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Grape pomace is a by-product of the winemaking process, rich in bioactive compounds such as resveratrol and has antioxidant and antimicrobial properties. Recent evidence has demonstrated the protective effects of grape against impairment of the phosphoryl transfer network, a system linked to energetic homeostasis of high-energy phosphoryl transfer, to maintain energetic balance. Also, Pseudomonas aeruginosa infection in fish has been related to severe impairment of the branchial phosphoryl transfer network via inhibition of creatine kinase (CK), adenylate kinase (AK) and pyruvate kinase (PK) activities. Thus, the aim of this study was to evaluate whether dietary supplementation with grape pomace flour (GPF) is able to reduce or prevent the impairment of cellular energetic homeostasis in grass carp (Ctenopharyngodon idella) experimentally infected by P. aeruginosa. Branchial CK (cytosolic and mitochondrial), AK, and PK activities were lower in infected animals on day 15 post-infection (PI) compared to uninfected fish, while lactate dehydrogenase (LDH) activity was higher. Branchial reactive oxygen species (ROS), thiobarbituric acid reactive substances (TBARS), lipid oxidation (LOOH) and protein carbonylation levels were higher, while the level of non-proteic (NPSH) and proteic (PSH) thiols was lower in infected fish on day 15 PI, as compared to uninfected fish. The use of 300 mg of GPF/kg of feed was able to ameliorate the activities of CK, AK, PK, and LDH as compared to infected fish, but their activities remained different from the control group. On the other hand, dietary supplementation with 300 mg of GPF/kg of feed was able to prevent all alterations linked to oxidative damage elicited by disease. Based on this evidence, inhibition of the phosphoryl transfer network may contribute to the impairment of intracellular energetic communication between cellular ATP synthesis and consumption and is mediated by oxidative damage in the gills of fish infected by P. aeruginosa. The use of 300 mg of GPF/kg of feed exerted protective effects on branchial energetic metabolism linked to ATP metabolism by reducing the impairment in cellular energetic homeostasis; its effects can be mediated by the prevention of SH group oxidation. • Pseudomonas aeruginosa caused branchial bioenergetics dysfunction. • Grape pomace flour ameliorated the phosphotransfer network. • Grape pomace flour exerted protective effects on energetic metabolism. [ABSTRACT FROM AUTHOR]

The use of probiotics is a strategy employed to improve host health status and to prevent infectious diseases. The current study was aimed at investigating the diversity of lactic acid bacteria (LAB) and Bacillus species in the gastrointestinal tracts of wild marine fishes, as well as the beneficial effects of Lactococcus lactis WFLU12 as a host-derived probiotics in olive flounder. In marine fishes, wild olive flounder and rock bream were shown to be good sources of LAB and Bacillus isolation, respectively. Some isolates, including the strain WFLU12, have shown stronger inhibitory activity against various aquatic bacterial pathogens and more tolerance to low pH and bile acids compared to some strains isolated from sources other than marine. Lc. lactis WFLU12 was found to confer to olive flounder protection against streptococcosis caused by Streptococcus parauberis through competitive exclusion and increased innate immune responses. Interestingly, the natural infection rate in the probiotic fed group (33% = 10/30) was significantly lower than that in the control group (60% = 18/30). None of the nisin Z and colicin V-producing probiotic-fed fish were naturally infected by S. parauberis during the feeding period. In addition, more importantly, this promising probiotic strain significantly promoted fish growth along with better feed conversion and specific growth rate. This study demonstrates that the use of host-derived probiotics can offer a significant advantage in terms of optimum survival and function in the gastrointestinal tract of the intended host. Statement of relevance In this study, host-derived probiotic strain outperforms elimination of pathogen through competitive exclusion in the gastrointestinal tract and increased innate immune responses. More importantly, this promising probiotic strain significantly promoted fish growth along with better feed conversion. This study will provide insight into how optimal probiotics should be selected and developed. It might facilitate the replacement of commercial fish probiotic products originated from terrestrial sources with host-derived probiotics in the near future. [ABSTRACT FROM AUTHOR]

