EFFICACY OF AN INACTIVATED VACCINE AGAINST MEDITERRANEAN ISOLATES OF Lactococcus garviae ISOLATED FROM EUROPEAN SEABASS Dicentrarchus labrax

Introduction

Lactococcus garvieae (LG) is an emerging bacterial pathogen of European seabass (Dicentrarchus labrax), causing significant economic losses in Mediterranean aquaculture. Recent outbreaks have been reported in several producing countries, including Spain and Italy. Previous studies demonstrated that an inactivated vaccine licensed for trout, provides significant protection against a Spanish seabass isolate of LG [1]. The objective of the present study was to evaluate the protective efficacy of the same vaccine against genetically and geographically distinct LG isolates obtained from seabass outbreaks in the Mediterranean region.

Materials and Methods

European seabass fingerlings (15 g) were randomly allocated into 3 vaccinated and 3 placebo groups (n= 55 each). Fish received a single intraperitoneal injection of either ICTHIOVAC® LG (HIPRA, Spain) or phosphate-buffered saline (PBS). Twenty-eight days post-vaccination, separate experimental challenges were performed using three LG isolates recovered from disease outbreaks produced in different seabass aquaculture farms: one previously characterized Spanish isolate (Atlantic origin) and two recent Italian isolates recovered from the Mediterranean area. Challenges were conducted by intraperitoneal injection following anaesthesia. Mortality was monitored daily for 21 days post-challenge. Vaccine efficacy was assessed by comparing cumulative mortality, Relative Percent Survival (RPS), and Kaplan–Meier survival curves between groups. Statistical analyses were performed using the Gehan-Breslow-Wilcoxon test.

Results

All placebo groups developed severe disease following experimental infection, confirming the pathogenicity of the three isolates in seabass. In all challenge studies, cumulative mortality in placebo fish exceeded 60%, demonstrating the robustness and validity of the experimental infection models.

Vaccinated fish showed significantly reduced mortality in all challenge models. Relative Percent Survival values of 52.38%, 88.89%, and 86.67% were obtained against the Spanish isolate, Italian isolate 1, and Italian isolate 2, respectively at 13 days post challenge.

Discussion and Conclusions

The results confirm and extend previous evidence of protection conferred by ICTHIOVAC® LG in European seabass [1]. Importantly, the vaccine was effective not only against the previously evaluated Spanish isolate but also against two recent Mediterranean isolates recovered from clinical outbreaks in Italy.

The observed cross-protection suggests a substantial degree of antigenic conservation among contemporary LG strains affecting seabass and supports the use of this vaccine as a practical preventive tool in Mediterranean aquaculture. These findings provide further evidence that the vaccine originally developed for trout can contribute to reducing the impact of lactococcosis in seabass production systems.

References

[1] Baratelli, M., Verdaguer-Sentmartí, J., Sánchez-Leiva, R., Callol, A., Espinosa, T., Merino-Pérez, R.M. (2025). An inactivated vaccine indicated for trout protects seabass against an experimental challenge with Lactococcus garvieae. Abstract presented at Aquaculture Europe 2025.