Parasites and their impact on thick-shelled river mussels Unio crassus from two populations in Luxembourg

The thick-shelled river mussel Unio crassus Philipsson, 1788 is a species native to many European habitats, with declining populations. The impact of parasite communities on health status of this species is poorly understood. In this study, parasites of 30 U. crassus specimens from the Our and Sauer Rivers in Luxembourg were identified morphologically and, in some cases, using molecular genetic methods. The findings were correlated to selected parameters (total length, visceral weight, shell lesions, gonadal stage). The 2 populations did not differ in shell length, visceral weight, number of males and females, gonadal scoring, shell lesions, and the occurrence of glochidia. The prevalence and infestation intensities of detected Trichodina sp., Conchophthirus sp., and freshwater mite larvae did not differ between the 2 populations, whereas the prevalence and infestation intensities of mite eggs, nymphs, and adults were significantly higher in the Sauer River. Rhipidocotyle campanula and European bitterling Rhodeus amarus larvae were only present in the Sauer. Histopathology revealed the destruction of the gonads by R. campanula and tissue damage by the mites. The only significant correlation of the selected parameters was a positive correlation between R. amarus occurrence and total length as well as a negative correlation between R. amarus occurrence and gonadal stage. In the Sauer River, 2 mussels were found to be hermaphrodites.

Advanced vacuolation indicates propagation of various salmonid alphavirus type 2 isolates in Acholeplasma-infected BF-2 cells

During previous routine inspections of bluegill fry (BF-2) and rainbow trout gonad (RTG-2) cells incubated with organ samples from asymptomatic Arctic char Salvelinus alpinus, brook trout Salvelinus fontinalis, and rainbow trout Oncorhynchus mykiss, a distinctive, reproducible cytopathic effect (CPE) appeared. The striking CPE, involving progressive vacuolation turning into slowly proceeding pyknotic degeneration, was originally attributed exclusively to enhanced growth of Acholeplasma sp. However, at a recent re-examination of re-infected BF-2 cells using electron microscopy (EM), conventional PCR, and quantitative PCR (qPCR), a virus was also detected. Two days post inoculation (dpi), EM revealed characteristic virions inside cytoplasmic vacuoles and next to bacteria outside the cells. The nucleotide sequences of the viral nsP3 gene fragment obtained from supernatants of infected cells were 100% identical and representative for salmonid alphavirus type 2 (SAV 2). The 16S RNA gene (16S rDNA) fragment sequences of the Mollicutes-specific PCR product obtained from SAV-infected as well as virus-free BF-2 control cells were identical with Acholeplasma laidlawii. In addition, qPCR results indicated enhanced propagation of virus and bacteria increasing with vacuolation between 5 and 8 dpi. Advanced vacuolation can be regarded as a CPE of both SAV and A. laidlawii, suggesting a viral impact on the bacterial infection that turns a latent intracellular stage into an apparent degenerative condition.

Renibacterium salmoninarum and Mycobacterium spp.: two bacterial pathogens present at low levels in wild brown trout (Salmo trutta fario) populations in Austrian rivers

BACKGROUND

Renibacterium salmoninarum and Mycobacterium sp. are important bacterial pathogens of fish. R. salmoninarum is the causative agent of bacterial kidney disease, a Gram-positive bacterium mostly known for causing chronic infections in salmonid fish, while multiple species belonging to the Mycobacterium genus have been associated with mycobacteriosis in fish as well as in human. The objective of this study was to determine the prevalence of these two bacterial pathogens in populations of wild brown trout (Salmo trutta fario) in four rivers (Kamp, Wulka, Traun and Ybbs) in Austria.

RESULTS

A total of 457 kidney samples were examined for both bacterial agents using nested and conventional PCR as well as bacterial cultivation on KDM-2, histological examination and immunohistochemistry. Molecular evidence showed an estimated prevalence level of 0.94% for R. salmoninarum in 2017 while the bacterium could not be detected in 2018 and histology showed signs consistent with a low-level chronic inflammation in the kidney of infected fish. Similarly, no fish were found positive for Mycobacterium in 2017 but in 2018, the prevalence was found to be 37.03% in the Kamp river (4.08% across all rivers). The sequencing data confirmed that these fish carried Mycobacterium sp. although the precise species of Mycobacterium could not be ascertained.

