The vaccines cause a transient defect in the vertebral growth zone. How the environment impacts further growth of the vertebrae decides whether a deformity develops or not.
In the autumn and winter of 2016-17, there were reports of a new type of spinal deformity in harvested fish, and fish farmers suspected a relation to the use of a new vaccine against PD.
The deformities were particularly severe, with a high number of abnormal vertebrae, reduced weight gain and reduced fillet quality. X-rays clearly showed that these deformities differed from other types of spinal defects, and they were called ‘cross-stitch vertebrae’ in order to distinguish them from other, more common deformities.
Analysis of production data from producers who were affected by the problem clearly showed that the vaccine was a risk factor. Scientists at Nofima also confirmed what fish farmers had already observed; that autumn smolts were much more susceptible than spring smolts. However, there was considerable variation between otherwise comparable groups, so it was obvious that something more than just the vaccine had influenced the reaction in the vertebrae.
The scientists gained access to sample material from slaughterhouses. It clearly showed that this was a specific problem that was similar from fish to fish and between different fish groups and fish farms.
“The key to understanding what went wrong was found through histology. In vertebrae that had the typical appearance on X-rays, we found a ring-shaped weakness running symmetrically around the centre of the vertebrae, sometimes also with a fracture of the vertebral endplate”, said senior scientist Grete Bæverfjord, who led the research.
At Nofima’s research station at Sunndalsøra, Bæverfjord and her colleagues set up a long-term trial in order to follow the development of this pathology, from vaccination and through to slaughter. They had four different vaccine groups: unvaccinated fish, fish vaccinated with a standard vaccine without a PD component, and two groups that were vaccinated with the two oil-based PD vaccines that were on the market at the start of the trial.
The trial fish developed normally, showing good growth and low mortality rates. They were then slaughtered at an average weight of 3.8 kilograms. No X-ray pathology was seen until the last sampling. A clear, albeit low, prevalence of cross-stitch vertebrae was found in the two PD-vaccinated groups, and not in the other two.
Defects after vaccination
In tissue studies (histology), the scientists found vertebral growth zone changes in the period after vaccination. There were increased deposits of the protein fibrin and irregularities in the bone-producing cells in the growth zone. As the fish grew, normal bone continued to form, while the weakness that were induced in the period after vaccination remained in the location where it had formed. After the fish were slaughtered, the scientists found the typical cross-stitch changes using X-rays and histology. X-ray control measurements showed that the ring-shaped weakness that had formed was consistently located where the vertebral growth zone was at the time of vaccination.
Temperature during post-smolt stage
A subgroup of fish was withheld at seawater transfer and placed in seawater tanks at 6 and 12 degrees, respectively. The scientists found several indications, both in growth patterns and on histology, that the reaction to the vaccine was stronger at the higher temperature. This may indicate that temperature and growth patterns after transferring to sea are factors that have an impact on whether the pathology develops or not, but this is something that needs to be investigated further.
“We don’t have any answers as to what it is about these vaccines that triggers the cross-stitch reaction. Is it the PD component, or is it something else? We have reason to believe that bone cell trials are an approach that can be used to get closer to an answer,” said Bæverfjord.
The project was financed by the Norwegian Seafood Research Fund (FHF), and was a collaboration between Nofima, NMBU Faculty of Veterinary Medicine, INAQ AS, Pharmaq AS and Pharmaq Analytiq AS.