Confirmation of Piscirickettsia salmonis as a pathogen in European sea bass Dicentrarchus labrax and phylogenetic comparison with salmonid strains

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.

Comparative Analysis of Salmon Cell Lines and Zebrafish Primary Cell Cultures Infection with the Fish Pathogen Piscirickettsia salmonis

Piscirickettsia salmonis is the etiologic agent of piscirickettsiosis, a disease that causes significant losses in the salmon farming industry. In order to unveil the pathogenic mechanisms of P. salmonis, appropriate molecular and cellular studies in multiple cell lines with different origins need to be conducted. Toward that end, we established a cell viability assay that is suitable for high-throughput analysis using the alamarBlue reagent to follow the distinct stages of the bacterial infection cycle. Changes in host cell viability can be easily detected using either an absorbance- or fluorescence-based plate reader. Our method accurately tracked the infection cycle across two different Atlantic salmon-derived cell lines, with macrophage and epithelial cell properties, and zebrafish primary cell cultures. Analyses were also carried out to quantify intracellular bacterial replication in combination with fluorescence microscopy to visualize P. salmonis and cellular structures in fixed cells. In addition, dual gene expression analysis showed that the pro-inflammatory cytokines IL-6, IL-12, and TNFα were upregulated, while the cytokines IL1b and IFNγ were downregulated in the three cell culture types. The expression of the P. salmonis metal uptake and heme acquisition genes, together with the toxin and effector genes ospD3, ymt, pipB2 and pepO, were upregulated at the early and late stages of infection regardless of the cell culture type. On the other hand, Dot/Icm secretion system genes as well as stationary state and nutrient scarcity-related genes were upregulated only at the late stage of P. salmonis intracellular infection. We propose that these genes encoding putative P. salmonis virulence factors and immune-related proteins could be suitable biomarkers of P. salmonis infection. The infection protocol and cell viability assay described here provide a reliable method to compare the molecular and cellular changes induced by P. salmonis in other cell lines and has the potential to be used for high-throughput screenings of novel antimicrobials targeting this important fish intracellular pathogen.

First description and diagnostics of disease caused by Piscirickettsia salmonis in farmed European sea bass (Dicentrarchus labrax Linnaeus) from Croatia

During the winter of 2013 and 2016, several Croatian fish farms experienced mortalities in the fry of European sea bass, Dicentrarchus labrax. Affected fish showed abnormal swimming behaviour and reduced appetite, and death ensued several days after the onset of clinical signs of disease. Necropsy revealed pale liver, empty digestive tract, distended gall bladder, and hyperaemia and congestion of the meninges. Routine bacteriological examination tested negative, and virological examination ruled out nodavirus infection. Histological examination revealed multifocal necrosis and extensive inflammation in the brain with abundant cellular debris in the ventricles. Inflammatory cells displayed intra-cytoplasmic basophilic vacuoles leading to suspicion of Piscirickettsia salmonis infection. Fluorescent in situ hybridization using an oligonucleotide probe targeting Domain Bacterium applied to tissue sections tested positive. The pathogen was identified by 16S rRNA gene sequencing of brain material, and the sequence showed 99% similarity with P. salmonis. This result enabled the design of an oligonucleotide probe specifically targeting P. salmonis. In 2016, P. salmonis was successfully isolated on CHAB from the brain of an affected specimen and identified using 16S rRNA gene sequencing and MALDI-TOF. This study describes the first outbreak of disease caused by P. salmonis in sea bass in Croatia, while new diagnostic tools will enable further research on its epidemiology and pathogenicity.

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.

Development of a Multiplex PCR Assay for Genotyping the Fish Pathogen Piscirickettsia salmonis Through Comparative Genomics

Piscirickettsia salmonis is a bacterial pathogen that severely impact the aquaculture in several countries as Canada, Scotland, Ireland, Norway, and Chile. It provokes Piscirickettsiosis outbreaks in the marine phase of salmonid farming, resulting in economic losses. The monophyletic genogroup LF-89 and a divergent genogroup EM-90 are responsible for the most severe Piscirickettsiosis outbreaks in Chile. Therefore, the development of methods for quick genotyping of P. salmonis genogroups in field samples is vital for veterinary diagnoses and understanding the population structure of this pathogen. The present study reports the development of a multiplex PCR for genotyping LF-89 and EM-90 genogroups based on comparative genomics of 73 fully sequenced P. salmonis genomes. The results revealed 2,322 sequences shared between 35 LF-89 genomes, 2,280 sequences in the core-genome of 38 EM-90 genomes, and 331 and 534 accessory coding sequences each genogroup, respectively. A total of 1,801 clusters of coding sequences were shared among all tested genomes of P. salmonis (LF-89 and EM-90), with 253 and 291 unique sequences for LF-89 and EM-90 genogroups, respectively. The Multiplex-1 prototype was chosen for reliable genotyping because of differences in annealing temperatures and respective reaction efficiencies. This method also identified the pathogen in field samples infected with LF-89 or EM-90 strains, which is not possible with other methods currently available. Finally, the genome-based multiplex PCR protocol presented in this study is a rapid and affordable alternative to classical sequencing of PCR products and analyzing the length of restriction fragment polymorphisms.

