Rainbow trout (Onchorhynchus mykiss) farms are periodically affected by outbreaks of lactococcosis caused by a gram-positive bacterium, Lactococcus garvieae, where the coexistence of healthy and symptomatic fish is observed simultaneously. The present study, being part of an “in field” sanitary survey, was mainly designed to provide insight on the expression of immune related genes in infected versus healthy fish, as well as to explore a Near Infrared Spectroscopy (NIR) based analysis (SCiO molecular sensor) as a novel approach to discriminate between them. The measurement/sampling was performed at a single time point in September 2021, in a commercial trout farm located in northern Italy, of two adult fish groups weighing between 150 and 300 g: asymptomatic (n=10) and symptomatic (n=10) sharing the same basin. Fry (n=5) weighing 15 g sampled from the L. garvieae free hatchery of the same farm were considered as reference group. Selected fish were sacrificed by the farmer and rapidly examined externally, then submitted to the SCiO molecular sensor in the abdomen region. After dissection, spleen and head kidney (HK) were collected for molecular analyses. Part of the spleen and HK were used to identify L. garvieae DNA by PCR, and part of HK was used for immune gene expression studies. The results obtained from SCiO readings allowed to detect the presence of two different spectral populations, almost coinciding with the two adult fish groups considered (asymptomatic and symptomatic). Seven out of nine (7/9) symptomatic rainbow trout were positive (L. garvieae DNA presence) while all the asymptomatic (10/10) and fry (2/2) were negative (L. garvieae DNA absence) by PCR detection. Concerning cytokine genes expression, symptomatic adult rainbow trout showed a significant up-regulation of IL-1β, IL-8 and IL-10 while no differences were found for IL-6 and TNF-α compared to fry (P ≤ 0.05). On the contrary, asymptomatic rainbow trout did not show differences for IL-1β, IL-6, IL-8, IL-10 and TNF-α compared to fry (P ≤ 0.05). Among the immune relevant receptors, analysed genes (TLR5, TCR-β, MHC-I and MHC-II) did not show differences in both asymptomatic and symptomatic fishes except MHC-II which has been down regulated in trout displaying the clinical signs compared to fry (P < 0.01). As regards the IgM expression, it was significantly enhanced in asymptomatic trout, compared to fry (P ≤ 0.05), but no difference was found in the symptomatic group compared to fry. In addition, the level of mRNA transcript of IgT was significantly higher in both symptomatic and asymptomatic adult trout compared to fry (P ≤ 0.05). This investigation tried for the first time to propose a NIR based approach in the field of fish pathology/immunology with the aim of differentiating between healthy and diseased fish. Moreover, our results on immune related genes modulation highlighted how the progression of a L. garvieae infection might be influenced by a specific gene expression pattern, which is crucial in the mode of action of trout immune system against L. garvieae.

The present investigation has been conducted under the SUPERTROUT European Project (https://mel.cgiar.org/projects/supertrout) funded by PRIMA.