SUPERTROUT is a project funded in the context of PRIMA (Partnership for Research and Innovation in the Mediterranean Area) programme. The goal of SUPERTROUT is to improve sustainability and profitability of small-scale farming systems facing lactococcosis in rainbow trout by applying an innovative strategy focussed primarily on the selection of genetic/genomic resistant fish and the development of a recombinant subunit-protein based vaccine. The main results obtained until now show a high prevalence in the Mediterranean area of Lactococcus petauri previously identified as Lactococcus garvieae, indicating that lactococcosis could be sustained by both bacterial species, apparently inducing the same symptoms. Interestingly, it has been demonstrated that conventional biochemical and biomolecular diagnostic techniques, usually applied, are not efficient for species discrimination. Thus, a new method based on the amplification of the ITS 16S-23S region was developed (Stoppani et al., 2023). Hemolysins and capsule have been studied as virulence factors, showing differences related to the presence of hemolysin 3 in the two species. Antimicrobial resistance was assessed by testing a panel of 12 antibiotics with disc diffusion test and e-test, while the presence of antimicrobial resistance genes (ARGs) was also examined. Overall, antimicrobial susceptibility tests indicate that 93.8% of the isolates are resistant to oxolinic acid, followed by flumequine (91.3%), streptomycin (88.8%), kanamycin (72.5%), and neomycin (66.3%). The study of the genetic of the host, focussing on MHC class IIB gene, is ongoing, but preliminary results based on different outbreaks selected in the countries involved in the project, showed a few statistically significant polymorphisms associated with trout resistance to lactococcosis. Host immune response has been studied in the Italian natural outbreaks and differences in head kidney expression of IL-1β, IL-8, IL-10, TCR- β, MHC-II, IgT have been reported (Khalil et al., 2023a). Moreover, in symptomatic fish blood lymphocytes, serum antiprotease and peroxidase activity were lower than in asymptomatic fish, while thrombocytes were higher (Khalil et al., 2023b). The development of the recombinant subunit-protein vaccine is being carried out using the reverse vaccinology approach and different epitope targets able to induce immunogenic responses have been predicted to be used for vaccine production using plasmids. Matings of broodstock that had been genetically selected according to specific haplotypes of the MHC class IIB gene were evaluated for their reproductive performances using a factorial design. It was shown that gametes with similar MHC genetics produce offspring with a higher survival rate and lower larval deformities.

The results obtained until now are promising for the management of the outbreaks, mainly for the possibility of having a specific etiological diagnosis and applying the proper control measures.