032 | Natural exposure to Lactococcus garvieae in rainbow trout: evidence of susceptibility and clinical outcomes

Annalisa Cotugno¹, Sara Glorio Patrucco¹, Giorgia Zicarelli¹, Silvia Colussi^1|2, Martina Gini¹, Pier Luigi Acutis¹, Giulia Milanese¹, Giulia Stilo¹, Caterina Faggio^3|4, Giuseppe Esposito¹, Paolo Pastorino¹, Marino Prearo¹ | ¹Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d’Aosta, Turin, Italy; ²Department of Animal, Nutritional and Food Sciences, Università Cattolica del Sacro Cuore, Piacenza, Italy; ³Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of Messina, Italy; ⁴Department of Ecosustainable Marine Biotechnology, Stazione Zoologica Anton Dohrn, Naples, Italy.

Rainbow trout (Oncorhynchus mykiss) is considered one of the most important and widely farmed freshwater species in global aquaculture, playing a central role in European production systems due to its rapid growth, high adaptability and excellent flesh quality. However, intensification of farming practices exposes trout to multiple environmental and physiological stressors which can compromise immune competence and markedly increase susceptibility to infectious disease, leading to significant economic losses. Lactococcosis, caused by Lactococcus garvieae, represents one of the major health problems in rainbow trout farms, associated with high mortality rates (50-80%) that can strongly reduce farms productivity. Recent studies have also reported Lactococcus petauri and Lactococcus formosensis as new emerging etiological agents for lactococcosis, showing clinical signs comparable to those typically associated to L. garvieae. The present study aimed to evaluate the susceptibility of rainbow trout exposed to aquaculture water naturally contaminated with L. garvieae. The research is part of RESILTROUT project, which investigates the resilience of rainbow trout to various farming challenges, including lactococcosis. A total of 1000 healthy rainbow trout belonging to autochthonous genetic line, sourced from a L. garvieae free farm in Piedmont, were introduced into a trout farm in Trentino-Alto Adige where L. garvieae was naturally present. Post-mortem analyses were performed on 240 specimens and included necropsy and bacteriological sampling of the kidney and eye tissues, the primary target organs for detecting Lactococcus spp. Pure bacterial isolates were obtained and identified using MALDI-TOF, followed by molecular analysis to discriminate Lactococcus species. Data processing and statistical analyses were performed in R/RStudio. L. garvieae was isolated in 26.8% of kidney and 31.5% of eye samples. Other bacterial species, including Hafnia alvei, Carnobacterium maltaromaticum, C. divergens, Lactococcus lactis, and Acinetobacter spp. were detected at lower frequencies. Notably, L. petauri was also isolated in ocular samples, although only in a minimal proportion of cases (0.5%). Spearman correlation analysis revealed a significant positive association between L. garvieae detection (in both kidney and eye) and typical clinical signs of lactococcosis, including exophthalmos, ocular enucleation, and hepatic petechiae. Conversely, L. garvieae negative individuals showed a significant correlation with the absence of clinical alterations and a negative association with exophthalmos, indicating an overall normal condition. These findings highlight the variable susceptibility and resilience of rainbow trout to L. garvieae under natural farming conditions, with clear clinical correlations. As part of the RESILTROUT project, this study contributes to a deeper understanding of host-pathogen interactions and supports the development of strategies to enhance trout resilience against lactococcosis.
Research funded by Italian Ministry of Agriculture, Food and Forestry, RESILTROUT-6CFP Supply Chain Program, IZSPLV Project Code: 23E03.