CONCLUSIONS

This survey constitutes the first insight into the prevalence rate of R. salmoninarum and Mycobacterium sp. in wild brown trout (Salmo trutta fario) populations in Austria. Both of these pathogens were only detected in the summer months (June and July), which might suggest that the stress linked to increased water temperature could act as stressor factor and contribute to the outbreak of these diseases. The age of the fish might also play a role, especially in the case of Mycobacterium sp. as all the infected fish were in their first summer (June).

First confirmation of salmonid alphavirus infection in Arctic char Salvelinus alpinus and in Austria

To date, sleeping disease (SD) caused by salmonid alphavirus 2 (SAV 2) has been reported in freshwater rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar. This study describes for the first time the occurrence of SD in farm-reared Arctic char Salvelinus alpinus and the occurrence of SAV in Austria. Clinical symptoms were indicative of the disease, and the diagnosis was confirmed by histopathology, infectivity in first passages of CHSE-214 cells and PCR. The phylogenetic analysis of the amplified SAV-nonstructural protein-3 (nsP3) fragment revealed the affiliation to the SAV 2 genotype.

Rapid detection and differentiation of carp oedema virus and cyprinid herpes virus-3 in koi and common carp

Carp oedema virus (CEV) and koi herpes virus (KHV) are of major concern to common carp breeders and koi enthusiasts worldwide. The viruses cause diseases that exhibit similar external signs; thus, it is difficult to distinguish between them clinically. In this study, we developed and optimized rapid and accurate single- and multiplex isothermal diagnostic tools, based on recombinase polymerase amplification (RPA), for detection and differentiation of CEV and KHV. The assays were combined with a lateral flow dipstick to enable visual detection of amplification products and simplify post-amplification analysis. Both CEV- and KHV-RPA assays were specific for their target virus. The lower detection limits of the assays were similar to those of established diagnostic PCR tests for the viruses. A sample preparation method was optimized to eliminate the need for total DNA extraction from fish tissues. The estimated time to perform these RPA assays, from receiving the sample to having a result, is 50 min, compared to 10 and 7 hr for CEV- and KHV-PCR tests, respectively. The assays can be performed in field situations to improve screening of fish and reduce spread of these viruses and thereby enhance the common carp and koi industries.

Emergence of carp edema virus (CEV) and its significance to European common carp and koi Cyprinus carpio

Carp edema virus disease (CEVD), also known as koi sleepy disease, is caused by a poxvirus associated with outbreaks of clinical disease in koi and common carp Cyprinus carpio. Originally characterised in Japan in the 1970s, international trade in koi has led to the spread of CEV, although the first recognised outbreak of the disease outside of Japan was not reported until 1996 in the USA. In Europe, the disease was first recognised in 2009 and, as detection and diagnosis have improved, more EU member states have reported CEV associated with disease outbreaks. Although the structure of the CEV genome is not yet elucidated, molecular epidemiology studies have suggested distinct geographical populations of CEV infecting both koi and common carp. Detection and identification of cases of CEVD in common carp were unreliable using the original PCR primers. New primers for conventional and quantitative PCR (qPCR) have been designed that improve detection, and their sequences are provided in this paper. The qPCR primers have successfully detected CEV DNA in archive material from investigations of unexplained carp mortalities conducted >15 yr ago. Improvement in disease management and control is possible, and the principles of biosecurity, good health management and disease surveillance, applied to koi herpesvirus disease, can be equally applied to CEVD. However, further research studies are needed to fill the knowledge gaps in the disease pathogenesis and epidemiology that, currently, prevent an accurate assessment of the likely impact of CEVD on European koi and common carp aquaculture and on wild carp stocks.

