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The major histocompatibility complex (MHC) consists of genes involved in immune response and molecular discrimination between self and non-self. MHC genes are the most polymorphic in vertebrates. The origin and maintenance of polymorphism in MHC genes in populations is still unresolved. Mechanisms such as sexual selection and heterozygote advantage have been suggested as explanations for this high variability. In this study, a farmed population of rainbow trout (Oncorhynchus mykiss) characterized by the presence of specific MHC class IIB gene haplotypes at a frequency higher (30%) than that expected from random matings was investigated. Therefore, it was hypothesized that disassortative matings occur with an adaptive advantage for females, resulting in improved reproductive performance when mated with individuals with similar MHC haplotypes. Genetic analyses of the breeders were performed to define the MHC haplotypes and to perform specific matings. The effect of mating was evaluated by analyzing the survival rate of the offspring at various stages of incubation until swim-up. The reproductive performance of the offspring derived from specimens with similar haplotypes showed a better survival trend during the first life stages and reduced malformations. The results obtained are in contrast with the heterozygous advantage theory, therefore it was hypothesized, as for other salmonid species, the presence of a positive selection towards locally adapted MHC genes that promotes reproduction between genetically similar individuals. [ABSTRACT FROM AUTHOR]

Lactococcosis, caused by the Gram‐positive bacterium Lactococcus garvieae, is a major concern in rainbow trout (Oncorhynchus mykiss) farms, which are regularly affected by outbreaks especially during the summer/fall months. In these farms, unvaccinated healthy and symptomatic fish can coexist with vaccinated fish. In the present study, innate (leukogram, serum lysozyme activity, peroxidase activity, antiprotease activity, bactericidal activity, total IgM and total proteins), and specific immune parameters (serum antibodies to L. garvieae) were assessed in unvaccinated adult rainbow trout naturally exposed to the pathogen, with or without evidence of clinical signs, or subjected to vaccination. Blood was drawn from all three groups, and blood smears were prepared. Bacteria were found in the blood smears of 70% of the symptomatic fish but not in any of the asymptomatic fish. Symptomatic fish showed lower blood lymphocytes and higher thrombocytes than asymptomatic fish (p ≤.05). Serum lysozyme and bactericidal activity did not vary substantially among groups; however, serum antiprotease and peroxidase activity were significantly lower in the unvaccinated symptomatic group than in the unvaccinated and vaccinated asymptomatic groups (p ≤.05). Serum total proteins and total immunoglobulin (IgM) levels in vaccinated asymptomatic rainbow trout were significantly higher than in unvaccinated asymptomatic and symptomatic groups (p ≤.05). Similarly, vaccinated asymptomatic fish produced more specific IgM against L. garvieae than unvaccinated asymptomatic and symptomatic fish (p ≤.05). This preliminary study provides basic knowledge on the immunological relationship occurring between the rainbow trout and L. garvieae, potentially predicting health outcomes. The approach we proposed could facilitate infield diagnostics, and several non‐specific immunological markers could serve as reliable indicators of the trout's innate ability to fight infection. [ABSTRACT FROM AUTHOR]

Lactococcus garvieae is the etiological agent of lactococcosis, a clinically and economically significant infectious disease affecting farmed rainbow trout. L. garvieae had been considered the only cause of lactococcosis for a long time; however, L. petauri, another species of the genus Lactococcus, has lately been linked to the same disease. The genomes and biochemical profiles of L. petauri and L. garvieae have a high degree of similarity. Traditional diagnostic tests currently available cannot distinguish between these two species. The aim of this study was to use the transcribed spacer (ITS) region between 16S rRNA and 23S rRNA as a potential useful molecular target to differentiate L. garvieae from L. petauri, saving time and money compared to genomics methods currently used as diagnostic tools for accurate discrimination between these two species. The ITS region of 82 strains was amplified and sequenced. The amplified fragments varied in size from 500 to 550 bp. Based on the sequence, seven SNPs were identified that separate L. garvieae from L. petauri. The 16S-23S rRNA ITS region has enough resolution to distinguish between closely related L. garvieae and L. petauri and it can be used as a diagnostic marker to quickly identify the pathogens in a lactococcosis outbreak. [ABSTRACT FROM AUTHOR]

