Lactococcus garvieae is an important emergent bacterial pathogen of farmed fish in the USA. This study aimed to evaluate the safety and efficacy of a n autogenous immersion (IM) vaccine against L. garvieae in rainbow trout (Oncorhynchus mykiss ) as well as enhanced protection afforded by boost er IM versus injectable (IC) vaccination .
I nitial IM vaccination using a formalin-killed vaccine augmented by the adjuvant MontanideTM was followed by booster vaccination via IM or IC injection routes 273 degree days (dd) post-immunization along with appropriate PBS controls. The various vaccination protocol efficacies were evaluated by challenging fish with L. garvieae by cohabitation with diseased fish 399 dd post-booster administration.
Rainbow trout tolerated the vaccines well with no mortalities during vaccination, boost and challenge . A relative percent survival (RPS) of 98%, 14%, 3% and -8% was recorded in the IM immunized + IC injection boosted, IM immunized + mock IC injection boosted, IM immunized + IM boosted, and IM immunized + mock IM boosted treatments, respectively. Only the IM immunized + IC injection boosted treatments provided significant protection when compared to positive control treatments (p<0.05). Approximately 0%, 50%, 20% and 30% bacterial persistence was recorded in the IM immunized + IC injection boosted, IM immunized + mock IC injection boosted, IM immunized + IM boosted and IM immunized + mock IM boosted treatments at the end of the challenge, respectively. Twenty survivors were maintained at 13°C for 30d, and temperature-stressed by increasing temperature to 18°C during a one-week period. After this period, bacterial persistence and gene expression of immune- related genes were explored in the different treatments.
Both IM and IC vaccines induce an immune response in gills and spleen of immunized rainbow trout. Immune response in vaccinated rainbow trout is stronger than in adjuvant and control treatments. Although b oth vaccines appear to be safe to trout fingerlings and protective against piscine lactococcosis , IC immunized fish develop a significantly stronger protective response.