Immune response induced by coinfection of the sea louse Caligus rogercresseyi and the intracellular bacteria Piscirickettsia salmonis in vaccinated Atlantic salmon

Recently, we showed that Atlantic salmon vaccinated against Piscirickettsia salmonis lose their protection upon coinfection with Caligus rogercresseyi (sea lice). However, the causes of the overriding effect of C. rogercresseyi infection have not been elucidated, and the molecular basis of the cellular and humoral immune responses upon C. rogercresseyi infection has not been described for vaccinated salmon. Therefore, we studied changes in the transcription of immune genes in vaccinated Atlantic salmon that were experimentally challenged by co-infecting them with C. rogercresseyi and P. salmonis. In general, coinfection treatments showed immune gene expression similar to treatments with a single P. salmonis infection, showing a decreased cellular response. However, a high variance was found between individual fish in the case of crucial cellular immune genes, with a few fish reacting overwhelmingly highly compared to the majority. This supports our previous findings on vaccination response variation and reinforces the idea that vaccination failures in the field might be caused by an overwhelming amount of vaccinated fish that display a deficient immune response to the infection.

Automated summary

Recently, it was found that Atlantic salmon vaccinated against Piscirickettsia salmonis lose their protection when co-infected with Caligus rogercresseyi. The reasons for this effect and the molecular basis of the immune responses in vaccinated salmon during C. rogercresseyi infection have not been determined. In this study, changes in immune gene expression were investigated in vaccinated Atlantic salmon co-infected with C. rogercresseyi and P. salmonis. The results showed that the co-infection had a similar effect on immune gene expression as a single P. salmonis infection, suggesting a decreased cellular response. However, some individual fish exhibited a significantly higher immune response, indicating variation in vaccine efficacy.