Routes of entry of Piscirickettsia salmonis in rainbow trout Oncorhynchus mykiss

Salmo salar Skin and Gill Microbiome during Piscirickettsia salmonis Infection

Maintaining the high overall health of farmed animals is a central tenant of their well-being and care. Intense animal crowding in aquaculture promotes animal morbidity especially in the absence of straightforward methods for monitoring their health. Here, we used bacterial 16S ribosomal RNA gene sequencing to measure bacterial population dynamics during P. salmonis infection. We observed a complex bacterial community consisting of a previously undescribed core pathobiome. Notably, we detected Aliivibrio wodanis and Tenacibaculum dicentrarchi on the skin ulcers of salmon infected with P. salmonis, while Vibrio spp. were enriched on infected gills. The prevalence of these co-occurring networks indicated that coinfection with other pathogens may enhance P. salmonis pathogenicity.

Dynamics of co-infection in fish: A review of pathogen-host interaction and clinical outcome

Co-infections can affect the transmission of a pathogen within a population and the pathogen's virulence, ultimately affecting the disease's dynamics. In addition, co-infections can potentially affect the host's immunological responses, clinical outcomes, survival, and disease control efficacy. Co-infections significantly impact fish production and can change several fish diseases' progression and severity. However, the effect of co-infection has only recently garnered limited attention in aquatic animals such as fish, and there is currently a dearth of studies on this topic. This study, therefore, presents an in-depth summary of the dynamics of co-infection in fish. This study reviewed the co-infection of fish pathogens, the interaction of pathogens and fish, clinical outcomes and impacts on fish immune responses, and fish survival. Most studies described the prevalence of co-infections in fish, with various parameters influencing their outcomes. Bacterial co-infection increased fish mortality, ulcerative dermatitis, and intestinal haemorrhage. Viral co-infection resulted in osmoregulatory effects, increased mortality and cytopathic effect (CPE). More severe histological alterations and clinical symptoms were related to the co-infection of fish than in single-infected fish. In parasitic co-infection, there was increased mortality, high kidney swelling index, and severe necrotic alterations in the kidney, liver, and spleen. In other cases, there were more severe kidney lesions, cartilage destruction and displacement. There was a dearth of information on mitigating co-infections in fish. Therefore, further studies on the mitigation strategies of co-infections in fish will provide valuable insights into this research area. Also, more research on the immunology of co-infection specific to each fish pathogen class (bacteria, viruses, fungi, and parasites) is imperative. The findings from such studies would provide valuable information on the relationship between fish immune systems and targeted responses.

Biofilm formation of pathogenic bacteria isolated from aquatic animals

Bacterial biofilm formation is one of the dynamic processes, which facilitates bacteria cells to attach to a surface and accumulate as a colony. With the help of biofilm formation, pathogenic bacteria can survive by adapting to their external environment. These bacterial colonies have several resistance properties with a higher survival rate in the environment. Especially, pathogenic bacteria can grow as biofilms and can be protected from antimicrobial compounds and other substances. In aquaculture, biofilm formation by pathogenic bacteria has emerged with an increased infection rate in aquatic animals. Studies show that Vibrio anguillarum, V. parahaemolyticus, V. alginolyticus, V. harveyi, V. campbellii, V. fischeri, Aeromonas hydrophila, A. salmonicida, Yersinia ruckeri, Flavobacterium columnare, F. psychrophilum, Piscirickettsia salmonis, Edwardsiella tarda, E. ictaluri, E. piscicida, Streptococcus parauberis, and S. iniae can survive in the environment by transforming their planktonic form to biofilm form. Therefore, the present review was intended to highlight the principles behind biofilm formation, major biofilm-forming pathogenic bacteria found in aquaculture systems, gene expression of those bacterial biofilms and possible controlling methods. In addition, the possibility of these pathogenic bacteria can be a serious threat to aquaculture systems.

Why Does Piscirickettsia salmonis Break the Immunological Paradigm in Farmed Salmon? Biological Context to Understand the Relative Control of Piscirickettsiosis

Piscirickettsiosis (SRS) has been the most important infectious disease in Chilean salmon farming since the 1980s. It was one of the first to be described, and to date, it continues to be the main infectious cause of mortality. How can we better understand the epidemiological situation of SRS? The catch-all answer is that the Chilean salmon farming industry must fight year after year against a multifactorial disease, and apparently only the environment in Chile seems to favor the presence and persistence of Piscirickettsia salmonis. This is a fastidious, facultative intracellular bacterium that replicates in the host’s own immune cells and antigen-presenting cells and evades the adaptive cell-mediated immune response, which is why the existing vaccines are not effective in controlling it. Therefore, the Chilean salmon farming industry uses a lot of antibiotics—to control SRS—because otherwise, fish health and welfare would be significantly impaired, and a significantly higher volume of biomass would be lost per year. How can the ever-present risk of negative consequences of antibiotic use in salmon farming be balanced with the productive and economic viability of an animal production industry, as well as with the care of the aquatic environment and public health and with the sustainability of the industry? The answer that is easy, but no less true, is that we must know the enemy and how it interacts with its host. Much knowledge has been generated using this line of inquiry, however it remains insufficient. Considering the state-of-the-art summarized in this review, it can be stated that, from the point of view of fish immunology and vaccinology, we are quite far from reaching an effective and long-term solution for the control of SRS. For this reason, the aim of this critical review is to comprehensively discuss the current knowledge on the interaction between the bacteria and the host to promote the generation of more and better measures for the prevention and control of SRS.

Transcriptome Profiling of Atlantic Salmon (Salmo salar) Parr With Higher and Lower Pathogen Loads Following Piscirickettsia salmonis Infection

Salmonid rickettsial septicemia (SRS), caused by Piscirickettsia salmonis, is one of the most devastating diseases of salmonids. However, the transcriptomic responses of Atlantic salmon (Salmon salar) in freshwater to an EM-90-like isolate have not been explored. Here, we infected Atlantic salmon parr with an EM-90-like isolate and conducted time-course qPCR analyses of pathogen load and four biomarkers (campb, hampa, il8a, tlr5a) of innate immunity on the head kidney samples. Transcript expression of three of these genes (except hampa), as well as pathogen level, peaked at 21 days post-injection (DPI). Multivariate analyses of infected individuals at 21 DPI revealed two infection phenotypes [lower (L-SRS) and higher (H-SRS) infection level]. Five fish from each group (Control, L-SRS, and H-SRS) were selected for transcriptome profiling using a 44K salmonid microarray platform. We identified 1,636 and 3,076 differentially expressed probes (DEPs) in the L-SRS and H-SRS groups compared with the control group, respectively (FDR = 1%). Gene ontology term enrichment analyses of SRS-responsive genes revealed the activation of a large number of innate (e.g. “phagocytosis”, “defense response to bacterium”, “inflammatory response”) and adaptive (e.g. “regulation of T cell activation”, “antigen processing and presentation of exogenous antigen”) immune processes, while a small number of general physiological processes (e.g. “apoptotic process”, development and metabolism relevant) was enriched. Transcriptome results were confirmed by qPCR analyses of 42 microarray-identified transcripts. Furthermore, the comparison of individuals with differing levels of infection (H-SRS vs. L-SRS) generated insights into the biological processes possibly involved in disease resistance or susceptibility. This study demonstrated a low mortality (~30%) EM-90-like infection model and broadened the current understanding of molecular pathways underlying P. salmonis-triggered responses of Atlantic salmon, identifying biomarkers that may assist to diagnose and combat this pathogen.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

