Lactococcosis caused by Lactococcus garvieae has emerged as one of the most devastating bacterial disease affecting rainbow trout (Oncorhynchus mykiss) worldwide. Understanding the immune response to this streptococcal pathogen is crucial for diagnosing and managing the associated disease and for developing vaccines and preventive measures, such as the selection of naturally resistant trout stocks. Over the past few decades, a significant number of studies have focused on several aspects of lactococcosis, including pathological features, immune responses and vaccination protocols. However, the disease's geographical distribution and its spread over time, the routes of entry and dissemination of the pathogen within the host's body, the progression of infection, and the subsequent inflammatory response of the host still need to be fully explored. This review describes the extant knowledge on pathogenesis, immune response and prevention tools for lactococcosis and identify areas that require further investigations and dedicated studies. Particular emphasis has been placed to describe the current understanding of how immune parameters and inflammatory mechanisms are affected in rainbow trout when they are exposed to natural or experimental infections or undergo vaccination treatments. Ultimately, a more comprehensive understanding of the immune interaction between L. garvieae and rainbow trout will contribute to developing effective disease prevention strategies in aquaculture settings.
