Molecular detection of Aphanomyces astaci - An improved species specific qPCR assay.

The parasitic oomycete Aphanomyces astaci is the causative agent of crayfish plague, a devastating disease for European freshwater crayfish. Species specific quantitative real-time PCR (qPCR) can offer rapid detection of the pathogen. However, the well established A. astaci qPCR assay recommended by the World Organization for Animal Health (WOAH) amplifies the recently described Aphanomyces fennicus. Consequently, false-positive results may occur. This calls for the improvement of the established species specific A. astaci qPCR assay in order to avoid amplifying A. fennicus while screening for A. astaci. We developed an improved species specific A. astaci qPCR assay and validated the assay across three laboratories, using established procedures including different qPCR master mixes for each respective laboratory. Genomic DNA from A. astaci, A. fennicus and closely related Aphanomyces spp. was analysed and compared with both the improved and established assay. Additionally, DNA from crayfish tissue and environmental samples were analysed with both assays. The improved assay showed similar sensitivity with the established assay for all sample types, while proving highly specific for A. astaci avoiding amplification of A. fennicus and the other tested Aphanomyces spp. Environmental DNA (eDNA) samples collected at River Lierelva in Norway amplified with the established assay, but not with the improved assay indicating false positive. We were able to sequence a 530 bp fragment of the ITS region from these eDNA samples and the consensus sequence showed 99.9-100 % pairwise identity with A. fennicus and 97.2-98 % pairwise identity with A. astaci, suggesting that the occurrence of A. fennicus is not limited to Finland, where it was first discovered.

Identification and genetic characterization of Saprolegnia parasitica, isolated from farmed and wild fish in Finland.

Oomycete infections in farmed fish are one of the most significant disease issues in salmonid aquaculture worldwide. In the present study, Saprolegnia spp. in different farmed fish species in Finland were identified, and the molecular epidemiology of especially Saprolegnia parasitica was examined. We analysed tissue samples from suspected oomycete-infected salmonids of different life stages from a number of fish farms, as well as three wild salmonids. From collected oomycete isolates, the ITS1, 5.8S and ITS2 genomic regions were amplified, analysed phylogenetically and compared with corresponding sequences deposited in GenBank. Of the sequenced isolates, 91% were identified as S. parasitica. Isolates of yolk sac fry were identified as different Saprolegnia spp. Among the isolates from rainbow trout eggs Saprolegnia diclina dominated. In order to determine potential dominating clones among the S. parasitica, isolates were analysed using Multi Locus Sequence Typing (MLST). The results showed that one main clone contained the majority of the isolates. The MLST analysis showed four main sequence types (ST1-ST4) and 13 unique STs. This suggests that the Saprolegnia infections in farmed fish in Finland are not caused by different strains originating in the farm environment. Instead, one main clone of S. parasitica is present in Finnish fish farms.

Genetic diversity and phenotypic characterization of Iodobacter limnosediminis associated with skin lesions in freshwater fish.

The relatively unknown genus Iodobacter sp. has been repeatedly isolated from skin ulcers and saprolegniosis on freshwater fish in Finland, especially farmed salmonids. Genetic characterization verified that all 23 bacterial isolates studied here belonged to the species Iodobacter limnosediminis, previously undescribed from the fish microbiota. Whole-genome pulsed-field gel electrophoresis revealed variability between the I. limnosediminis strains, suggesting that they were most likely of environmental origin. Two I. limnosediminis strains caused lesions in 27%-53% of brown trout (Salmo trutta) injected intramuscularly (p ≤ .05). The lesions represented moderate to severe tissue damage, but for most fish, the tissues had been repaired by the end of the experiment through the accumulation of fibrocytes and macrophages at the site of the lesion. I. limnosediminis was reisolated from some lesions and/or internal organs. Phenotypically and biochemically, I. limnosediminis resembles several common bacterial species found in the aquatic environment, as it grows well on several media as whitish medium-sized colonies, is Gram negative and rod-shaped. Here, we characterized I. limnosediminis strains with several methods, including MALDI-TOF. This characterization will help in further investigations into the occurrence and possible involvement of I. limnosediminis in skin lesions of freshwater fish.