Protein-Based Vaccine Protect Against Piscirickettsia salmonis in Atlantic Salmon (Salmo salar)

An effective and economical vaccine against the Piscirickettsia salmonis pathogen is needed for sustainable salmon farming and to reduce disease-related economic losses. Consequently, the aquaculture industry urgently needs to investigate efficient prophylactic measures. Three protein-based vaccine prototypes against Piscirickettsia salmonis were prepared from a highly pathogenic Chilean isolate. Only one vaccine effectively protected Atlantic salmon (Salmo salar), in correlation with the induction of Piscirickettsia-specific IgM antibodies and a high induction of transcripts encoding pro-inflammatory cytokines (i.e., Il-1β and TNF-α). In addition, we studied the proteome fraction protein of P. salmonis strain Austral-005 using multidimensional protein identification technology. The analyzes identified 87 proteins of different subcellular origins, such as the cytoplasmic and membrane compartment, where many of them have virulence functions. The other two prototypes activated only the innate immune responses, but did not protect Salmo salar against P. salmonis. These results suggest that the knowledge of the formulation of vaccines based on P. salmonis proteins is useful as an effective therapy, this demonstrates the importance of the different research tools to improve the study of the different immune responses, resistance to diseases in the Atlantic salmon. We suggest that this vaccine can help prevent widespread infection by P. salmonis, in addition to being able to be used as a booster after a primary vaccine to maintain high levels of circulating protective antibodies, greatly helping to reduce the economic losses caused by the pathogen.

Histological features of Rickettsia-like organisms in the European flat oyster (Ostrea edulis L.)

The European flat oyster (Ostrea edulis L.) represents an economically important oyster production in Southern Italy, widespread in natural beds along the coast. The practice to be eaten raw is an everlasting concern for possible health risk with a need to stringently monitor the health of aquatic environment. A screening survey using histopathological examination was undertaken by harvesting O. edulis from different sites along the Apulian coast of Italy. Tissue samples of the digestive gland, kidney, gonad, and gill were provided for morphologic study in light microscopy (LM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) analysis. The LM observations revealed spherical cytoplasmic inclusions as basophilic prokaryote colonies in 13/250 oysters. The TEM and SEM confirmed the presence of intracytoplasmic inclusions of Rickettsia-like organisms (RLOs), merely in the epithelial cells of the digestive gland tubule tissues in the 13 oysters. Within intracytoplasmic vacuoles, RLOs exhibited a prokaryotic characteristic ultrastructure with transverse binary fission, a DNA zone full of chromatin fibers and a granular periplasmic ribosome zone. O. edulis were found positive for RLOs in wild oysters from Manfredonia, while the other sites were found free of pathological inclusions. Thus, we present the first report of a Rickettsia-like infection in the Apulian wild oyster (O. edulis) from Italy, including an ultrastructural description and pathological characterization.

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.

Apoptosis inhibition of Atlantic salmon (Salmo salar) peritoneal macrophages by Piscirickettsia salmonis

To improve the understanding of the piscirickettsiosis pathogenesis, the in vivo apoptosis modulation of peritoneal macrophages and lymphocytes was studied in juvenile Salmo salar intraperitoneally injected with Piscirickettsia salmonis. Five fish were sampled at post-exposure days 1, 5, 8 (preclinical), 20 (clinical) and 40 (post-clinical period of the disease), and the leucocytes of their coelomic washings were analysed by flow cytometry (using the JC-1 cationic dye), TUNEL and cytology to detect apoptotic cells. A selective and temporal pattern of apoptosis modulation by P. salmonis infection was observed. Apoptosis in lymphocytes was not affected, whereas it was inhibited in macrophages but only during the preclinical stage of the induced piscirickettsiosis. Hence, it is postulated that P. salmonis inhibits macrophage apoptosis at the beginning of the disease development to survive, multiply and probably be transported inside these phagocytes; once this process is complete, macrophage apoptosis is no longer inhibited, thus facilitating the exit of the bacteria from the infected cells for continuing their life cycle.

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.

