The severity of side-effects after salmon vaccination depends on the antigen composition

Vaccines are used in aquaculture to avoid outbreaks of infection from bacteria and viruses. If given at the start of the sea-water phase, oil-based vaccines provide protection against bacterial infection for the entire life in the animal.

However, oil-based vaccines produce local side-effects in the form of pigmentation and adhesions between internal organs, which in some cases are severe enough to reduce the carcase quality at slaughter. The underlying mechanisms of side-effect development are little known.

Stephen Mutoloki discovered that the reaction to the vaccine, and especially to the bacterial component of the vaccine, comes in several “waves”. These waves consist of cells that migrate to the injection site, and in the van of these is the “rapid-response troop”, the neutrophilic granulocytes. These are followed by the “clean-up team”, the macrophages, and later still by the cells that provide the actual protection against disease later in life, the lymphocytes.

This is the same sequence one finds in a natural infection and is the result of a collaboration between the processes of inflammation and of immunity. How aggressive the reaction within the tissue is, depends on how many granulocytes that are involved and how many clean-up cells arrive.

The main findings in Dr. Mutoloki’s work show that the species has a lot to say for how effectively the “clean-up” progresses. The rainbow trout has in general more effective cleaning-up cells, while the Atlantic salmon does a poorer job with a correspondingly greater and longer-lasting tissue reaction. The clean-up phase is also affected by the vaccine’s composition, that is, the more unrefined a vaccine, the more inflammatory cells that will accumulate and the greater the tissue reaction.

The type of antigen in the vaccine is also significant. Moritella viscosa is, for example, more difficult to clean up after than Aeromonas salmonicida. And if the vaccinated fish uses more resources to clean up than to create immunity against future infections, the tissue relation may become too dominating and produce unwanted side-effects.