the clinic, we work hand in hand with our customers every step of the way. Our singular focus is to empower those who care for animals, helping them manage their vital responsibility with confidence. Because when it comes to animal health, no one sees it like we do. For more information, visit www.merck-animal-health.com and connect with us on LinkedIn, Facebook, X (formerly Twitter) and Instagram.

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