Lactococcosis in fish has been associated with Lactococcus garvieae and the recently described L. petauri. However, the relevance of these emerging fish pathogens to Nile tilapia still requires thorough understanding. This study investigated lactococcosis outbreaks in Nile tilapia on Brazilian farms and characterized the isolates through multilocus sequence typing (MLST), virulence to Nile tilapia, and antimicrobial susceptibility. Lactococcosis outbreaks were monitored from 2019 to 2022 throughout Brazil. The outbreaks occurred mainly during warmer months, and co-infections were observed in four farms, whereas concurrent bacterial infections were identified in all farms. Since the sequence of the 16S rRNA was not capable of differentiating between L. petauri and L. garvieae , Lactococcus spp. isolates were identified at the species level using the gyrB gene sequence. In total, 30 isolates were classified as L. petauri and two as L. garvieae. According to the MLST, all L. petauri isolates were grouped in the sequence type (ST) 24, except for one isolate which belonged to the newly described ST47. A new ST was also described for the L. garvieae isolates identified, ST46. The L. petauri ST24 and ST47 were characterized as singletons, whereas L. garvieae ST46 was grouped with ST16 and ST17 and formed CC17. For the challenge trial, an L. petauri ST24 isolate was chosen considering that this MLST lineage was the most frequently observed. Fish were challenged by intraperitoneal injection and different bacterial doses were used (106 to 102 CFU per fish). The infection in the challenged fish progressed very rapidly, and within 48 h post-challenge clinical signs and the first mortalities were observed. The pathogenicity of L. petauri to Nile tilapia was confirmed. The estimated LD50 was 5.74 × 103 CFU 15 days post-challenge. Provisional epidemiological cutoff values were determined for L. petauri for six antimicrobial agents from different drug classes. All isolates were characterized as wild type (WT) for neomycin and oxytetracycline, whereas 96.67% of the isolates were characterized as WT for amoxicillin, erythromycin, and florfenicol, and 83.33% were WT for norfloxacin. The L. petauri ST24 was observed in different regions of the country, illustrating a rapid expansion of this bacterial lineage. • Lactococcus petauri is pathogenic to Nile tilapia. • L. petauri was implicated in most lactococcosis outbreaks in tilapia in Brazil. • The most frequent and spread MLST lineage was L. petauri ST24. • The gyrB sequence allowed for the differentiation of L. petauri and L. garvieae. [ABSTRACT FROM AUTHOR]

Colombia due to its geographical location is provided with different climatic zones, that are suitable for continental and marine aquaculture of a wide range of species with a high possibility of commercialisation. Currently, the production comes mainly from ponds and floating cage aquaculture systems, supplying more than 90% of the freshwater and saltwater organisms consumed in the country, where the production is based on tilapia, trout, cachama and shrimp. This review aims to provide information of the current aquaculture value chain and development in Colombia by describing the current production systems and highlighting their limitations that make difficult to achieve high efficiency combined with necessary sustainability. Colombia has all the potential to develop tropical aquaculture, which should focus on native fish breeding, genetic improvement, high-tech aquaculture systems, agro-industrial and fish biomass development valorisation. Future development trends and perspectives are also analysed in this review. • Colombia has water availability for breeding and cultivation of tropical fish species. • Lack in knowledge and skilled workers affects high-tech aquaculture systems. • Academic and governmental actors can translate strategies and research into actions. • Native fish genetic breeding and agro-industrial valorisation are current challenges. [ABSTRACT FROM AUTHOR]