Nutrient Scarcity in a New Defined Medium Reveals Metabolic Resistance to Antibiotics in the Fish Pathogen Piscirickettsia salmonis

Extensive use of antibiotics has been the primary treatment for the Salmonid Rickettsial Septicemia, a salmonid disease caused by the bacterium Piscirickettsia salmonis. Occurrence of antibiotic resistance has been explored in various P. salmonis isolates using different assays; however, P. salmonis is a nutritionally demanding intracellular facultative pathogen; thus, assessing its antibiotic susceptibility with standardized and validated protocols is essential. In this work, we studied the pathogen response to antibiotics using a genomic, a transcriptomic, and a phenotypic approach. A new defined medium (CMMAB) was developed based on a metabolic model of P. salmonis. CMMAB was formulated to increase bacterial growth in nutrient-limited conditions and to be suitable for performing antibiotic susceptibility tests. Antibiotic resistance was evaluated based on a comprehensive search of antibiotic resistance genes (ARGs) from P. salmonis genomes. Minimum inhibitory concentration assays were conducted to test the pathogen susceptibility to antibiotics from drug categories with predicted ARGs. In all tested P. salmonis strains, resistance to erythromycin, ampicillin, penicillin G, streptomycin, spectinomycin, polymyxin B, ceftazidime, and trimethoprim was medium-dependent, showing resistance to higher antibiotic concentrations in the CMMAB medium. The mechanism for antibiotic resistance to ampicillin in the defined medium was further explored and was proven to be associated to a decrease in the bacterial central metabolism, including the TCA cycle, the pentose-phosphate pathway, energy production, and nucleotide metabolism, and it was not associated with decreased growth rate of the bacterium or with the expression of any predicted ARG. Our results suggest that nutrient scarcity plays a role in the bacterial antibiotic resistance, protecting against the detrimental effects of antibiotics, and thus, we propose that P. salmonis exhibits a metabolic resistance to ampicillin when growing in a nutrient-limited medium.

The importance of the Atlantic salmon peritoneal cavity B cell response: Local IgM secreting cells are predominant upon Piscirickettsia salmonis infection

The intraperitoneal route is favored for administration of inactivated and attenuated vaccines in Atlantic salmon. Nevertheless, the immune responses in the teleost peritoneal cavity (PerC) are still incompletely defined. In this study, we investigated the B cell responses after intraperitoneal Piscirickettsia salmonis (P. salmonis) challenge of Atlantic salmon, focusing on the local PerC response versus responses in the lymphatic organs: spleen and head kidney. We observed a major increase of leukocytes, total IgM antibody secreting cells (ASC), and P. salmonis-specific ASC in the PerC at 3- and 6-weeks post infection (wpi). The increase in ASC frequency was more prominent in the spleen and PerC compared to the head kidney during the observed 6 wpi. The serum antibody response included P. salmonis-specific antibodies and non-specific antibodies recognizing the non-related bacterial pathogen Yersinia ruckeri and the model antigen TNP-KLH. Finally, we present evidence that supports a putative role for the adipose tissue in the PerC immune response.

Horizontal transmission of Piscirickettsia salmonis from the wild sub-Antarctic notothenioid fish Eleginops maclovinus to rainbow trout (Oncorhynchus mykiss) under experimental conditions

Piscirickettsia salmonis is the aetiological agent of piscirickettsiosis, a bacterial disease that affects farmed salmonids, causing high mortalities and significant economic losses in the Chilean salmon farm industry. Given the Chilean native fish species Patagonian blenny, Eleginops maclovinus, lives in the vicinity of salmon farms, it is relevant to clarify the epidemiological role that this species could play in the transmission and/or dissemination of this pathogen. This study aimed to evaluate the bidirectional transmission of P. salmonis between the Patagonian blenny and Oncorhynchus mykiss (rainbow trout), via a cohabitation challenge model. The results of this study demonstrated the transmission of the bacteria from Patagonian blennies to rainbow trout, considering the specific mortality in cohabitant rainbow trout, reaching 46%: the necropsy of these specimens, evidencing the characteristic pathological lesions of the disease and the positive results of the qPCR analysis for P. salmonis, in the same individuals. In contrast, no mortalities of Patagonian blenny specimens were recorded in the challenged experimental groups. This study is the first report showing the horizontal transmission of P. salmonis from a native non-salmonid species, such as the Patagonian blenny, to a salmonid species, generating the disease and specific mortality in rainbow trout, using a cohabitation challenge.

Investigating mechanisms underlying genetic resistance to Salmon Rickettsial Syndrome in Atlantic salmon using RNA sequencing

Background

Salmon Rickettsial Syndrome (SRS), caused by Piscirickettsia salmonis, is one of the primary causes of morbidity and mortality in Atlantic salmon aquaculture, particularly in Chile. Host resistance is a heritable trait, and functional genomic studies have highlighted genes and pathways important in the response of salmon to the bacteria. However, the functional mechanisms underpinning genetic resistance are not yet well understood. In the current study, a large population of salmon pre-smolts were challenged with P. salmonis, with mortality levels recorded and samples taken for genotyping. In parallel, head kidney and liver samples were taken from animals of the same population with high and low genomic breeding values for resistance, and used for RNA-Sequencing to compare their transcriptome profile both pre and post infection.

Results

A significant and moderate heritability (h² = 0.43) was shown for the trait of binary survival. Genome-wide association analyses using 38 K imputed SNP genotypes across 2265 animals highlighted that resistance is a polygenic trait. Several thousand genes were identified as differentially expressed between controls and infected samples, and enriched pathways related to the host immune response were highlighted. In addition, several networks with significant correlation with SRS resistance breeding values were identified, suggesting their involvement in mediating genetic resistance. These included apoptosis, cytoskeletal organisation, and the inflammasome.

Conclusions

While resistance to SRS is a polygenic trait, this study has highlighted several relevant networks and genes that are likely to play a role in mediating genetic resistance. These genes may be future targets for functional studies, including genome editing, to further elucidate their role underpinning genetic variation in host resistance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07443-2.