Simple Summary: Lactococcus garvieae is the etiological agent of lactococcosis, a bacterial disease affecting many species of fish and causing major economic losses in aquaculture. In this study we described, for the first time, the isolation of L. garvieae in brook trout farmed in northwestern Italy by performing a molecular and epidemiological characterization. Results confirmed water as vehicle of infection, favoring the transmission of the pathogen between rainbow trout farmed in the upstream compartments of a raceways system and the brook trout located in downstream tanks. Lactococcosis is a fish disease of major concern in Mediterranean countries caused by Lactococcus garvieae. The most susceptible species is the rainbow trout (Oncorhynchus mykiss), suffering acute disease associated with elevated mortalities compared to other fish species. References reported that other salmonids are also susceptible to the disease, but no mortality outbreak has been described to date. The aim of this study was to present a mortality outbreak that occurred in brook trout (Salvelinus fontinalis) farmed in northwestern Italy during the summer of 2018. Fish exhibited clinical signs, such as exophthalmos, diffused hemorrhages localized in the ocular zone, hemorrhagic enteritis, and enlarged spleen. L. garvieae was isolated in all fish. Molecular and epidemiological characterization of the isolates, through Pulsed Field Gel Electrophoresis (PFGE), confirmed the initial hypothesis of water as vehicle of infection favoring transmission between rainbow trout farmed in upstream compartments and brook trout located in downstream tanks. Moreover, several environmental conditions affected and promoted the outbreak, among them the high-water temperature, which probably induced a physiological stress in brook trout, being way above the optimal temperature for this species, increasing the susceptibility to infection. [ABSTRACT FROM AUTHOR]

Lactococcosis is one of the main bacterial diseases affecting rainbow trout (Oncorhynchus mykiss), with significant economic and sanitary repercussion. Vaccination and antibiotic treatments are commonly used to prevent and control the infection outbreaks; however, these strategies have some drawbacks including limited coverage, handling costs, induction of antibiotic resistance and chemical residues in the environment. Selective breeding programs represent a promising complementary approach for increasing fish disease resistance in commercial farms and some immunological parameters may be tentatively used as indirect indicators for this purpose. The present study investigated for the first time some innate and adaptive immune responses in two groups of rainbow trout derived from selected lines (susceptible and resistant) showing a different "in field" phenotypical resistance to Yersinia ruckeri , Flavobacterium branchiophilum , F. psychrophilum , and Ichthyophthirius multifiliis , after an immersion-dilution based exposure to Lactococcus garvieae carried out in controlled experimental conditions. Twenty-six resistant and twenty-six susceptible female rainbow trout (mean body weight 80 g, 9 months aged, F5 generation) were obtained from an intensive farm considered L. garvieae free and were exposed to the pathogen. Moreover, 10 resistant and 10 susceptible fish were used as uninfected controls. After 5 days, blood and tissue samples were collected for immunological analyses. A significantly higher serum and mucus lysozyme activity was recorded in resistant rainbow trout compared to susceptible fish (P ≤ 0.05), both before and after exposure to L. garvieae. Similarly, respiratory burst activity of head kidney leukocytes resulted more intense in resistant fish (P ≤ 0.05), suggesting that phagocytes could more quickly activate their microbicidal mechanisms to counteract the bacterial spread. Resistant group displayed also an up-regulation of immunoglobulins M (IgM), major histocompatibility complex II (MHC-II) and interleukin 8 (IL-8) gene expression (P ≤ 0.05) and a significantly higher blood lymphocytes count (P ≤ 0.05), highlighting their potential better ability to trigger the recruitment of defensive cells and the initiation of specific immune processes such as antigen presentation to CD4+ T lymphocytes and IgM synthesis. The results herein presented might be useful for the identification of immunological markers to be used as indirect indicators in rainbow trout selective breeding programs. • Selected rainbow trout lines exhibited different attitude to control lactococcosis. • Resistant fish showed higher lysozyme activity to resist L. garvieae infection. • Resistant fish more effectively trigger specific immune response against L. garvieae. [ABSTRACT FROM AUTHOR]