Susceptibility of common carp and sunfish to a strain of Francisella noatunensis subsp. orientalis in a challenge experiment

Francisellosis, an emerging disease in many fish species, can cause high mortality in affected populations. Here we investigated the susceptibility of common carp Cyprinus carpio and sunfish Lepomis gibbosus to Francisella noatunensis subsp. orientalis (Fno), and possible transmission of the bacteria between the 2 fish species. In a challenge experiment, 3 groups of each species were injected intraperitoneally (IP) with 3 different doses of an Fno strain no. 9449 of the Norwegian Veterinary Institute, recovered from naturally infected ornamental Malawi cichlids. Infected carp were cohabitated with sunfish and vice versa. Control groups were injected with 0.9M phosphate-buffered saline and cohabitated accordingly. Fish were sampled at different time points. Mortality of challenged sunfish was observed during the first 96 h and reached 56.1%. In the control sunfish, 4 of 16 fish (25%) died within 48 h. In carp, no mortalities or clinical signs were observed during the experiment. General clinical and patho-anatomical disease signs of affected sunfish were observed. We detected granulomas in 2 cohabitated sunfish and 1 challenged carp, but could not re-isolate Fno from these fish. Fno was successfully cultured from 6 sunfish and 3 carp specimens until 35 d post injection. PCR of spleen and kidney with 16S rDNA Francisella-like bacterium primers 180f and 485r yielded amplicons in 68.3% of challenged sunfish and only 12.2% of challenged carp. We demonstrated that sunfish were susceptible to Fno infection while the carp were not. Horizontal transmission of the agent between the 2 fish species could not be demonstrated.

Effect of a phytogenic feed additive on the susceptibility of Onchorhynchus mykiss to Aeromonas salmonicida

In recent years, feed additives have increasingly been adopted by the aquaculture industry. These supplements not only offer an alternative to antibiotics but have also been linked to enhanced growth performance. However, the literature is still limited and provides contradictory information on their effectiveness. This is mainly due to the wide variety of available products and their complex mechanisms of action. Phytogenic feed additives have been shown to have antimicrobial effects and can improve growth performance. In the present study, we investigated the susceptibility of several fish pathogenic bacteria to a phytogenic essential oil product in vitro. In addition, we determined the protective effect of a commercial phytogenic feed additive containing oregano, anis and citrus oils on the resistance of rainbow trout Oncorhynchus mykiss to infection by Aeromonas salmonicida. The bacterium was administered through 3 different routes: intra-peritoneal injection, immersion in a bacterial solution and cohabitation with infected fish. Mortality rates were significantly lower in infected rainbow trout that had received the feed additive: the overall mortality rate across all routes of infection was 18% in fish fed a diet containing the additive compared to 37% in fish that received unsupplemented feed. The route of infection also significantly impacted mortality, with average mortality rates of 60, 17.5 and 5% for intra-peritoneal injection, immersion and cohabitation, respectively. In general, fish were better protected against infection by immersion than infection by injection.

Inhibition of spring viraemia of carp virus replication in an Epithelioma papulosum cyprini cell line by RNAi

Spring viraemia of carp virus (SVCV) is an aetiological agent of a serious disease affecting carp farms in Europe and is a member of the Rhabdoviridae family of viruses. The genome of SVCV codes for five proteins: nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G) and RNA-dependent RNA polymerase (L). RNA-mediated interference (RNAi) by small interfering RNAs (siRNAs) is a powerful tool to inhibit gene transcription and is used to study genes important for viral replication. In previous studies regarding another member of Rhabdoviridae, siRNA inhibition of the rabies virus nucleoprotein gene provided in vitro and in vivo protection against rabies. In this study, synthetic siRNA molecules were designed to target SVCV-N and SVCV-P transcripts to inhibit SVCV replication and were tested in an epithelioma papulosum cyprini (EPC) cell line. Inhibition of gene transcription was measured by real-time quantitative reverse-transcription PCR (RT-qPCR). The efficacy of using siRNA for inhibition of viral replication was analysed by RT-qPCR measurement of a reporter gene (glycoprotein) expression and by virus endpoint titration. Inhibition of nucleoprotein and phosphoprotein gene expression by siRNA reduced SVCV replication. However, use of tandem siRNAs that target phosphoprotein and nucleoprotein worked best at reducing SVCV replication.