Case definitions for skin lesion syndromes in chinook salmon farmed in Marlborough Sounds, New Zealand

Skin lesions are commonly reported in farmed salmonids. Chinook salmon (Oncorhynchus tshawytscha), introduced from California USA, is the only salmonid species commercially farmed in New Zealand, although trout are cultured for release by Fish and Game New Zealand. There are several farming areas in New Zealand, including Marlborough Sounds, Canterbury and Stewart Island. While the industry has not been affected by any of the major production diseases seen internationally, skin lesions have been recorded with an increase in prevalence from 2012 onwards in the Marlborough Sounds and were associated with elevated summer mortalities on farms in 2015. Here we are proposing case definitions for different types of skin lesions for future monitoring and research. Based on our current understanding of the above skin conditions, we developed case definitions for spots, spreading spots, ulcerated spreading spots and Regular Outline Flank Ulcers defining a positive case for three study units, including individual fish, pen/unit and farm.

Assessing the impacts of skin mucus from Salmo salar and Oncorhynchus mykiss on the growth and in vitro infectivity of the fish pathogen Piscirickettsia salmonis

Piscirickettsiosis is a fish disease caused by the facultative intracellular bacterium, Piscirickettsia salmonis. Even though entry routes of P. salmonis in fish are not fully clear yet, the skin seems to be the main portal in some salmonid species. Despite the importance of fish mucous skin barrier in fighting waterborne pathogens, the interaction between salmonid skin mucus and the bacterium is unknown. This study seeks to determine the in vitro changes in the growth of two Chilean P. salmonis strains (LF-89-like and EM-90-like genotypes) and the type strain LF-89^T under exposures to skin mucus from Salmo salar and Oncorhynchus mykiss, as well as changes in the cytotoxic effect of P. salmonis on the SHK-1 cells following exposures. The results suggest that the growth of three P. salmonis strains was not significantly negatively affected under exposures to skin mucus (adjusted at 100 μg total protein ml^-1 ) of O. mykiss (69 ± 18 U lysozyme ml^-1 ) and S. salar (48 ± 33 U lysozyme ml^-1 ) over time. However, the cytotoxic effect of P. salmonis, pre-exposed to salmonid skin mucus, on the SHK-1 cell line was reliably identified only towards the end of the incubation period, suggesting that the mucus had a delaying effect on the cytotoxic response of the cell line to the bacterium. These results represent a baseline knowledge to open new avenues of research intended to understand how P. salmonis faces the fish mucous skin barrier.

Development of a Piscirickettsia salmonis immersion challenge model to investigate the comparative susceptibility of three salmon species

Piscirickettsia salmonis, the aetiological agent of salmonid rickettsial septicaemia (SRS), is a global pathogen of wild and cultured marine salmonids. Here, we describe the development and application of a reproducible, standardized immersion challenge model to induce clinical SRS in juvenile pink (Oncorhynchus gorbuscha), Atlantic (Salmo salar) and sockeye salmon (O. nerka). Following a 1-hr immersion in 10⁵ colony-forming units/ml, cumulative mortality in Atlantic salmon was 63.2% while mortality in sockeye salmon was 10%. Prevalence and levels of the bacterium in kidney prior to onset of mortality were lower in sockeye compared with Atlantic or pink salmon. The timing and magnitude of bacterial shedding were estimated from water samples collected during the exposure trials. Shedding was estimated to be 82-fold higher in Atlantic salmon as compared to sockeye salmon and peaked in the Atlantic salmon trial at 36 d post-immersion. These data suggest sockeye salmon are less susceptible to P. salmonis than Atlantic or pink salmon. Finally, skin lesions were observed on infected fish during all trials, often in the absence of detectable infection in kidney. As a result, we hypothesize that skin is the primary point of entry for P. salmonis during the immersion challenge.

Investigation of routes of entry and dispersal pattern of RGNNV in tissues of European sea bass, Dicentrarchus labrax

Viral encephalopathy and retinopathy (VER) is a serious neuropathological fish disease affecting in the Mediterranean aquaculture mainly European sea bass, Dicentrarchus labrax. It is well known that betanodaviruses are neurotropic viruses that replicate in nerve tissues, preferentially brain and retina. However, routes of entry and progression of the virus in the central nervous system (CNS) remain unclear. The role of four tissues-eye, oesophagus, gills and skin-as possible gateways of a betanodavirus, the redspotted grouper nervous necrosis virus (RGNNV), was investigated after experimental challenges performed on European seabass juveniles. The dispersal pattern of Betanodavirus at primarily stages of the disease was also assessed, using a real-time qPCR assay. The development of typical clinical signs of VER, the presence of characteristic histopathological lesions in the brain and retina and the detection of viral RNA in the tissues of all experimental groups ascertained that successful invasion of RGNNV under all experimental routes was achieved. Transneuronal spread along pathways known to be connected to the initial site of entry seems to be the predominant scenario of viral progression in the CNS. Furthermore, viraemia appeared only after the installation of the infection in the brain.

Ultrastructure of intracytoplasmic Rickettsia-like infection of the gills of the teleost Archosargus probatocephalus (Sparidae) in northeastern Brazil

A histopathological survey was conducted to investigate the presence of microparasites in fish Archosargus probatocephalus in a river near Maceió, Brazil. Light microscope observations of fragments of gill showed the presence of small cysts containing numerous myxospores that were morphologically identified as Henneguya. Transmission electron microscopy observations further revealed several gill cells containing groups of prokaryotic cells within large cytoplasmic vacuoles. Each infected host cell displayed a single vacuole containing a variable number of Rickettsia-like cells (up to 11), some of which presented the dumbbell shape characteristic of binary fission. The Rickettsia-like cells were pleomorphic, without a nucleus and with chromatin dispersed in the cytoplasm. They had a thin electron-dense wall of Gram-negative type. The morphology of these prokaryotic was similar to those of the order Rickettsiales and was described as a Rickettsia-like organism. Histopathological evaluation showed that several vacuole membranes had a lysed appearance. Some had ruptured, thus allowing direct contact between the Rickettsia-like organism and the cytoplasm of the host cell. The rupturing of the branchial epithelium may have contributed towards reduction of the surface area of the gills, but it is not possible to say that this was the cause of the host's death.

