In field study on immune-genes expression during a lactococcosis outbreak in rainbow trout (Oncorhynchus mykiss)

Italian rainbow trout (Oncorhynchus mykiss) farms are periodically affected by outbreaks of lactococcosis caused by Lactococcus garvieae, and rearing basins reveal simultaneously asymptomatic and symptomatic fish. The present study, as part of an in field sanitary survey, was designed to describe the expression of immune related genes in infected versus healthy fish, as well as to explore a Near Infrared Spectroscopy (NIR) as a novel approach to discriminate between them. Measurements/sampling were performed in a farm of Northern Italy and included two groups of fish (150-300 g), sharing the same basin: asymptomatic (n = 10) and symptomatic (n = Fingerling (n = 5, 15 g) were also sampled from the farm internal hatchery and considered as L. garvieae-free reference group. NIR spectra were collected from the abdominal region of the fish through the SCiO molecular device. Spleen and head kidney were sampled for bacterial detection (PCR). Head kidney was also subjected gene expression analysis (qPCR). Seven out of nine (7/9) symptomatic trout were L.garvieae positive (bacterial DNA) while all the asymptomatic (10/10) and fingerling (2/2) were L. garvieae negative. SCiO scanning provided two different spectral populations, almost coinciding with the two fish groups considered (asymptomatic symptomatic). Among cytokines, IL-1 beta showed significant up-regulation in symptomatic group compared asymptomatic group (P < 0.05) while IL-8 showed very high (P < 0.01) and high significant (P < 0.05) ferences in its up-regulation in symptomatic group compared to asymptomatic group and fingerling, respectively. IL-10 showed a tendency of significant over-expressed in symptomatic fish compared to fingerling (P < 0.1). TCR-beta showed significant (P < 0.05) down-regulation in symptomatic fish compared to asymptomatic ones and MHC-II resulted in down-regulation in a very significant (P < 0.01) manner in symptomatic trout compared fingerling and asymptomatic. Concerning immunoglobulin genes expression, the level of mRNA transcript of was significantly higher in symptomatic trout compared to fingerling (P < 0.05) and tended to be higher asymptomatic trout compared to fingerling (P < 0.1). No significant differences were observed for IL-6, TNF-alpha, TLR5, MHC-I and IgM. In this study, a NIR based approach was used for the first time in the field of fish thology/immunology with the aim of differentiating between healthy and diseased fish. Moreover, our results immune related genes modulation highlighted how the outcome of a L. garvieae infection might be influenced a specific gene expression pattern, which is crucial in the mode of action of trout immune system against garvieae.

Automated summary

Italian rainbow trout farms often experience outbreaks of lactococcosis caused by Lactococcus garvieae, with both asymptomatic and symptomatic fish present in rearing basins. This study aimed to examine immune-related gene expression and investigate Near Infrared Spectroscopy (NIR) as a tool for distinguishing between infected and healthy fish. The results showed significant differences in gene expression between symptomatic and asymptomatic fish, particularly for cytokines IL-1 beta and IL-8. NIR scanning successfully differentiated between the two fish groups. This study also demonstrated the importance of specific gene expression patterns in the trout immune system's response to L. garvieae infection.