Carp edema virus/Koi sleepy disease: an emerging disease in Central-East Europe

Koi sleepy disease (KSD), also known as carp edema virus (CEV), was first reported from juvenile colour carp in Japan in the 1970s. Recently, this pox virus was detected in several European countries, including Germany, France and the Netherlands. In England, in addition to colour carp, outbreaks in common carp are reported. KSD/CEV is an emerging infectious disease characterized by a typical sleepy behaviour, enophthalmia, generalized oedematous condition and gill necrosis, leading to hypoxia. High mortality, of up to 80-100%, is seen in juvenile koi collected from infected ponds. In Austria, this disease had not been detected until now. In spring 2014, diagnostic work revealed the disease in two unrelated cases. In one instance, a pond with adult koi was affected; in the other, the disease was diagnosed in adult common carp recently imported from the Czech Republic. A survey was carried out on recent cases (2013/2014), chosen from those with similar anamnestic and physical examination findings, revealing a total of 5/22 cases positive for KSD/CEV. In this study, two paradigmatic cases are presented in detail. Results together with molecular evidence shaped the pattern of the first diagnosis of KSD/CEV in fish from Austrian ponds. In the light of the positive cases detected from archived material, and the spread of the disease through live stock, imported from a neighbouring country, the need for epidemiological investigations in Austria and surrounding countries is emphasized.

Development of a novel in vitro method for drug development for fish; application to test efficacy of antimicrosporidian compounds

Few drugs are approved for treating diseases caused by parasites in minor species such as fish. This is due, in part, to the expense of drug development and to the comparatively small market. In vivo effectiveness trials for antiparasitic drugs are costly, time consuming and require ethics approval, therefore an in vitro screening approach is a cost-effective alternative to finding promising drug candidates. We developed an in vitro testing system to test antimicrosporidial compounds against a microsporidian pathogen Heterosporis saurida. Five antiparasitic compounds, albendazole, fumagillin, TNP-70, nitazoxanide and lufenuron, were assayed for antimicrosporidial activity. All compounds reduced the number of H saurida spores in infected cells when applied at a concentration that did not appear to be toxic to the host cells. Albendazole inhibited replication of H saurida by >60 per cent, fumagillin and its analogue TNP-470 inhibited H saurida >80 per cent, nitazoxanide and lufenuron inhibited growth >70 per cent. The data suggest that both fumagillin and its analogous TNP-70 hold the best promise as therapeutic agents against H saurida. The ability to use fish cell cultures to assess drugs against H saurida demonstrates an approach that may be helpful to evaluate other drugs on different microsporidia and host cells.

In vitro cultivation model for Heterosporis saurida (Microsporidia) isolated from lizardfish, Saurida undosquamis (Richardson)

Heterosporis saurida is a microsporidian that infects lizardfish, Saurida undosquamis (Richardson, 1848), in the Arabian Sea. Spores were isolated from infected lizardfish and used to infect derived fish cell lines: common carp brain (CCB), epithelioma papulosum cyprinid (EPC), fathead minnow epithelial (FHM), rainbow trout gonad (RTG), bluegill fry (BF-2) and chinook salmon embryo (CHSE). Non-fish cell lines were also tested that include: insect (SF-9), rabbit (RK-13) and African green monkey (Vero E6). No growth of H. saurida was observed in any fish cell line, SF-9 or Vero E6 cell lines. H. saurida spores grew only in RK-13 cell line and were detected by immunofluorescence. Developmental stages of H. saurida were seen in RK-13 cells by light and transmission electron microscopy, and species identification was confirmed by sequencing. This study demonstrated that H. saurida was able to proliferate in the mammalian RK-13 cell line, which thus represents an in vitro model for conducting molecular genetics and cell-pathogen interaction studies of Heterosporis.

Pesticide-contaminated feeds in integrated grass carp aquaculture: toxicology and bioaccumulation

Effects of dissolved pesticides on fish are widely described, but little is known about effects of pesticide-contaminated feeds taken up orally by fish. In integrated farms, pesticides used on crops may affect grass carp that feed on plants from these fields. In northern Vietnam, grass carp suffer seasonal mass mortalities which may be caused by pesticide-contaminated plants. To test effects of pesticide-contaminated feeds on health and bioaccumulation in grass carp, a net-cage trial was conducted with 5 differently contaminated grasses. Grass was spiked with 2 levels of trichlorfon/fenitrothion and fenobucarb. Unspiked grass was used as a control. Fish were fed at a daily rate of 20% of body mass for 10 d. The concentrations of fenitrothion and fenobucarb in pond water increased over time. Effects on fish mortality were not found. Fenobucarb in feed showed the strongest effects on fish by lowering feed uptake, deforming the liver, increasing blood glucose and reducing cholinesterase activity in blood serum, depending on feed uptake. Fenobucarb showed increased levels in flesh in all treatments, suggesting bio-concentration. Trichlorfon and fenitrothion did not significantly affect feed uptake but showed concentration-dependent reduction of cholinesterase activity and liver changes. Fenitrothion showed bioaccumulation in flesh which was dependant on feed uptake, whereas trichlorfon was only detected in very low concentrations in all treatments. Pesticide levels were all detected below the maximum residue levels in food. The pesticide-contaminated feeds tested did not cause mortality in grass carp but were associated with negative physiological responses and may increase susceptibility to diseases.