Improved understanding of biofilm development by Piscirickettsia salmonis reveals potential risks for the persistence and dissemination of piscirickettsiosis

Piscirickettsia salmonis is the causative agent of piscirickettsiosis, a disease with high socio-economic impacts for Chilean salmonid aquaculture. The identification of major environmental reservoirs for P. salmonis has long been ignored. Most microbial life occurs in biofilms, with possible implications in disease outbreaks as pathogen seed banks. Herein, we report on an in vitro analysis of biofilm formation by P. salmonis Psal-103 (LF-89-like genotype) and Psal-104 (EM-90-like genotype), the aim of which was to gain new insights into the ecological role of biofilms using multiple approaches. The cytotoxic response of the salmon head kidney cell line to P. salmonis showed interisolate differences, depending on the source of the bacterial inoculum (biofilm or planktonic). Biofilm formation showed a variable-length lag-phase, which was associated with wider fluctuations in biofilm viability. Interisolate differences in the lag phase emerged regardless of the nutritional content of the medium, but both isolates formed mature biofilms from 288 h onwards. Psal-103 biofilms were sensitive to Atlantic salmon skin mucus during early formation, whereas Psal-104 biofilms were more tolerant. The ability of P. salmonis to form viable and mucus-tolerant biofilms on plastic surfaces in seawater represents a potentially important environmental risk for the persistence and dissemination of piscirickettsiosis.

Gene expression associated with immune response in Atlantic salmon head-kidney vaccinated with inactivated whole-cell bacterin of Piscirickettsia salmonis and pathogenic isolates

Piscirickettsiosis is the most challenging disease present in the Chilean salmon industry. The aim of this study was to describe the expression of genes associated with immune response of Atlantic salmon intraperitoneally infected with LF-89 and EM-90 Piscirickettsia salmonis and vaccinated with inactivated whole-cell bacterin of P. salmonis. The fish infected with PS-LF-89 showed an anti-inflammatory response, whereas this finding was not observed in the PS-EM-90-infected fish and vaccinated fish. Fish infected with both P. salmonis isolates showed mhc1-mhc2, cd4-cd8b and igm overexpression, suggesting that P. salmonis promotes a T CD4⁺ and T CD8⁺ cell response and a humoral immune response. The vaccinated-fish exhibited mhc1, mhc2 and cd4 overexpression but a significant downregulation of cd8b and igm, suggesting that the vaccine supported the CD4⁺ T-cell response but did not induce an immune response mediated by CD8⁺ T cells or a humoral response. In conclusion, the expression pattern of genes related to the humoral and cell-mediated adaptive immune response showed upregulation in fish infected with P. salmonis and down-regulation in vaccinated fish. The results of this study contribute to our understanding of the immune response against P. salmonis and can be used in the optimization of SRS prevention and control measures.

Pathogenicity of the bacterium New Zealand rickettsia-like organism (NZ-RLO2) in Chinook salmon Oncorhynchus tshawytscha smolt

Farmed New Zealand Chinook salmon Oncorhynchus tshawytscha Walbaum have been found to be infected by rickettsia-like organisms (NZ-RLO). While these Gram-negative intra-cellular bacteria are closely related to Piscirickettsia salmonis, a significant pathogen for farmed salmon globally, the pathogenicity of NZ-RLO is unknown. The aim of the present study was to determine if one strain, NZ-RLO2, causes disease in Chinook salmon. Post-smolt salmon were inoculated with NZ-RLO2 by intraperitoneal injection at high, medium and low doses and observed for 30 d. All fish in the high and medium dosed groups died by the end of the study and 63% of the low dose group died within 30 d of inoculation. Necropsy revealed the fish inoculated with NZ-RLO2 had internal multifocal haemorrhages. The most consistent histological finding in fish inoculated with NZ-RLO2 was neutrophilic and necrotizing pancreatitis and steatitis with intra-cytoplasmic organisms often visible within areas of inflammation. Other histological lesions included multifocal hepatic necrosis, haematopoietic cell necrosis and splenic and renal lymphoid depletion. The presence of NZ-RLO2 within the inoculated fish was confirmed by replication in cell culture and qPCR. The results suggest NZ-RLO2 can cause disease in Chinook salmon and therefore could be a significant pathogen in farmed Chinook salmon.

Significant Differences in Bacterial and Potentially Pathogenic Communities Between Sympatric Hooded Crane and Greater White-Fronted Goose

The gut microbiota of vertebrates play a crucial role in shaping the health of their hosts. However, knowledge of the avian intestinal microbiota has arguably lagged behind that of many other vertebrates. Here, we examine the intestinal bacterial communities of the hooded crane and the greater white-fronted goose at the Shengjin Lake of China, using high-throughput sequencing (Illumina Mi-Seq), and infer the potential pathogens associated with each species. Intestinal bacterial alpha-diversity in the greater white-fronted goose was significantly higher than that in hooded crane. The intestinal bacterial community compositions were significantly different between the two hosts, suggesting that host interactions with specific communities might have profound implications. In addition, potential pathogens were detected in both guts of the two hosts, suggesting that these wild birds might be at risk of disease and probably spread infectious disease to other sympatric vertebrates. The gut of hooded crane carried more potential pathogens than that of the greater white-fronted goose. The potentially pathogenic community compositions were also significantly different between the two hosts, suggesting the divergence of potentially pathogenic communities between hooded crane, and greater white-fronted goose. Finally, bacterial and potentially pathogenic structures showed strong evidence of phylogenic clustering in both hosts, further demonstrating that each host was associated with preferential and defined bacterial and potentially pathogenic communities. Our results argue that more attention should be paid to investigate avian intestinal pathogens which might increase disease risks for conspecifics and other mixed species, and even poultry and human beings.

Piscirickettsia salmonis infection in cultured lumpfish (Cyclopterus lumpus L.)

A Piscirickettsia salmonis infection was diagnosed in lumpfish (Cyclopterus lumpus L.) juveniles held in a marine research facility on the west coast of Ireland. The main clinical signs and pathology included marked ascites, severe multifocal liver necrosis and severe diffuse inflammation and necrosis of the exocrine pancreas and peri-pancreatic adipose tissue. Numerous Piscirickettsia-like organisms were observed by histopathology in the affected organs, and the bacterial species was characterized by molecular analysis. Sequencing of the partial 16S rDNA gene and internal transcribed spacer region showed the lumpfish sequences to be closely related to previously identified Atlantic salmon (Salmo salar L.) sequences from Ireland. To the authors' knowledge, this is the first detection of P. salmonis in lumpfish worldwide. The infection is considered potentially significant in terms of lumpfish health and biosecurity.

Identification of genomic islands in Chilean Piscirickettsia salmonis strains and analysis of gene expression involved in virulence

Piscirickettsia salmonis, an agent of Piscirickettsiosis, is the cause of major losses in the Chilean salmon industry. We identified, characterized and bioinformatically analysed genomic islands in field strains of P. Salmonis, using the bioinformatic software PIPS, that uses the characteristics of the islands of pathogenicity to identify them. We analysed nine partially sequenced genomes in different new field strains, and compared them with the LF-89 (Type strain) genome, selecting a genomic island present in all of them. We then evaluated the relative expression of three genes present in that island. From the obtained results, we conclude that the expression of the tcf gene is directly proportional to the cytopathogenicity in vitro of the bacteria; the product of the dnsa gene could contribute to its pathogenicity, but would be potentiated by one or more factors. The product of the gene liso is necessary for the virulence process and could have functions in early stages of infection. Regarding the strains, the IBM-040 strain showed a significant increase in the expression of all the genes in the study. Contrarily, LF-89 only presented a significant increase in expression of the gene liso, which correlates with the cytopathogenicity in vitro observed in the SHK-1 cells.