In 2009, the largest steelhead trout conservation hatchery in the state of Idaho, Dworshak National Fish Hatchery (NFH), lost over 50% of the juvenile steelhead trout ( Oncorhynchus mykiss ) population being reared for release. The causative agent of this high mortality was the viral pathogen infectious hematopoietic necrosis virus (IHNV). This was neither the first nor the worst epidemic of IHNV to occur at the hatchery, but it was the worst in over a decade. Genetic analysis of IHNV isolates taken from juveniles suffering epidemic IHN disease in 2009 revealed that the virus was of the M group of IHNV viruses, known to have high virulence for trout. The water supply for steelhead trout rearing at Dworshak NFH is untreated water taken directly from the Clearwater River. Further genetic analysis of IHNV isolates from adults spawned in 2009 indicated that adult steelhead trout in the river (in the hatchery water supply) were the most probable transmission source for the epidemic IHN disease in the juvenile fish. Previously, Dworshak NFH had been able to gain access to reservoir water from behind the Dworshak Dam for nursery egg incubation and the earliest stage of fry rearing, which nearly eliminated incidence of IHN disease in that stage of rearing. Additionally, the nearby Clearwater State Fish Hatchery (SFH), which operates entirely with reservoir water, has never had a case of IHN disease in juvenile steelhead trout. Therefore, staff at Dworshak NFH sought and obtained access to a limited supply of reservoir water for the first few months of outdoor rearing of juvenile steelhead trout, beginning in 2010. This strategy delayed the exposure of juvenile steelhead trout to river water for several months. The effects of this program change were: drastic reduction in IHN disease in juvenile steelhead trout; interruption in the transmission of highly virulent M group IHNV from adult steelhead trout; no interruption in the transmission of low virulent U group IHNV from adult Chinook salmon; and a shift of IHNV types in adult fish spawned at Dworshak NFH in subsequent years from M to U group viruses. While juvenile steelhead trout may still be infected via exposure to IHNV in river water, the disruption of virulent M group IHNV has been successful in dramatically reducing IHN disease in steelhead trout every year since 2010. Statement of relevance Effective reduction of severe mortality due to IHNV. [ABSTRACT FROM AUTHOR]

The transport of live fish is a major sanitary issue for aquaculture. This study addresses two critical steps regarding natural recurrent mortality outbreaks after transport of tambaqui Colossoma macropomum. At first, five mortality outbreaks post-transport of juveniles showing clinical signs suggestive of columnaris were investigated. Flavobacterium columnare was ascertained as the etiological agent in all outbreaks, causing massive mortality up to 72 h after transport. Based on these findings, the next step consisted of a trial to investigate the use of three additives (8 g/L sodium chloride, 11.12 mg/L Melaleuca alternifolia essential oil and 60 mg/L oxytetracycline) in the water of transport, simulating fish farmers' practices to prevent disease outbreak. Juveniles (≅ 8 cm) were transported for 3 h under trade-mimicking conditions, then transferred to 20 tanks (5 groups, 4 replicates) and monitored for 30 days for analysis of health status. Zootechnical parameters (weight gain, total feed intake, feed conversion, specific growth rate and growth analysis) were assessed on days 1, 15 and 30. The best health status was attributed to the group transported with 0.8% NaCl, which showed 100% survival, in addition to being the only group that did not show behavioral changes and clinical signs, indicating the absence of any infectious biological source after transport. A poly-infection (Aeromonas jandaei and F. columnare) was detected in fish from all other groups (M. alternifolia , oxytetracycline and control). Furthermore, fish exposed to sodium chloride showed better weight gain and feed conversion compared to those transported with oxytetracycline (p ≤ 0.05). Then, the pathogenicity of the two isolated strains (A. jandaei and F. columnare) was ascertained by Koch's Postulate assay, which established two emerging pathogens for C. macropomum. This study addresses fundamental information for the development of fish farming and suggests the addition of salt at 0.8% in the water of transport to prevent columnaris and motile Aeromonas septicemia (MAS) associated disease. • Columnaris was identified as a major threat for Colossoma macropomum. • This is the first report of Flavobacterium columnare and Aeromonas jandaei as pathogens for tambaqui. • Salt at 0.8% prevents columnaris and MAS-associated disease after fish transport. • This strategic measure ensures 100% of survival and higher productivity parameters. [ABSTRACT FROM AUTHOR]