[Ovarian tumor in a koi carp (Cyprinus carpio): Diagnosis, surgery, postoperative care and tumour classification]

Although ovarian tumour in the koi (Cyprinus carpio) does not appear to be an uncommon condition, its occurrence and therapy has rarely been reported. In the present case, the decision for surgery was based on clinical and sonographic findings of an intracoelomic mass. We used tricaine methansulfonate for the anaesthesia. Laparotomy was performed by ventral access and an ovarian tumour of 12-cm diameter was removed. The wound was sutured in two layers using Vicryl®. In addition to the application of an analgesic, an antibiotic and vitamins, the postoperative conditions the patient was kept under were adapted to support wound healing. The fish recovered uneventfully and was clinically healthy during the 16-month observation period. Based on the histological findings, the tumour was diagnosed as a thecoma. Investigations using antibodies against vimentin, cytokeratin, S 100 and glial fibrillary acidic protein (GFAP) failed to provide reliable results.

Antibody screening identifies 78 putative host proteins involved in Cyprinid herpesvirus 3 infection or propagation in common carp, Cyprinus carpio L

Cyprinid herpesvirus 3 (CyHV-3) is the aetiological agent of a serious and notifiable disease afflicting common and koi carp, Cyprinus carpio L., termed koi herpesvirus disease (KHVD). Significant progress has been achieved in the last 15 years, since the initial reports surfaced from Germany, USA and Israel of the CyHV-3 virus, in terms of pathology and detection. However, relatively few studies have been carried out in understanding viral replication and propagation. Antibody-based affinity has been used for detection of CyHV-3 in enzyme-linked immunosorbent assay and PCR-based techniques, and immunohistological assays have been used to describe a CyHV-3 membrane protein, termed ORF81. In this study, monoclonal antibodies linked to N-hydroxysuccinimide (NHS)-activated spin columns were used to purify CyHV-3 and host proteins from tissue samples originating in either CyHV-3 symptomatic or asymptomatic fish. The samples were next analysed either by polyacrylamide gel electrophoresis (PAGE) and subsequently by electrospray ionization coupled to mass spectrometry (ESI-MS) or by ESI-MS analysis directly after purification. A total of 78 host proteins and five CyHV-3 proteins were identified in the two analyses. These data can be used to develop novel control methods for CyHV-3, based on pathways or proteins identified in this study.

Establishment of medium for laboratory cultivation and maintenance of Fredericella sultana for in vivo experiments with Tetracapsuloides bryosalmonae (Myxozoa)

The freshwater bryozoan Fredericella sultana (Blumenbach) is the most common invertebrate host of the myxozoan parasite Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonid fish. Culture media play an important role in hatching of statoblasts and maintaining clean bryozoan colonies for Malacosporea research. We developed a novel culture medium, Bryozoan Medium C (BMC), for the cultivation and maintenance of F. sultana under laboratory conditions. Statoblasts of F. sultana were successfully hatched to produce transparent-walled, specific pathogen-free (SPF) colonies that were maintained >12 months in BMC at pH 6.65. Tetracapsuloides bryosalmonae was successfully transmitted from infected brown trout, Salmo trutta L., to newly hatched F. sultana colonies in BMC, then from the infected bryozoan to SPF brown trout. This study demonstrated the utility of BMC (pH 6.65) for hatching statoblasts, long-term cultivation of clean and transparent bryozoan colonies and maintenance of the Tetracapsuloides bryosalmonae life cycle in the laboratory for molecular genetic research and other studies such as host-parasiteinteraction.