Lactococcosis is a globally prevalent infectious disease that has a significant economic and sanitary impact on the rainbow trout industry. Lactococcus garvieae has traditionally been considered the only species implicated in the etiology of this disease, but Lactococcus petauri , a new species, has recently been implicated as another etiological agent. Both species cannot be distinguished by routine methods commonly used in diagnostic laboratories, resulting in their misidentification. In the present study, the identification of 48 isolates initially identified as L. garvieae was studied by determining their in-silico DNA–DNA hybridization (dDDH) and average nucleotide identity (ANI) values using pairwise comparisons of their whole genome sequences and the genomes of the type strains of L. garvieae and L. petauri. The genome sequences of 37 isolates from countries in which lactococcosis can be considered endemic (Spain, Italy, Türkiye, and Greece) were obtained in this study, and the genomes of 11 isolates were retrieved from the GenBank database. Isolates from Italy, Singapore, Japan, South Korea, India, one Turkish isolate from 2013 and two Spanish isolates recovered in 1992 and 1996 were confirmed as L. garvieae. The remaining isolates from Spain and Türkiye, as well as those from Portugal, Israel, USA, and Greece were identified as L. petauri. Overall, 60.4% of isolates previously identified as L. garvieae were found to be L. petauri. These results confirm the implication of both species in the etiology of lactococcosis and suggest that L. petauri plays a significant role in the epidemiology of this disease. Some of the isolates identified as L. petauri in the present study were isolated three decades ago, indicating that its association with lactococcosis is older than might be expected from the recent descriptions. The commercial Rapid ID32 Strep system was unable to discriminate between L. garvieae and L. petauri. However, both species exhibited some biochemical differences that might serve as phenotypic markers for their presumptive recognition. Consequently, isolates that hydrolyze hippurate and produce acid from sucrose and tagatose could be presumptively recognized as L. petauri , while those that fail these tests could be identified as L. garvieae. The results of this work indicate that great attention should be given to L. petauri in the epidemiology of lactococcosis. • Identification of L. garvieae isolates was determined by dDDH and ANI analysis • The 60.4% of isolates previously identified as L. garvieae were found to be L. petauri. • These results confirm the implication of L. petauri in the etiology of lactococcosis. • L. petauri plays a significant role in the epidemiology of lactococcosis. • Some biochemical differences might serve as markers for their presumptive recognition. [ABSTRACT FROM AUTHOR]

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 = 9). 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 to 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 and symptomatic). Among cytokines, IL-1β showed significant up-regulation in symptomatic group compared to asymptomatic group (P < 0.05) while IL-8 showed very high (P < 0.01) and high significant (P < 0.05) differences 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-β 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 to fingerling and asymptomatic. Concerning immunoglobulin genes expression, the level of mRNA transcript of IgT was significantly higher in symptomatic trout compared to fingerling (P < 0.05) and tended to be higher in asymptomatic trout compared to fingerling (P < 0.1). No significant differences were observed for IL-6, TNF-α, TLR5, MHC-I and IgM. In this study, a NIR based approach was used for the first time 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 outcome 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. • Profile of immune related genes in L. garvieae infected rainbow trout may vary between fish displaying symptoms or not. • Influence the mode of action of trout immune system against the pathogen and the disease outcome. • Pilot proposal of a NIR based approach in the field of fish pathology to differentiate between infected and uninfected fish. [ABSTRACT FROM AUTHOR]