Comparative pathogenesis of piscirickettsiosis in Atlantic salmon (Salmo salar L.) post-smolt experimentally challenged with LF-89-like and EM-90-like Piscirickettsia salmonis isolates

Piscirickettsiosis (SRS) is the most prevalent bacterial disease in Chilean salmon aquaculture and is responsible for high economic losses. The aim of this study was to comparatively characterize the pathogenesis of SRS in post-smolt Atlantic salmon during the early and late stages of infection with Piscirickettsia salmonis LF-89-like (PS-LF-89) and EM-90-like (PS-EM-90) using a cohabitation challenge. The pathogenesis of cohabitant fish infected with the two isolates was relatively different due to cohabitant fish infected with PS-EM-90 showing higher cumulative mortality and shorter time until death compared with PS-LF-89 fish. PS-LF-89 caused an SRS infection characterized by kidney and liver lesions, whereas PS-EM-90 caused systemic and haemorrhagic disease characterized by kidney, liver, heart, brain, skeletal muscle and intestine lesions. Decreased serum concentration of total proteins and albumin as well as increased serum ALT, AST and creatinine levels in fish infected with both isolates confirmed that changes in liver and kidney function occurred during infection. Tissue damage, expressed as an SRS histoscore, showed a strong positive correlation with the bacterial load expressed as abundance of P. salmonis 16S rRNA transcripts in the livers and kidneys of fish affected with either isolate, but the correlation was significantly higher in fish infected with PS-EM-90. The results contribute to improving the understanding of the bacteria-host interaction.

Search and analysis of genes involved in antibiotic resistance in Chilean strains of Piscirickettsia salmonis

Piscirickettsia salmonis is the pathogen causing Piscirickettsiosis. For treatment, the industry mainly uses oxytetracycline and florfenicol, so it is essential to understand the degree of susceptibility of this pathogen to these drugs. But this is still unknown for a large number of P. salmonis strains, as are the molecular mechanisms responsible for greater or lesser susceptibility. However, genes that confer resistance to these antimicrobials have been reported and characterized for this and other bacterial species, among which are membrane proteins that take out the drug. Our results identified differences in the degree of susceptibility to both antibiotics among different Chilean isolated of these bacteria. We analysed 10 available genomes in our laboratory and identified ~140 genes likely to be involved in antibiotic resistance. We analysed six specific genes, which suggests that some of them would eventually be relevant in conferring resistance to both antibiotics, as they encode for specific transporter proteins, which increase the number of transcripts when grown in media with these antibiotics. Our results were corroborated with EtBr permeability analysis, which revealed that the LF-89 strain accumulates this compound and has a reduced capacity to expulse it compared with the field strains.

Transcriptome Analysis of the Intracellular Facultative Pathogen Piscirickettsia salmonis: Expression of Putative Groups of Genes Associated with Virulence and Iron Metabolism

The intracellular facultative bacteria Piscirickettsia salmonis is one of the most important pathogens of the Chilean aquaculture. However, there is a lack of information regarding the whole genomic transcriptional response according to different extracellular environments. We used next generation sequencing (NGS) of RNA (RNA-seq) to study the whole transcriptome of an isolate of P. salmonis (FAVET-INBIOGEN) using a cell line culture and a modified cell-free liquid medium, with or without iron supplementation. This was done in order to obtain information about the factors there are involved in virulence and iron acquisition. First, the isolate was grown in the Sf21 cell line; then, the bacteria were cultured into a cell-free liquid medium supplemented or not with iron. We identified in the transcriptome, genes associated with type IV secretion systems, genes related to flagellar structure assembly, several proteases and sigma factors, and genes related to the development of drug resistance. Additionally, we identified for the first time several iron-metabolism associated genes including at least two iron uptake pathways (ferrous iron and ferric iron uptake) that are actually expressed in the different conditions analyzed. We further describe putative genes that are related with the use and storage of iron in the bacteria, which have not been previously described. Several sets of genes related to virulence were expressed in both the cell line and cell-free culture media (for example those related to flagellar structure; such as basal body, MS-ring, C-ring, proximal and distal rod, and filament), which may play roles in other basic processes rather than been restricted to virulence.

Immunization Strategies against Piscirickettsia salmonis Infections: Review of Vaccination Approaches and Modalities and Their Associated Immune Response Profiles

Salmonid rickettsial septicemia (SRS) is a serious, infectious disease in Chilean salmon farming caused by Piscirickettsia salmonis, causing heavy losses to the salmonid industry. P. salmonis belongs to the Gammaproteobacteria, order Thiotrichales. SRS was first described in Chile in 1989, and infection with P. salmonis has since been described from a high number of fish species and in several geographic regions globally. P. salmonis infection of salmonids causes multifocal, necrotic areas of internal organs such as liver, kidney, and spleen. Histologically and immunologically, the tissue response is the formation of granulomas, often with central suppuration. The exact sequence of infection is not known, but bacteria likely gain access to internal organs through mucosal surfaces and when infected, fish carry bacteria in macrophages. It has not been fully determined if the bacterium resides in the cytosol or “hide” within vesicular structures intracellularly, although there are indications that in vitro infection results in actin reorganization and formation of actin-coated vesicle within which the bacterium resides. Protection against lethal challenge is well documented in lab scale experiments, but protection from vaccination has proven more difficult to attain long term under field conditions. Current vaccination protocols include whole cell, inactivated and adjuvanted vaccines for injection for primary immunization followed by oral boost where timing of boost delivery is followed by measuring circulating antibody levels against the pathogen. Documentation also exist that there is correlation between antibody titers and protection against mortality. Future vaccination regimes will likely also include live-attenuated vaccines or other technologies such as DNA vaccination. So far, there is no documentation available for live vaccines and, for DNA vaccines, studies have been unsuccessful under laboratory conditions.

The impact of co-infections on fish: a review

Co-infections are very common in nature and occur when hosts are infected by two or more different pathogens either by simultaneous or secondary infections so that two or more infectious agents are active together in the same host. Co-infections have a fundamental effect and can alter the course and the severity of different fish diseases. However, co-infection effect has still received limited scrutiny in aquatic animals like fish and available data on this subject is still scarce. The susceptibility of fish to different pathogens could be changed during mixed infections causing the appearance of sudden fish outbreaks. In this review, we focus on the synergistic and antagonistic interactions occurring during co-infections by homologous or heterologous pathogens. We present a concise summary about the present knowledge regarding co-infections in fish. More research is needed to better understand the immune response of fish during mixed infections as these could have an important impact on the development of new strategies for disease control programs and vaccination in fish.