Antibody-coated gold nanoparticles immunoassay for direct detection of Aeromonas salmonicida in fish tissues

Aeromonas salmonicida is the causative agent of furunculosis, a disease that affects both salmonid and non-salmonid fish. Detection of A. salmonicida can be labour intensive and time consuming because of the difficulties in distinguishing the bacterium from other species given the wide variety of existing biochemical profiles and the slow growth characteristics which allow other organisms to overgrow the A. salmonicida. Herein, we report the development of a specific immunoassay using gold-conjugated polyclonal antibodies for the rapid detection of A. salmonicida in fish tissues. Monodispersible 13-nm gold nanoparticles were coated with polyclonal antibodies specific to A. salmonicida. Reddish purple agglutination of gold particles indicated the presence of A. salmonicida in samples. Positive reactions were detected visually with the naked eye. No agglutination was observed when A. salmonicida antibody-coated gold nanoparticles were tested with other common bacterial fish pathogens, thereby verifying the specificity of the assay. The assay could detect A. salmonicida in fish tissues down to 1 × 10(4)  CFU mL(-1) , and results were obtained within 45 min. The antibody-coated gold nanoparticles were stable for at least 2 months at 4 ° C. The immunoassay using antibody-coated gold nanoparticles represents a promising tool for the rapid and specific detection of A. salmonicida in fish tissues.

Analysis of rainbow trout Oncorhynchus mykiss epidermal mucus and evaluation of semiochemical activity for polar filament discharge in Myxobolus cerebralis actinospores

As myxozoan actinospores are stimulated by fish epidermal mucus to attach to their hosts via extrusion of filaments from specialized organelles, the polar capsules, mucus components were tested for discharge triggering activity on Myxobolus cerebralis actinospores. Using various methodological approaches, a selective exclusion of candidate substances based on experimental outcome is provided and the physiochemical traits of the putative agents are explored to create a basis for the isolation of the host recognition chemostimuli. Activity was detected in compounds that can be characterized as small molecular, amphiphilic to slightly hydrophobic organic substances. They were separable by chromatographic methods using reversed phase C18 supports. An active fraction was isolated by solid phase extraction comprising at least nine UV-detectable constituents as shown by thin-layer chromatography. By means of biochemical fractionation and analysis of host fish mucus, non-volatile inorganic electrolytes, all volatiles, free L-amino acids, glycoproteins, bound and free hexoses, sialic acids, glycans, proteins, urea, amines and inositols were shown not to trigger polar filament discharge. The results contribute to the identification of the attachment host cues and enable a more focused laboratory activation of myxozoan actinospores.

Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea) portal of entry into the fish host

The portal of entry and the penetration process of the myxozoan parasite Tetracapsuloides bryosalmonae are still poorly understood. In the present study, spores of T. bryosalmonae derived from the bryozoan host (malacospores) were activated chemically and mechanically to investigate their reaction after attachment to the fish host in vitro. Amoeboid movement of both sporoplasms was shown for the first time. The morphology of malacospores was assessed using scanning electron microscopy (SEM). Openings of the polar capsules and released polar filaments were visible. One sporoplasm was observed leaving the spore shell. Laboratory exposure experiments of juvenile rainbow trout Oncorhynchus mykiss to spores of T. bryosalmonae were also conducted. Single fish were incubated with 1000 to 2000 spores in 100 ml of water for 5 to 60 min. Immunohistochemically stained sections of skin and gills were examined using light microscopy, as well as ultra thin sections using transmission electron microscopy (TEM), to investigate attachment and early penetration. Whole fish and excised gills of fish exposed to a spore suspension were fixed and prepared for scanning electron microscopy. Attached and penetrating stages were found only on or in the gills, and not in the skin. Due to the low overall number of spores, only a few spores were found adjacent to the gill epithelium in TEM. No parasite stages were found on the samples used for SEM. These results indicate that the gills are the preferred entry loci for the amoeboid sporoplasms of T. bryosalmonae into the fish host.

