Lactococcus petauri is an emerging pathogen in rainbow trout (Oncorhynchus mykiss) aquaculture in the Americas that has resulted in significant economic loss. Piscicine lactococcosis often presents as an acute septicemia with clinical signs and gross changes including: hyperpigmentation, lethargy, exophthalmia, hemorrhage of the skin, eyes, fins, and/or gills and death. Vaccination is key to preventing and controlling lactococcosis outbreaks.
In California, a two-step vaccination protocol using immersion followed by injection months later of autogenous whole-cell killed vaccines is being used to prevent the disease. To create alternative vaccines, the potential of sub-unit vaccines and live-attenuated vaccines (LAVs) was recently investigated. To generate the LAVs candidates, a recently isolated L. petauri was passaged on nutrient media with increasing increments of rifampin to generate random mutations. After 21 passages, nine strains were generated. Significant in vitro attenuation was observed in four strains. However, only one strain resulted in >90% survival during in vivo trials.
Additionally, we characterized immunodominant L. petauri antigens with potential applications for recombinant antigen subunit vaccine development. Rainbow trout sub-adults, were inoculated with formalin-killed whole-bacteria preparations in Montanide ISA 763 A adjuvant to generate anti- L. petauri IgM. Enzyme-linked immunoassays identified that vaccinated fish produced a significantly higher anti-L. petauri antibody response compared to control fish. Serum from the rainbow trout was used in a shotgun immunoproteomic approach for discovery of immunogenic L. petauri peptides via LC/MS-MS. This analysis identified two main immunogenic proteins using the trout serum. Further ELISA analyses revealed that serum from rainbow trout exposed to formalin-killed L. petauri, L. formosensis and L. garvieae preparations were strongly reactive to the selected proteins, suggesting that these proteins are highly conserved across the different piscine lactococcosis causing agents.
Further oral, immersion and injectable challenges suggests mucosal route provided immunostimulation against L. petauri, but further research is needed before these potential vaccines can be used in the field. Although LAV and sub-unit vaccines appear to be safe to trout fingerlings and protective against piscine lactococcosis, intra-coelomically immunized fish with LAV or whole-cell killed vaccines develop a significantly stronger protective response.