Genome-wide association analysis reveals loci associated with resistance against Piscirickettsia salmonis in two Atlantic salmon (Salmo salar L.) chromosomes

Background

Pisciricketssia salmonis is the causal agent of Salmon Rickettsial Syndrome (SRS), which affects salmon species and causes severe economic losses. Selective breeding for disease resistance represents one approach for controlling SRS in farmed Atlantic salmon. Knowledge concerning the architecture of the resistance trait is needed before deciding on the most appropriate approach to enhance artificial selection for P. salmonis resistance in Atlantic salmon. The purpose of the study was to dissect the genetic variation in the resistance to this pathogen in Atlantic salmon.

Methods

2,601 Atlantic salmon smolts were experimentally challenged against P. salmonis by means of intra-peritoneal injection. These smolts were the progeny of 40 sires and 118 dams from a Chilean breeding population. Mortalities were recorded daily and the experiment ended at day 40 post-inoculation. Fish were genotyped using a 50K Affymetrix® Axiom® myDesign^TM Single Nucleotide Polymorphism (SNP) Genotyping Array. A Genome Wide Association Analysis was performed on data from the challenged fish. Linear regression and logistic regression models were tested.

Results

Genome Wide Association Analysis indicated that resistance to P. salmonis is a moderately polygenic trait. There were five SNPs in chromosomes Ssa01 and Ssa17 significantly associated with the traits analysed. The proportion of the phenotypic variance explained by each marker is small, ranging from 0.007 to 0.045. Candidate genes including interleukin receptors and fucosyltransferase have been found to be physically linked with these genetic markers and may play an important role in the differential immune response against this pathogen.

Conclusions

Due to the small amount of variance explained by each significant marker we conclude that genetic resistance to this pathogen can be more efficiently improved with the implementation of genetic evaluations incorporating genotype information from a dense SNP array.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2038-7) contains supplementary material, which is available to authorized users.

Identification and genetic characterization of Piscirickettsia salmonis in native fish from southern Chile

Piscirickettsia salmonis is the etiological agent of piscirickettsiosis, a severe disease causing high mortalities in salmonids. This bacterium has been previously identified and isolated in all cultivated salmonids in Chile and worldwide, including Salmo salar, Oncorhynchus kisutch, and O. mykiss, in addition to being found in non-salmonid species such as Dicentrarchus labrax and Atractoscion nobilis. In this study, the 16S rRNA gene and intergenic spacer ITS-1 of P. salmonis were amplified by PCR from DNA samples extracted from the native Chilean fish species Eleginops maclovinus, Odontesthes regia, Sebastes capensis, and Salilota australis. Analysis of the 16S rRNA sequences from O. regia demonstrated a close phylogenetic relationship with the 16S rRNA gene in the Chilean EM-90 strain. The 16S rRNA sequences from E. maclovinus, S. capensis, and S. australis were related to the Chilean LF-89 sequence and Scottish strains. To confirm these findings, analysis of P. salmonis ITS-1 sequences obtained from the 4 sampled native species demonstrated a high degree of identity and a close phylogenetic relationship with Chilean P. salmonis sequences, including LF-89 and EM-90. These results suggest a strong relationship between the nucleotide sequences from the 16S rRNA and ITS-1 genes amplified from native fish with those sequences described in the first P. salmonis strains to be identified and isolated in Chile.

Patterns of Piscirickettsia salmonis load in susceptible and resistant families of Salmo salar

The pathogen Piscirickettsia salmonis produces a systemic aggressive infection that involves several organs and tissues in salmonids. In spite of the great economic losses caused by this pathogen in the Atlantic salmon (Salmo salar) industry, very little is known about the resistance mechanisms of the host to this pathogen. In this paper, for the first time, we aimed to identify the bacterial load in head kidney and muscle of Atlantic salmon exhibiting differential familiar mortality. Furthermore, in order to assess the patterns of gene expression of immune related genes in susceptible and resistant families, a set of candidate genes was evaluated using deep sequencing of the transcriptome. The results showed that the bacterial load was significantly lower in resistant fish, when compared with the susceptible individuals. Based on the candidate genes analysis, we infer that the resistant hosts triggered up-regulation of specific genes (such as for example the LysC), which may explain a decrease in the bacterial load in head kidney, while the susceptible fish presented an exacerbated innate response, which is unable to exert an effective response against the bacteria. Interestingly, we found a higher bacterial load in muscle when compared with head kidney. We argue that this is possible due to the availability of an additional source of iron in muscle. Besides, the results show that the resistant fish could not be a likely reservoir of the bacteria.

Disease resistance in Atlantic salmon (Salmo salar): coinfection of the intracellular bacterial pathogen Piscirickettsia salmonis and the sea louse Caligus rogercresseyi

Background

Naturally occurring coinfections of pathogens have been reported in salmonids, but their consequences on disease resistance are unclear. We hypothesized that 1) coinfection of Caligus rogercresseyi reduces the resistance of Atlantic salmon to Piscirickettsia salmonis; and 2) coinfection resistance is a heritable trait that does not correlate with resistance to a single infection.

Methodology

In total, 1,634 pedigreed Atlantic salmon were exposed to a single infection (SI) of P. salmonis (primary pathogen) or coinfection with C. rogercresseyi (secondary pathogen). Low and high level of coinfection were evaluated (LC = 44 copepodites per fish; HC = 88 copepodites per fish). Survival and quantitative genetic analyses were performed to determine the resistance to the single infection and coinfections.

Main Findings

C. rogercresseyi significantly increased the mortality in fish infected with P. salmonis (SI mortality = 251/545; LC mortality = 544/544 and HC mortality = 545/545). Heritability estimates for resistance to P. salmonis were similar and of medium magnitude in all treatments (h²_SI = 0.23±0.07; h²_LC = 0.17±0.08; h²_HC = 0.24±0.07). A large and significant genetic correlation with regard to resistance was observed between coinfection treatments (r_g LC-HC = 0.99±0.01) but not between the single and coinfection treatments (r_g SI-LC = −0.14±0.33; r_g SI-HC = 0.32±0.34).

Conclusions/Significance

C. rogercresseyi, as a secondary pathogen, reduces the resistance of Atlantic salmon to the pathogen P. salmonis. Resistance to coinfection of Piscirickettsia salmonis and Caligus rogercresseyi in Atlantic salmon is a heritable trait. The absence of a genetic correlation between resistance to a single infection and resistance to coinfection indicates that different genes control these processes. Coinfection of different pathogens and resistance to coinfection needs to be considered in future research on salmon farming, selective breeding and conservation.