A tentative new species Aphanomyces fennicus sp. nov. interferes with molecular diagnostic methods for crayfish plague.

Several isolates of an unknown oomycete resembling the genus Aphanomyces were obtained into laboratory culture from samples of noble crayfish (Astacus astacus) in 2016-2017. The crayfish were kept in cages in connection with a study on an eventually persistent crayfish plague infection in a small Finnish lake, following an acute episode of the disease in 2010. Despite the close resemblance of the isolates to the causative agent of crayfish plague, Aphanomyces astaci, and the positive results obtained in OIE recommended A. astaci-specific ITS-based conventional PCR and qPCR molecular assays, the isolates can be distinguished from A. astaci by morphological features concerning hyphal structure and chlamydospore formation, as well as using the randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) method, microsatellite-based genotyping, the pathogenicity test and phylogenetic analysis based on ITS sequencing. The name Aphanomyces fennicus sp. novum is proposed for this close relative of A. astaci. The detection of this tentative novel species giving false-positive results in existing diagnostic assays for the crayfish plague highlights the importance of careful interpretation of the results from molecular methods, especially concerning crayfish with low-level infections, excluding the possibility to verify the results from clinical or sequencing data.

Infectious pancreatic necrosis virus (IPNV) strain with genetic properties associated with low pathogenicity at Finnish fish farms.

Infectious pancreatic necrosis (IPN) is a contagious viral disease of fish that causes economic losses in aquaculture worldwide. In Finland, IPN virus (IPNV) has been isolated since 1987 from adult fish showing no signs of clinical disease at fish farms located in the coastal areas of the Baltic Sea. The inland area of Finland, however, remained free of IPN until 2012, when fish on several rainbow trout farms were diagnosed IPNV-positive. The fish mortalities detected at the farms were low, but clinical signs and histopathological changes typical for IPNV infection were seen in juvenile salmonids. IPNV was isolated at high water temperatures up to 22°C. In 2013 and 2014, IPNV detections continued at inland farms, indicating that infections have spread. The aim of this study was to describe the epidemiology of the outbreak and to characterise the Finnish inland IPNV isolates using histopathological, immunohistochemical and genetic approaches. In order to determine the epidemiological origin of the inland IPNV infections, the partial viral capsid protein (VP2) gene sequences of the inland IPNV isolates were compared with the sequences of the isolates from the coastal farms. Based on the genetic analysis, the inland isolates belong to IPNV Genogroup 2 (Serotype A3/Ab), and the origin of the isolates appears to be one or several coastal fish farms.

Effect of experimental exposure to differently virulent Aphanomyces astaci strains on the immune response of the noble crayfish Astacus astacus.

European crayfish are sensitive to the crayfish plague pathogen, Aphanomyces astaci, carried by North American crayfish species due to their less effective immune defence mechanisms against this disease. During a controlled infection experiment with a susceptible crayfish species Astacus astacus using three A. astaci strains (representing genotype groups A, B, and E), we investigated variation in their virulence and in crayfish immune defence indicators (haemocyte density, phenoloxidase activity, and production of reactive oxygen species). Experimental crayfish were exposed to two dosages of A. astaci spores (1 and 10 spores mL(-1)). The intensity and timing of the immune response differed between the strains as well as between the spore concentrations. Stronger and faster change in each immune parameter was observed in crayfish infected with two more virulent strains, indicating a relationship between crayfish immune response and A. astaci virulence. Similarly, the immune response was stronger and was observed earlier for the higher spore concentration. For the first time, the virulence of a strain of the genotype group E (isolated from Orconectes limosus) was experimentally tested. Total mortality was reached after 10 days for the two higher spore dosages (10 and 100 spores mL(-1)), and after 16 days for the lowest (1 spore mL(-1)), revealing equally high and rapid mortality as caused by the genotype group B (from Pacifastacus leniusculus). No mortality occurred after infection with genotype group A during 60 days of the experimental trial.