Lateral flow immunoassays (LFIAs) are point-of-care detection systems based on the principle of immunochromatography. They have several advantages, including a relatively low manufacturing cost and a simple, rapid, and user-friendly application. A standard LFIA strip generally consists of two target-specific antibodies (one anti-species antibody and one bio-recognition element) and one reaction platform comprised of overlapped membranes, viz., sample pad, conjugate pad, reaction pad, and absorption pad. Depending on the pathogen of interest, preferences of manufacturing materials, reaction formats, selection of target tissue, choice of application buffer, and concentrations of antibodies can vary. Recently, as many as 22 LFIA detection devices have been reported for use in aquaculture, including nine devices for detecting fish pathogens and 13 devices that target shrimp pathogens. In this review article, we will summarize reported findings in terms of their technical aspects, manufacturing elements, and quality parameters for pathogen detection to present cumulative baseline data that might help us develop new LFIA devices for other significant pathogens in aquaculture. • An LFIA is a simple, fast, but very easy-to-use point-of-care detection system. • 22 LFIAs for detecting aquatic pathogens are reviewed in terms of their technical aspect of development. • The quality of LFIAs depends on several factors mostly the selection of antibody, label, and buffer. • Future studies of LFIAs are focused on improving the sensitivity and quality of the assay. [ABSTRACT FROM AUTHOR]

Abstract: The use of plants for herbal medicine in the Indonesian aquaculture is still poorly known. The present study aimed to provide an inventory of the plants used by fish farmers, establishing their respective ethnobotanic importance and identifying the variables that determine the use and the choice of these plants in fish health management. A survey based on a semi-directive questionnaire was conducted using a representative sample of fish farmers (n=504 from 176 villages) from the province of West Java. Of these fish farmers, 46% [41%, 50%; CI95%] of them use plants in their farms and 79 species of plants belonging to 36 families have been identified. Most of these plants were common plants used also traditionally in human pharmacopeia. Four categories of plant use were identified namely: improvement of water quality; reduction of fish stress; increase of fish resistance to pathogens; and treatment of fish diseases (when an outbreak occurred). In order to appraise the significance of plant usage, the following ethnobotanic parameters were determined: the Use Value (UV), the Fidelity Level (FL) and the Informant Consensus Factor (ICF). The Use Value (UV) was generally low for plants except for Carica papaya which reached the highest UV scores. The majority of the plants were used according to personal experience of the fish farmers and the knowledge related to herbal therapy appeared variable among fish farmers. Only 26 species of plants had a UV>0.025. The highest Fidelity Level (FL) value was obtained for C. papaya. The Informant Consensus Factor (ICF) of each plant usage was relatively high for all four categories of use – ranging from 0.78 to 0.88 – but the same plant may have several therapeutic indications. The use of plants, as well as their variety and number, depended not only on fish species, production systems and production areas but also on social characteristics such as the professional experience of fish farmers and their ethnic origin. To our knowledge, this is the first ethnobotanic survey specifically applied to aquaculture. It was able to identify the species of plants used in the West Java Province, and how they are used. It also highlighted the significance of traditional Indonesian herbal therapy in aquaculture. The use of several plants in aquaculture is reported for the first time in this study. [Copyright &y& Elsevier]

Abstract: Salt is commonly recommended as an inexpensive treatment against many fish parasites in freshwater fish culture; however, few studies have scientifically evaluated its efficacy. Amongst the monogeneans, salt has only been previously tested against Gyrodactylus salaris infecting Atlantic salmon (Salmo salar) and G. derjavini infecting rainbow trout (Oncorhynchus mykiss). Here we tested the efficacy of salt treatments against G. bullatarudis and G. turnbulli on guppies (Poecilia reticulata), both commercially important pathogens in the ornamental fish industry. In vitro survival of both parasites was negatively correlated with increasing salinities of 3, 5, 7 and 33gL−1. Parasite establishment on guppies maintained at 0, 3 and 7gL−1 salinity decreased drastically for G. turnbulli from 94% in the control to 78 and 0% on fish in 3 and 7gL−1 salinity, respectively. G. bullatarudis establishment was still 100% at 3gL−1 salinity but was reduced to 73% in 7gL−1. Throughout an infection, parasite populations of both species increased faster on guppies in 3gL−1 salinity compared to dechlorinated water, whereas population growth was severely affected at 7gL−1 salinity. Overall a short duration, high concentration salt bath was most effective at treating gyrodactylid infections: 15min exposure to 25gL−1 salinity for adults or 5min for juvenile fish removed 100% of G. turnbulli or 72% of G. bullatarudis. The results reflect the generalist characteristics of the more tolerant G. bullatarudis compared to G. turnbulli, but have wider implications for treatment application: clearly one treatment regime does not suit all even within a genus. [ABSTRACT FROM AUTHOR]