Piscirickettsiosis and Piscirickettsia salmonis in fish: a review

The bacterium Piscirickettsia salmonis is the aetiological agent of piscirickettsiosis a severe disease that has caused major economic losses in the aquaculture industry since its appearance in 1989. Recent reports of P. salmonis or P. salmonis-like organisms in new fish hosts and geographical regions have increased interest in the bacterium. Because this gram-negative bacterium is still poorly understood, many relevant aspects of its life cycle, virulence and pathogenesis must be investigated before prophylactic procedures can be properly designed. The development of effective control strategies for the disease has been limited due to a lack of knowledge about the biology, intracellular growth, transmission and virulence of the organism. Piscirickettsiosis has been difficult to control; the failure of antibiotic treatment is common, and currently used vaccines show variable long-term efficacy. This review summarizes the biology and characteristics of the bacterium, including its virulence; the infective strategy of P. salmonis for survival and evasion of the host immune response; the host immune response to invasion by this pathogen; and newly described features of the pathology, pathogenesis, epidemiology and transmission. Current approaches to the prevention of and treatment for piscirickettsiosis are discussed.

Evidence of exotoxin secretion of Piscirickettsia salmonis, the causative agent of piscirickettsiosis

Piscirickettsia salmonis is the aetiological agent of piscirickettsiosis, a disease which affects a variety of teleost species and that is particularly severe in salmonid fish. Bacterial-free supernatants, obtained from cultures of three isolates of Piscirickettsia salmonis, were inoculated in Atlantic salmon, Salmo salar L., and in three continuous cell lines in an effort to determine the presence of secretion of extracellular products (ECPs) by this microorganism. Although steatosis was found in some liver samples, no mortalities or clinical signs occurred in the inoculated fish. Clear cytotoxicity was observed after inoculation in the cell lines CHSE-214 and ASK, derived from salmonid tissues, but not in MDBK, which is of mammalian origin. The degree of cytotoxicity of the ECPs was different among the P. salmonis isolates tested. The isolate that evidenced the highest cytotoxicity in its ECPs exhibited only an intermediate virulence level after challenging fish with bacterial suspensions of the three P. salmonis isolates. Almost complete inhibition of the cytotoxic activity of ECPs was seen after proteinase K treatment, indicating their peptidic nature, and a total preclusion of the cytotoxicity was shown after their incubation at 50 °C for 30 min. Results show that P. salmonis can produce ECPs and at least some of them are thermolabile exotoxins that probably play a role in the pathogenesis of piscirickettsiosis.

Co-infection patterns of infectious salmon anaemia and sea lice in farmed Atlantic salmon, Salmo salar L., in southern Chile (2007-2009)

Infectious salmon anaemia virus (ISAV) caused a large epidemic in farmed Atlantic salmon in Chile in 2007-2009. Here, we assessed co-infection patterns of ISAV and sea lice (SL) based on surveillance data collected by the fish health authority. ISAV status and SL counts in all Atlantic salmon farms located in the 10th region of Chile were registered monthly from July 2007 through December 2009. Each farm was categorized monthly according to its ISAV and SL status. A multinomial time-space scan test using a circular window was applied to identify disease clusters, and a multivariate regression model was fitted to quantify the association between disease-clustering and farm-management factors. Most of the identified clusters (9/13) were associated with high SL burdens. There were significant associations (P < 0.05) between management factors and ISAV/SL status. Areas in which good management practices were associated with a reduced disease risk were identified. The findings of this study suggest that certain management practices can effectively reduce the risk of SL and ISAV in the face of an epidemic and will be helpful towards creating an effective disease control programme in Chile.

Injuries and deformities in fish: their potential impacts upon aquacultural production and welfare

Fish can be the recipients of numerous injuries that are potentially deleterious to aquacultural production performance and welfare. This review will employ a systematic approach that classifies injuries in relation to specific anatomical areas of the fish and will evaluate the effects of injury upon production and welfare. The selected areas include the (1) mouth, (2) eye, (3) epidermis and (4) fins. These areas cover a large number of external anatomical features that can be injured during aquacultural procedures and husbandry practices. In particular, these injuries can be diagnosed on live fish, in a farm environment. For each anatomical feature, this review addresses (a) its structure and function and (b) defines key injuries that can affect the fish from a production and a welfare perspective. Particular attention is then given to (c) defining known and potential aquacultural risk factors before (d) identifying and outlining potential short- and long-term farming practices and mitigation strategies to reduce the incidence and prevalence of these injuries. The review then concludes with an analysis of potential synergies between risk factors the type of injury, in addition to identifying potential synergies in mitigation strategies. The paper covers both aquaculture and capture-based aquaculture.

Multiple tissue transcriptomic responses toPiscirickettsia salmonisin Atlantic salmon (Salmo salar)

The bacterium Piscirickettsia salmonis is the etiological agent of salmonid rickettsial septicemia (SRS), a severe disease that causes major economic losses to the Atlantic salmon aquaculture industry every year. Little is known about the infective strategy of P. salmonis, which is able to infect, survive within, and replicate inside salmonid macrophages as an intracellular parasite. Similarly there is little knowledge concerning the fish host's response to invasion by this pathogen. We have examined the transcriptional response of postsmolt Atlantic salmon ( Salmo salar) to P. salmonis at 48 h following infection in three tissues, liver, head kidney, and muscle, using an Atlantic salmon oligonucleotide microarray (Salar_2, Agilent 4x44K). The infection led to a large alteration of transcriptional activity in all the tissues studied. In infected salmon 886, 207, and 153 transcripts were differentially expressed in liver, head kidney, and muscle, respectively. Assessment of enrichment for particular biological pathways by gene ontology analysis showed an upregulation of genes involved in oxidative and inflammatory responses in infected fish, indicative of the activation of the innate immune response. The downregulation of genes involved in the adaptive immune response, G protein signaling pathway, and apoptotic process in infected fish may be reflective of mechanisms used by P. salmonis to survive, replicate, and escape host defenses. There was also evidence of differential responses between studied tissues, with protein metabolism being decreased in muscle of infected fish and with a concomitant increase being shown in liver.

"Candidatus Cryptoprodotis polytropus," a novel Rickettsia-like organism in the ciliated protist Pseudomicrothorax dubius (Ciliophora, Nassophorea)

Rickettsia-like organisms (RLO) are obligate, often highly fastidious, intracellular bacterial parasites associated with a variety of vertebrate and invertebrate hosts. Despite their importance as causative agents of severe mortality outbreaks in farmed aquatic species, little is known about their life cycle and their host range. The present work reports the characterization of "Candidatus Cryptoprodotis polytropus," a novel Rickettsia-like bacterium associated with the common ciliate species Pseudomicrothorax dubius by means of the "Full-Cycle rRNA Approach" and ultrastructural observations. The morphological description by in vivo and scanning electron microscopy and the 18S rRNA gene sequence of the host species is provided as well. Phylogenetic analysis based on the 16S rRNA gene supports the inclusion of "Candidatus Cryptoprodotis polytropus" within the family Rickettsiaceae (cl. Alphaproteobacteria) together with the genera Rickettsia and Orientia. Observations on natural ciliate populations account for the occasional nature of this likely parasitic association. The presence of a previously unknown RLO in ciliates sheds a new light on the possible role of protists as transient hosts, vectors or natural reservoir for some economically important pathogens.