Characterization of perch rhabdovirus (PRV) in farmed grayling Thymallus thymallus.

Two Finnish fish farms experienced elevated mortality rates in farmed grayling Thymallus thymallus fry during the summer months, most typically in July. The mortalities occurred during several years and were connected with a few neurological disorders and peritonitis. Virological investigation detected an infection with an unknown rhabdovirus. Based on the entire glycoprotein (G) and partial RNA polymerase (L) gene sequences, the virus was classified as a perch rhabdovirus (PRV). Pairwise comparisons of the G and L gene regions of grayling isolates revealed that all isolates were very closely related, with 99 to 100% nucleotide identity, which suggests the same origin of infection. Phylogenetic analysis demonstrated that they were closely related to the strain isolated from perch Perca fluviatilis and sea trout Salmo trutta trutta caught from the Baltic Sea. The entire G gene sequences revealed that all Finnish grayling isolates, and both the perch and sea trout isolates, were most closely related to a PRV isolated in France in 2004. According to the partial L gene sequences, all of the Finnish grayling isolates were most closely related to the Danish isolate DK5533 from pike. The genetic analysis of entire G gene and partial L gene sequences showed that the Finnish brown trout isolate ka907_87 shared only approximately 67 and 78% identity, respectively, with our grayling isolates. The grayling isolates were also analysed by an immunofluorescence antibody test. This is the first report of a PRV causing disease in grayling in Finland.

Detection and Quantification of the Oomycete Saprolegnia parasitica in Aquaculture Environments.

Saprolegnia parasitica induces heavy mortality in aquaculture. The detection of S. parasitica is often time consuming and uncertain, making it difficult to manage the disease. We validated a previously published real-time quantitative PCR (qPCR) assay to confirm the presence of S. parasitica in fish and in water using environmental DNA (eDNA) quantification. Analytical sensitivity and specificity of the assay was assessed in silico, in vitro and the qPCR assay was compared with microbiological cultivation methods to detect and quantify S. parasitica in water samples from a controlled fish exposure experiment and from fish farms. Furthermore, we compared the use of an agar cultivation method and the qPCR assay to detect S. parasitica directly from mucus samples taken from the fish surface. The analytical sensitivity and specificity of the qPCR assay were high. The qPCR assay detected 100% of S. parasitica-positive water samples. In a field study, the qPCR assay and a microwell plate (MWP) enumeration method correlated significantly. Furthermore, the qPCR assay could be used to confirm the presence of S. parasitica in skin mucus. Thus, the qPCR assay could complement diagnostic methods in specifically detecting saprolegniosis in fish and used as a surveillance method for S. parasitica pathogen in aquaculture environments.

Spiny-cheek crayfish Orconectes limosus carry a novel genotype of the crayfish plague pathogen Aphanomyces astaci.

The oomycete Aphanomyces astaci causes mass mortalities of European crayfish. Different species of North American crayfish, original hosts of this parasite, seem to carry different strains of A. astaci. So far, four distinct genotype groups have been recognised using Random Amplification of Polymorphic DNA (RAPD-PCR). We succeeded in isolating A. astaci from the spiny-cheek crayfish Orconectes limosus, a widespread invader in Europe, and confirmed that this species carries a novel A. astaci genotype. Improving knowledge on the diversity of this parasite may facilitate identification of genotypes in mass mortalities of European crayfish, thus tracing the sources of infection.

Aphanomyces astaci mtDNA: insights into the pathogen's differentiation and its genetic diversity from other closely related oomycetes.