Abstract: The bacterial cold water disease caused by Flavobacterium psychrophilum is a concern not only for the aquaculture industry, but also the environment. We developed a novel method for detecting F. psychrophilum using high gradient magnetic separation (HGMS) with flow cytometry (FCM). HGMS is a magnetic separation method in which the magnetic force is strengthened by introducing a magnetic gradient between the magnetic filter and nearby column. F. psychrophilum cells that had reacted with immunomagnetic beads were loaded onto the column containing the magnetic filters. When the column was placed inside a neodymium ring magnet, the cells became stuck to the magnetic filters. When magnetic separation of a 500ml sample was performed, more than 80% of the beads were trapped by the magnetic filter. The collected cells were easily removed from the filter by turning off the magnetic field. The cells were reacted with fluorescein isothiocyanate-labeled antibody, and then fluorescence was measured using FCM. The detection sensitivity was three orders of magnitude higher than that of an FCM assay without HGMS. The cell numbers determined by FCM correlated with those obtained using the colony counting method in the range of approximately 101–105 colony forming units per milliliter. One FCM assay could be completed within 60s and the total assay time, including sample preparation, was less than 2.5h. The proposed method using HGMS and FCM allows for rapid and highly sensitive detection of F. psychrophilum. [ABSTRACT FROM AUTHOR]

Abstract: Globally, Salmonella is one of the most important food borne zoonotic pathogens. Norway has a favourable situation regarding Salmonella, as these bacteria are virtually absent in food producing animals and domestically produced food, including products from fish. Consequently, Norway has a low incidence of domestically acquired salmonellosis in the human population. Some Salmonella serovars can however occasionally be found in animal feed and its ingredients, as well as in the feed production facilities. During the years from 2000 to 2004, the prevalence of Salmonella in samples of feed materials varied from 0.14 to 0.33%. Of environmental samples obtained at Norwegian fish feed production facilities, 3.78% of the examined samples were positive. During the same period, the prevalence of Salmonella in Norwegian ready to use compound fish feed were shown to be 0.3%. The predominant serovars found in fish meal were S. Senftenberg and S. Montevideo. The same serovars were dominating in isolates from the production environment, and could in these cases be considered “house strains”. In ready to use compound fish feed, the most common serovars were found to be S. Senftenberg, S. Agona, S. Montevideo and S. Kentucky. Under natural rearing conditions for farmed Atlantic salmon in Norway, and with low concentrations of Salmonella in the feed, the risk of transmission to humans via fish products is minimal. Epidemiological data on salmonellosis from the Norwegian Surveillance System for Communicable Diseases, show that the most common serovars in fish feed ingredients, fish feed and fish feed factories accounts for approximately 2 % of clinical Salmonella isolates from domestically acquired cases in Norway. There is no evidence for transmission of Salmonella from fish feed to humans. However, there is little information on the risk of Salmonella cross-contamination from fish feed, the ingredients and the factories to other parts of society, including wildlife. A probable cross-contamination between fish feed factories and seagulls have been described. The prevalence of antimicrobial resistant Salmonella strains isolated from fish feed, its ingredients and the production environment have so far been very low. [Copyright &y& Elsevier]

The precision of the disc diffusion protocols previously published by Alderman and Smith (Aquaculture 196 (2002) 211) was analysed in a seven-laboratory trial using Escherichia coli ATCC 25922 as the test strain. Discs containing seven antimicrobial agents were employed and 2899 zone size measurements were generated. The total data generated in the trial was used to quantify the intra- and inter-laboratory precisions. The study design also facilitated the investigation of the influence of the source of media and the source of discs on zone sizes. A smaller two-laboratory trial was employed to investigate the influence of incubation time of zone size.The intra-laboratory precision was relatively high with the mean of the coefficients of variation calculated for each laboratory and each agent being 4.7%. In contrast, the inter-laboratory precision was very much lower with the mean of the values for each agent being 11.1%. Significant influences on zone size were detected for all three parameters of the protocol. Media source effects were particularly notable with respect to oxytetracycline and oxolinic acid discs, disc source effects with respect to ampicillin and sulphamethoxazole/trimethoprim discs and incubation times with the ampicillin and amoxycillin discs. ANOVA analysis of the total data set confirmed that inter-laboratory variation was the major factor influencing the low precision of the protocol.The overall precision of the protocols used here was found to be significantly lower than that implied by the control limits associated with the same bacterium in other validated disc diffusion protocols. The implications of these results, for the further development of the protocols under investigation, are discussed. [Copyright &y& Elsevier]

