“First report of Lactococcosis by Lactococcus garvieae in Atlantic Bluefin Tuna (Thunnus thynnus) (7179)”

First report of Lactococcosis by Lactococcus garvieae in Atlantic Bluefin Tuna (Thunnus thynnus)

Introduction

Lactococcus garvieae is one of the most relevant infectious agents of temperate finfish aquaculture, as it has been reported in over 40 freshwater and saltwater fish species. In southern European countries, rainbow trout (Oncorhynchus mykiss) has always been considered the most susceptible host for Lactococcosis. However, since 2023, L. garvieae has shown its potential as an emerging pathogen of Mediterranean marine aquaculture, with multiple massive outbreaks registered in gilthead sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax). Lactococcosis generally presents an acute or hyperacute course, with high mortality in sub-adult and adult occurring mainly when the water temperatures rise above 18 °C. Diseased fish display anorexia, melanosis and severe exophthalmia till eye rupture. The anatomopathological findings involve massive haemorrhagic septicaemia with enlarged spleen, epicarditis and opaque swim bladder.

Methodology

In September 2023, the Italian NRL of IZSVe was contacted by a net-pen farm located in Central-eastern Mediterranean for a mortality event in Atlantic bluefin tuna (Thunnus thynnus) (average size about 200 kg). Portion of organs were submitted for analyses on ice, fixed in buffered 10% formalin and in RNAlater. Direct culture from heart and brain was performed on 5% sheep blood agar and plates were incubated for 24 hours at 22°C. Fixed brain, spleen, liver, heart were processed for histological examination (H&E and Gram stains). Brain and spleen were analysed for viral nervous necrosis (VNN), viral haemorrhagic septicaemia (VHS) and red sea bream iridoviral disease (RSIVD) by molecular means.

Results

Bacteriological examination revealed an abundant growth of Lactococcus garvieae, identified through biochemical tests, MALDI-tof (Bruker), amplification and Sanger sequencing of the 16S rRNA and gyrB genes and of the 16S-23S rRNA Internal Transcribed Spacer Region (ITS). Histological examination highlighted diffuse inflammatory infiltration associated with multifocal foci of bacteria (Gram + cocci) in encephalic and splenic parenchyma and in the epicardium. Virological examination tested negative.

Furthermore, portion of eye, brain, heart, spleen and liver of a diseased wild little tunny (Euthynnus alletteratus) found inside an affected sea cage were analysed. Similarly to what observed in Atlantic bluefin tuna, all sampled organs were positive for L. garvieae.

Conclusions

This is the first report of Lactococcosis in Atlantic bluefin tuna and little tunny. The findings here described have been confirmed through further sample submissions in 2023 and 2024, underlining the threat posed by L. garvieae to this valuable fish species.

1. Cortinovis, Luana, Istituto Zooprofilattico Sperimentale delle Venezie, Presenter
2. Boscarato, Marilena, Istituto Zooprofilattico Sperimentale delle Venezie, Author
3. Basso, Andrea, Istituto Zooprofilattico Sperimentale delle Venezie, Author
4. Toffan, Anna, Istituto Zooprofilattico Sperimentale delle Venezie, Author
5. Manfrin, Amedeo, Istituto Zooprofilattico Sperimentale delle Venezie, Author
6. Pretto, Tobia, Istituto Zooprofilattico Sperimentale delle Venezie, Author