The causative agent of crayfish plague, Aphanomyces astaci (Saprolegniales, Oomycota), is one of the 100 world's worst invasive alien species and represents a major threat to freshwater crayfish species worldwide. A better understanding of the biology and epidemiology of A. astaci relies on the application of efficient tools to detect the pathogen and assess its genetic diversity. In this study, we validated the specificity of two recently developed PCR-based approaches used to detect A. astaci groups. The first relies on the analysis of mitochondrial ribosomal rnnS (small) and rnnL (large) subunit sequences and the second, of sequences obtained by using genotype-specific primers designed from A. astaci whole genome sequencing. For this purpose, we tested the specificity against 76 selected isolates, including other oomycete species and the recently described species Aphanomyces fennicus, which, when used in nrITS-based specific tests for A. astaci, is known to result in a false positive. Under both approaches, we were able to efficiently and accurately identify A. astaci and its genetic groups in both pure cultures and clinical samples. We report that sequence analysis of the rnnS region alone is sufficient for the identification of A. astaci and a partial characterization of haplogroups. In contrast, the rnnL region alone is not sufficiently informative for A. astaci identification as other oomycete species present sequences identical to those of A. astaci.

Microsatellite markers for direct genotyping of the crayfish plague pathogen Aphanomyces astaci (Oomycetes) from infected host tissues.

Aphanomyces astaci is an invasive pathogenic oomycete responsible for the crayfish plague, a disease that has devastated European freshwater crayfish. So far, five genotype groups of this pathogen have been identified by applying random amplified polymorphic DNA analysis on axenic cultures. To allow genotyping of A. astaci in host tissue samples, we have developed co-dominant microsatellite markers for this pathogen, tested them on pure cultures of all genotype groups, and subsequently evaluated their use on tissues of (1) natural A. astaci carriers, i.e., North American crayfish species, and (2) A. astaci-infected indigenous European species from crayfish plague outbreaks. Out of over 200 potential loci containing simple sequence repeat (SSR) motifs identified by 454 pyrosequencing of SSR-enriched library, we tested 25 loci with highest number of repeats, and finally selected nine that allow unambiguous separation of all known RAPD-defined genotype groups of A. astaci from axenic cultures. Using these markers, we were able to characterize A. astaci strains from DNA isolates from infected crayfish tissues when crayfish had a moderate to high agent level according to quantitative PCR analyses. The results support the hypothesis that different North American crayfish hosts carry different genotype groups of the pathogen, and confirm that multiple genotype groups, including the one originally introduced to Europe in the 19th century, cause crayfish plague outbreaks in Central Europe. So far undocumented A. astaci genotype seems to have caused one of the analysed outbreaks from the Czech Republic. The newly developed culture-independent approach allowing direct genotyping of this pathogen in both axenic cultures and mixed genome samples opens new possibilities in studies of crayfish plague pathogen distribution, diversity and epidemiology.

Monitoring the spore dynamics of Aphanomyces astaci in the ambient water of latent carrier crayfish.

The specialized crayfish parasite Aphanomyces astaci causes the devastating crayfish plague in European crayfish. Even though A. astaci sporulation has been thoroughly studied under pure culture conditions, little is known about the sporulation dynamic from its live host. Our purpose was to investigate the A. astaci spore dynamic in its native parasite-host relationship by monitoring the sporulation from carrier crayfish into the ambient water using agent specific qPCR. American signal crayfish (Pacifastacus leniusculus) with known positive carrier status were housed individually and communally in two experimental set-ups using multiple replicates and different temperatures. Water samples were collected weekly, and spore numbers were quantified. We demonstrate here that live latent carrier crayfish continuously released a moderate number of A. astaci spores (~2700 spores per crayfish/week) in the absence of death and moulting events. In contrast, a pronounced sporulation increase was seen already one week prior to death in moribund crayfish, suggesting a crayfish plague-like condition developing in weakened or stressed individuals. Significantly more spores were produced at 18°C compared to 4°C, while a negative correlation was detected between spore numbers and temperatures rising from 17 to 23°C. This study is the first attempt to quantify the spore release from carrier crayfish on the basis of qPCR applied on water samples, and demonstrate that the approach successfully unravel A. astaci sporulation patterns. The results emphasize that carrier crayfish pose a constant infection risk to highly susceptible crayfish species regardless of crayfish life cycle state.