Lipid hydroperoxide (LPO) and hydroxy lipid levels of diseased fishes were investigated. Although significant differences in LPO levels were not always observed between normal and diseased fish, hydroxy lipid levels in muscle and liver were significantly higher in diseased fish than in uninfected normal fish cultured with the same feed in the same cage or in wild normal fish. Hydroxy triglyceride levels of diseased yellowtails with streptococcal infection were significantly elevated in ordinary and dark muscle, and especially in the liver; 1072±78 in diseased fish compared to 632±45 (nmol/g tissue) in normal fish cultured with the same feed. In this case, the polyunsaturated fatty acid contents of the diseased fish were decreased. This was notable, especially with respect to the decrease of docosahexaenoic acid. These trends, namely, increased accumulation of hydroxy lipids in the liver of diseased fish and decreased levels of polyunsaturated fatty acids, were observed in sweetfish with ulcer disease, globefish with Heterobothrium okamotoi infection and/or emaciation disease, red sea bream infected with edwardsiellosis and/or Vibrio anguillarum, yellowtail with jaundiced disease, and carp with cold water disease. [Copyright &y& Elsevier]

Different vegetable oils have been investigated as potential substitutes of fish oil in aquaculture feed and several of them have proven to be successful in terms of fish growth rates, survival rates, biometric indices or feeding efficiency, even when used as the only oils in the feed. However, final fish product composition (fish whole body or fish fillet) usually reflects feed composition, and fish products from fish fed with only vegetable oils usually show a deficiency in long chain polyunsaturated fatty acids (PUFAs). The welfare and health consequences for the fish of these alterations of the fatty acid profile are not well known. In the current study, three experimental diets using supercritically defatted fish meal as well as rapeseed oil as a 100% replacement of fish oil, and differing in essential fatty acid (eicosapentaneoid acid - EPA, docosahexaenoic acid - DHA) supplementation, were tested in rainbow trout, Oncorhynchus mykiss. The growth performance, feed conversion ratio and digestive enzyme profiles were evaluated. Further, stress-and infection-challenge experiments were performed to assess the robustness of the fish against stress and disease. The trout fry fed the experimental diets showed a different fatty acid profile than fish fed a commercial diet, with lower EPA and DHA levels but higher linolenic acid (18:3n-3) and oleic acid (18:1n-9) levels. No significant effects were found in growth performance, feed conversion ratio or digestive enzymatic activity among diets. The fish stress responsiveness (brain and plasma stress marker response) to acute and repeated stressors was in general also similar irrespective of the diet. Infection trials, however, showed a differential ability of the fish to survive upon a controlled infection with the bacterium Flavobacterium psychrophilum. Fish fed with the experimental diets displayed higher mortalities than fish fed a commercial trout fry diet. The current results highlight the need of further research on nutritional requirements to optimize the disease resilience in farmed fish. These results also demonstrate that the adaptation of the fish to new formulated feed (in this case associated to plant oil ingredients) should not be assessed solely based on the growth performance of the fish. Other aquaculture relevant welfare indicators, as disease or stress resilience, should be part of a more complete assessment of feed adequacy for fish farming purposes. • Total dietary replacement of fish oil by rapeseed oil was tested on rainbow trout. • Effects of essential fatty acid supplementation (EPA and DHA) were also assessed. • Oil replacement or fatty acid supplementation did not affect early growth performance. • Survival upon an infection trial was compromised with the substituted diets. • Results demonstrate the need of a wide approach when testing innovative diets in fish. [ABSTRACT FROM AUTHOR]