Isolation of Tasmanian Rickettsia-like organism (RLO) from farmed salmonids: identification of multiple serotypes and confirmation of pathogenicity

Atlantic salmon Salmo salar L. farmed in south-east Tasmania, Australia, are susceptible to infection by the Tasmanian Rickettsia-like organism (TRLO), a Gram-negative bacterium. Here, we report the first isolation of TRLO from south-east Tasmania in pure culture and show that the bacterium is culturable on both specialised enriched agar and in cell culture using the CHSE-214 cell line. In vitro cultured TRLO was used to reproducibly elicit disease in Atlantic salmon parr held in fresh water. In inoculated fish, TRLO was observed intracytoplasmically in peripheral blood leucocytes, suggesting that these cells are responsible for haematogenous dispersal of the bacterium within the host. Fish with experimentally induced disease presented with gross and histopathological changes similar to TRLO-infected fish at commercial marine farms. TRLO was also isolated in culture from farmed Atlantic salmon in the Tamar River and Macquarie Harbour production areas in Tasmania, both of which have no history of TRLO-associated disease. These TRLO isolates appear to be serologically distinct from each other as well as from isolates obtained from south-east Tasmania, linking each serotype to a specific geographical location within Tasmania. Despite the lack of clinical evidence of TRLO-linked disease in fish grown in the Tamar River and Macquarie Harbour, experimental infection trials demonstrably showed the pathogenic potential of these TRLO serovars. Together, these data provide evidence that TRLO is a fastidious, facultative intracellular bacterium and confirm TRLO as a pathogen of Atlantic salmon, causing a disease designated Tasmanian salmonid rickettsiosis.

Draft Genome Sequence of a New Zealand Rickettsia-Like Organism Isolated from Farmed Chinook Salmon

We report here the draft genome sequence of a rickettsia-like organism, isolated from a New Zealand Chinook salmon farm experiencing high mortality. The genome is approximately 3 Mb in size, has a G+C content of approximately 39.2%, and is predicted to contain 2,870 coding sequences.

Mussel biofiltration effects on attached bacteria and unicellular eukaryotes in fish-rearing seawater

Mussel biofiltration is a widely used approach for the mitigation of aquaculture water. In this study, we investigated the effect of mussel biofiltration on the communities of particle-associated bacteria and unicellular eukaryotes in a sea bass aquaculture in southern North Sea. We assessed the planktonic community changes before and after biofiltration based on the diversity of the 16S and 18S rRNA genes by using next generation sequencing technologies. Although there was no overall reduction in the operational taxonomic units (OTU) numbers between the control (no mussels) and the test (with mussels) tanks, a clear reduction in the relative abundance of the top three most dominant OTUs in every sampling time was observed, ranging between 2-28% and 16-82% for Bacteria and Eukarya, respectively. The bacterial community was dominated by OTUs related to phytoplankton blooms and/or high concentrations of detritus. Among the eukaryotes, several fungal and parasitic groups were found. Their relative abundance in most cases was also reduced from the control to the test tanks; a similar decreasing pattern was also observed for both major higher taxa and functional (trophic) groups. Overall, this study showed the effectiveness of mussel biofiltration on the decrease of microbiota abundance and diversity in seawater fueling fish farms.

Fast detection of Piscirickettsia salmonis in Salmo salar serum through MALDI-TOF-MS profiling

Piscirickettsia salmonis is a pathogenic bacteria known as the aetiological agent of the salmonid rickettsial syndrome and causes a high mortality in farmed salmonid fishes. Detection of P. salmonis in farmed fishes is based mainly on molecular biology and immunohistochemistry techniques. These techniques are in most of the cases expensive and time consuming. In the search of new alternatives to detect the presence of P. salmonis in salmonid fishes, this work proposed the use of MALDI-TOF-MS to compare serum protein profiles from Salmo salar fish, including experimentally infected and non-infected fishes using principal component analysis (PCA). Samples were obtained from a controlled bioassay where S. salar was challenged with P. salmonis in a cohabitation model and classified according to the presence or absence of the bacteria by real time PCR analysis. MALDI spectra of the fish serum samples showed differences in its serum protein composition. These differences were corroborated with PCA analysis. The results demonstrated that the use of both MALDI-TOF-MS and PCA represents a useful tool to discriminate the fish status through the analysis of salmonid serum samples.

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.