In vivo transfer of plasmid pRAS1 between Aeromonas salmonicida and Aeromonas hydrophila in artificially infected Cyprinus carpio L

This study investigated the possible in vivo transfer of plasmid pRAS1 between Aeromonas salmonicida and A. hydrophila inhabiting two different organs of Cyprinus carpio L. To distinguish transconjugants from naturally occurring antibiotic resistant bacteria, twelve luminescent transposon-tagged A. hydrophila strains using mini Tn5luxCDABEKm2 transposon were generated. In conjugal transfer experiments, fish were conditioned with the donor bacteria and subsequently immersed in water containing the recipient strain. Bacteria were recovered from gills and intestines and isolated by growth on selective plates. Transconjugants were identified by their resistance to the pRAS1 encoded antimicrobials and by light emission. In vivo transfer frequencies ranged between 10(-3) and 10(-6) and were somewhat lower in intestines, compared to gills. Transfer frequencies were also smaller relative to those obtained in vitro. The minimal amount of donor and recipient bacteria needed to yield detectable transconjugants in vivo was 1 x 10(4) CFU mL(-1). Implications of this plasmid transfer in natural settings and its possible consequences to human health are discussed.

Atlantic salmon bath challenged with Moritella viscosa--pathogen invasion and host response

The Gram-negative bacterium Moritella viscosa is considered to be the main causative agent of winter ulcer, a disease that primarily affects salmonid fish in sea water during cold periods. The disease is initially characterised by localised swelling of the skin followed by development of lesions. To gain more knowledge of the role of M. viscosa in the pathogenesis of winter ulcer, 159 Atlantic salmon (80-110 g) were exposed to a bath challenge dose of 7 x 10(5) cfu ml(-1) for 1 h at 8.9 degrees C. The first mortalities were registered two days post-challenge and the mortality rate increased rapidly. Multi-organ samples were taken throughout the challenge for culture, immunohistochemistry and PCR analysis. Using real-time PCR, M. viscosa DNA was first detected in the gills of all fish examined 2, 6 and 12 h after challenge. From day 2, the bacterium was detected in the muscle/skin, head kidney, spleen and liver. This was in correlation with positive cultured samples and confirmed systemic infection. The early and consistent detection of M. viscosa DNA in gill samples, and less or not in muscle/skin or intestine, could suggest gills as a port of entry for the bacterium. Immunohistochemical analysis using a polyclonal antiserum against M. viscosa demonstrated generalised staining in the lumen of blood vessels and some positive mononuclear cells. The antigens recognised by the antiserum may have originated from extracellular bacterial products and be part of a bacterial invasion strategy. To better understand the immune response in salmon to M. viscosa infection, the expression profiles of the immune genes IL1 beta, C3, ISG15 and CD83 were studied. Increased expression of IL1 beta and C3 was not induced until day 7, which may suggest that M. viscosa might utilize escape mechanisms to evade the host's immune system by suppressing relevant immune responses.

Microarray analyses identify molecular biomarkers of Atlantic salmon macrophage and hematopoietic kidney response to Piscirickettsia salmonis infection

Piscirickettsia salmonis is the intracellular bacterium that causes salmonid rickettsial septicemia, an infectious disease that kills millions of farmed fish each year. The mechanisms used by P. salmonis to survive and replicate within host cells are not known. Piscirickettsiosis causes severe necrosis of hematopoietic kidney. Microarray-based experiments with QPCR validation were used to identify Atlantic salmon macrophage and hematopoietic kidney genes differentially transcribed in response to P. salmonis infection. Infections were confirmed by microscopy and RT-PCR with pathogen-specific primers. In infected salmon macrophages, 71 different transcripts were upregulated and 31 different transcripts were downregulated. In infected hematopoietic kidney, 30 different transcripts were upregulated and 39 different transcripts were downregulated. Ten antioxidant genes, including glutathione S-transferase, glutathione reductase, glutathione peroxidase, and cytochrome b558 alpha- and beta-subunits, were upregulated in infected macrophages but not in infected hematopoietic kidney. Changes in redox status of infected macrophages may allow these cells to tolerate P. salmonis infection, raising the possibility that treatment with antioxidants may reduce hematopoietic tissue damage caused by this rickettsial infection. The downregulation of transcripts involved in adaptive immune responses (e.g., T cell receptor alpha-chain and C-C chemokine receptor 7) in infected hematopoietic kidney but not in infected macrophages may contribute to infection-induced kidney tissue damage. Molecular biomarkers of P. salmonis infection, characterized by immune-relevant functional annotations and high fold differences in expression between infected and noninfected samples, may aid in the development of anti-piscirickettsial vaccines and therapeutics.

Piscirickettsia salmonis: a Gram-negative intracellular bacterial pathogen of fish

Piscirickettsia salmonis is the first Gram-negative, intracellular bacterial pathogen isolated from fish and is a significant cause of mortality in salmonid fish. Recent reports of P. salmonis or P. salmonis-like organisms from new fish hosts and geographic regions have increased the interest in the bacterium. In this review, the important characteristics of the bacterium including recent taxonomic changes, features of the disease caused by the bacterium including transmission, hosts, reservoirs, diagnostic procedures, and current approaches for prevention and treatment have been discussed. The reader is also directed to other reviews concerning the bacterium and the disease it causes (Fryer & Lannan 1994, 1996; Almendras & Fuentealba 1997; Lannan, Bartholomew & Fryer 1999; House & Fryer 2002; Mauel & Miller 2002).

Piscirickettsiosis and piscirickettsiosis-like infections in fish: a review

Piscirickettsia salmonis was the first "rickettsia-like" bacteria to be recognized as a pathogenic agent of fish. Since the first reports of piscirickettsiosis emerged from Chile in the late 1980s, Piscirickettsia-like bacteria have been recognized with increasing frequency in a variety of fish species, from both fresh and saltwaters around the world. Although the first reported incidents of Piscirickettsia were in salmonids, Piscirickettsia-like bacteria are now being frequently associated with disease syndromes in non-salmonid fish. Mortalities have occurred in white seabass (Atactoscion noblis), black seabass (Dicentrarchus sp.), tilapia (Oreochromis, Tilapia and Sarotherodon spp.) and blue-eyed plecostomus (Panaque suttoni). Piscirickettsiosis and piscirickettsiosis-like diseases have affected aquaculture productivity, profitability, the species of fish compatible with commercial rearing, and transportation of fish from site to site. Piscirickettsiosis and syndromes caused by similar bacteria are an emerging disease complex that will increasingly inhibit fish production.