Among the antimicrobial peptides (AMPs), which act as natural antibiotics in preventing the colonization of pathogens, LL-37 is noteworthy. It is a human cathelicidin characterized as a broad-spectrum antibiotic, with chemotactic and immunomodulatory properties. This study aimed to evaluate the antimicrobial activity and therapeutic effectiveness of peptide LL-37 against streptococcosis caused by Streptococcus agalactiae in Nile tilapia and to assess some of its potential adverse effects (in vivo haematological, biochemical, and immunological profiles) in the fish. After in vitro cytotoxicity studies of LL-37 against Nile tilapia red blood cells (RBCs) and antimicrobial susceptibility testing (minimum inhibitory concentration and minimum bactericidal concentration, MIC and MBC, respectively) against Aeromonas hydrophila and S. agalactiae , fish were divided into six experimental groups. G1 and G2 were challenged by intraperitoneal injection with S. agalactiae and received a single oral dose of LL-37 (15 mg/kg bw) and florfenicol (FFC) (15 mg/kg bw), respectively. G3 was challenged with S. agalactiae and received a single intraperitoneal dose of LL-37 (15 mg/kg bw). The other groups were controls for infection (G4) and antimicrobial administration (G5 and G6). LL-37 showed weak hemolytic activity against Nile tilapia RBCs (24.4% lysis at 625 μg/mL) and strong in vitro antimicrobial activity against S. agalactiae (MIC and MBC = 31.25 μg/mL). No significant effects were observed in the haematological, biochemical, and immunological profiles of fish medicated with LL-37 or FFC, compared to the control fish. Twenty days after the challenge, survival rates were 33.38% (G1), 38.51% (G2), 23.12% (G3), 25% (G4), and 100% (G5 and G6). Despite the strong in vitro antimicrobial activity, both antimicrobials were ineffective to control streptococcosis caused by S. agalactiae in Nile tilapia. These findings suggest that administration of LL-37 (15 mg/kg bw) is safe for Nile tilapia but not effective to control streptococcosis. • This is the first report of antimicrobial activity of LL-37 against S. agalactiae and its effects in Nile tilapia. • LL-37 showed strong antimicrobial activity against S. agalactiae and weak hemolytic activity against Nile tilapia RBCs. • The pathogen does not persist in surviving fish after administration of LL-37 (15 mg/kg). • Administration of LL-37 (15 mg/kg) is safe for Nile tilapia but not effective to control streptococose. [ABSTRACT FROM AUTHOR]

Microsporidia are emerging as a threat to productivity of commercial finfish aquaculture because of high-prevalence infections facilitated by direct transmission. Infection dynamics are poorly understood for most microsporidians and inability to reliably quantify differential infections impedes disease mitigation strategies that could select broodstock with inherent disease-resistant phenotypes. Moreover, investigations of suitable biomarkers that link immune function with disease susceptibility of fishes are limited. Herein, we observe an inverse relationship between fish growth and infection status with the microsporidium Loma morhua among 50 different Atlantic cod (Gadus morhua) family lines. We present a standardized template for quantifying parasite infection intensity, with the spleen being the most reliable organ for comparing natural infections and for monitoring experimental infections. Variability in infection intensities among 50 cod families suggests that differential susceptibility to L. morhua has a genetic basis. Ultra-deep Illumina sequencing identified 299 major histocompatibility class I (MH-I) variants from this regionally restricted 50-family cod population. Variation in Atlantic cod susceptibility might reflect hyper-diversification of MH-I genes and individual variability in the ability to present endogenous pathogen antigens during cell-mediated immune responses. Relevance of MH-I expansion to selective breeding for disease resistance remains poorly characterized. Importantly, differential susceptibility to L. morhua revealed impaired fish growth, resulting in a 14% reduction in fillet mass in susceptible individuals. These data imply that relative resistance to L. morhua exists in cod, that it is heritable, and that broodstock selection could be used to limit the impact of microsporidian infections on growth during finfish aquaculture. • Loma morhua infections in cultured Atlantic cod highlight novel approaches to quantify infections for broodstock selection. • Variation in L. morhua infection intensity among 50 Atlantic cod family lines supports differential genetic susceptibility. • Differential susceptibility to L. morhua reveals impaired growth and 14% reduction in fillet mass in heavily-infected fish. [ABSTRACT FROM AUTHOR]