Stocking density and Piscirickettsia salmonis infection effect on Patagonian blennie (Eleginops maclovinus, Cuvier 1830) skeletal muscle intermediate metabolism

The need to expand aquaculture production has led to other fish to be considered as potential species for culture, such as the sub-Antarctic notothenioid Eleginops maclovinus (Valenciennes, 1830). The aim of this study was to determine the cumulative effect of density and pathogen infection by protein extract of Piscirickettsia salmonis on skeletal muscle metabolism. In a first experiment, specimens were submitted to three different stocking densities: (1) 3.1 kg m(-3), (2) 15 kg m(-3) and (3) 60 kg m(-3), for a period of 10 days. In a second experiment, metabolic changes caused by an infection of P. salmonis protein extract (a single injection of 0.5 μL P. salmonis protein extract g body weight(-1) was inoculated in the fish) and its combined effect with stocking density was assessed during a period of 10 days. This study concludes that stress caused by high stocking density led to the reorganization of some metabolic routes to fulfill skeletal muscle energy needs. Furthermore, infection response by pathogen P. salmonis differed when stocking density increased, suggesting an increase of energy needs with density in skeletal muscle of infected fish.

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.

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.

Francisella infections in farmed and wild aquatic organisms

Over the last 10 years or so, infections caused by bacteria belonging to a particular branch of the genus Francisella have become increasingly recognised in farmed fish and molluscs worldwide. While the increasing incidence of diagnoses may in part be due to the development and widespread availability of molecular detection techniques, the domestication of new organisms has undoubtedly instigated emergence of clinical disease in some species. Francisellosis in fish develops in a similar fashion independent of host species and is commonly characterised by the presence of multi-organ granuloma and high morbidity, with varying associated mortality levels. A number of fish species are affected including Atlantic cod, Gadus morhua; tilapia, Oreochromis sp.; Atlantic salmon, Salmo salar; hybrid striped bass, Morone chrysops × M. saxatilis and three-lined grunt, Parapristipoma trilinineatum. The disease is highly infectious and often prevalent in affected stocks. Most, if not all strains isolated from teleost fish belong to either F. noatunensis subsp. orientalis in warm water fish species or Francisella noatunensis subsp. noatunensis in coldwater fish species. The disease is quite readily diagnosed following histological examination and identification of the aetiological bacterium by culture on cysteine rich media or PCR. The available evidence may indicate a degree of host specificity for the various Francisella strains, although this area requires further study. No effective vaccine is currently available. Investigation of the virulence mechanisms and host response shows similarity to those known from Francisella tularensis infection in mammals. However, no evidence exists for zoonotic potential amongst the fish pathogenic Francisella.

Association of red-mark syndrome with a Rickettsia-like organism and its connection with strawberry disease in the USA

Red-mark syndrome (RMS), a disease seen mostly in rainbow trout, Oncorhynchus mykiss, is of unknown aetiology. The research presented here indicates the presence of an intracellular bacterium in RMS-affected fish. A positive reaction was observed using immunohistochemistry (IHC) with skin lesions, liver, kidney and spleen of affected fish sampled from several locations within the United Kingdom using two different polyclonal antisera raised against Piscirickettsia salmonis. The same reaction was also seen with a number of different anti-P. salmonis monoclonal antibodies (MAbs). A disease with similar clinical signs to RMS, referred to as strawberry disease (SD), has been reported in the USA. A Rickettsia-like organism (RLO) has recently been associated with SD based on analysis of 16S rDNA sequences. Using the same panel of anti-P. salmonis antibodies used to screen the RMS samples, similar staining was obtained in tissue of SD-affected fish by IHC. A polymerase chain reaction (PCR) using RLO-specific primers was also performed on RMS-affected fish from the United Kingdom, and the samples were positive for the RLO 16S rRNA sequence. These findings suggest that the same aetiological agent may be responsible for RMS in the United Kingdom and SD in the USA.

Real time PCR detection of Piscirickettsia salmonis from formalin-fixed paraffin-embedded tissues

Piscirickettsia salmonis is the causative agent of piscirickettsiosis, a transmissible disease of salmonid fish. Diagnosis of piscirickettsiosis has traditionally been based upon identification of typical pathological changes by histological investigation, with confirmation by immunohistochemistry on formalin-fixed, paraffin-embedded tissues. However, implementation of more rapid confirmatory techniques, preferably with higher levels of sensitivity and possibilities for quantification, is desirable. A real-time polymerase chain reaction (PCR) assay was designed for specific detection of P. salmonis and tested on samples extracted from formalin-fixed paraffin-embedded material. Construction of a PCR-target mimic allowed determination of detection limits, linearity of the real-time PCR and quantitative detection of P. salmonis. The present study demonstrates the capability of the described real time PCR assay for detection of P. salmonis from paraffin-embedded material with a high degree of sensitivity and specificity. Implementation of this assay constitutes an important development for a rapid and secure diagnosis of piscirickettsiosis.