Characterization of glycans in the developmental stages of Myxobolus cerebralis (Myxozoa), the causative agent of whirling disease

Glycans and sugar-binding molecules (lectins) form an interactive recognition system, which may enable parasitic organisms to adhere to host cells and migrate into target tissues. The aim of the present study was to analyse surface-associated glycans in the developmental stages of Myxobolus cerebralis (Hofer), the causative agent of whirling disease. A panel of biotin-labelled plant lectins was used to detect a broad spectrum of glycan motifs with high specificity. Binding sites were detected histochemically in the tissue sections of infected rainbow trout, Oncorhynchus mykiss (Walbaum), and infected Tubifex tubifex (Müller), and were characterized by light, fluorescence and transmission electron microscopy. With mannose-specific lectins [Lens culinaris agglutinin, Pisum sativum agglutinin, Canavalia ensiformis agglutinin (LCA, PSA, CanA)] mannose-containing glycans were detected in all the developmental stages and host tissues. No binding sites for galactose-specific lectins were present in M. cerebralis spores but reactivity with host tissues occurred. Diversity in glycans was detected by N-acetyl-D-galactosamine-specific lectins in sporoplasm cells of M. cerebralis and triactinomyxon spores. In the group of lectins with monosaccharide-specificity for N-acetyl-D-glucosamine (GlcNAc), the reactivity of Datura stramonium agglutinin (DSA), Lycopersicon esculentum agglutinin (LEA) and Solanum tuberosum agglutinin (STA) was restricted to polar capsules whereas Griffonia simplicifolia agglutinin II (GSA II) also bound to sporoplasm cells of stages in the fish host but not in those present in infected T. tubifex. Moreover, Triticum vulgaris (wheat germ) agglutinin (WGA) and succinylated WGA indicated the presence of N-acetyl-D-glucosamine polymers in polar capsules. No specificity for spores was observed concerning 'bisected'N-glycans and no reactivity in parasitic stages was observed with the fucose-binding lectin Ulex europaeus agglutinin (UEA) I, Sambucus nigra agglutinin (SNA) (specific for alpha2,6-sialylated glycans) and Maackia amurensis agglutinin (MAAI) (specific for alpha2,3-sialylated glycans). Arachis hypogaea (peanut) agglutinin (PNA), Erythrina cristagalli agglutinin (ECA), GSA I, Sophora japonica agglutinin (SJA), Dolichos biflorus agglutinin (DBA) and GSA II detected reactive sites solely confined to the developmental stages of M. cerebralis and were not reactive in the fish host. These parasite-specific glycans may play a role in the adhesion process of the parasite to fish epidermis prior to infection, but may provide protection to the host by activating the complement system, or stimulating an adaptive immune response as putative antigens.

Myxozoan transmission via actinospores: new insights into mechanisms and adaptations for host invasion

Various mechanisms that enable and improve transmission success of myxozoan actinospore stages towards fish hosts are described, based upon a combination of experimental data and functional analysis of morphological characters. For this purpose, laboratory-reared actinospores of Myxobolus cerebralis, Myxobolus parviformis, Henneguya nuesslini and Myxobolus pseudodispar were employed to exemplarily investigate aspects of host attachment and invasion. The process of polar filament discharge of M. cerebralis actinospores was analysed, showing that full discharge occurs in less than 10 msec. Additionally, a mechanism that rapidly contracts the discharged filament after discharge is described for the first time. Its purpose is most likely to bring the actinospore apex rapidly into intimate contact with the surface of the host. Unlike M. cerebralis, M. parviformis actinospores did not discharge polar filaments after mechanical and chemical stimulation, suggesting a different mode of triggering. For H. nuesslini actinospores, experimental results indicated that polar filament discharge is independent of external calcium-ion concentration but is influenced by osmolality. After attachment of an actinospore and prior to penetration into the host, an ensheathed unit ('endospore'), containing the sporoplasm, was emitted from the valves in a manner which varied from species to species. Experimentally induced sporoplasm emission was time-dependent and was found to be independent of polar filament discharge in H. nuesslini. Remarkably, it could be concluded that the sporoplasm is able to recognize host-stimuli while still within the intact spore. An updated summary of the sequential course of events during host recognition and invasion by actinospores is given.

Molecular diagnostic methods for detection of Thelohania contejeani (Microsporidia), the causative agent of porcelain disease in crayfish

Diagnosis of Thelohania contejeani in the crayfish Astacus astacus is currently based on observation of gross clinical signs--opaque appearance of the abdomen and whitish colouration of the musculature--and confirmed by microscopic examination of histological sections of muscle. We have developed 2 molecular diagnostic methods for sensitive and rapid detection of porcelain disease in its early stages: PCR and loop-mediated isothermal amplification (LAMP). The PCR test utilises a primer based on the T. contejeani small subunit ssu ribosomal RNA (ssu rRNA) gene and amplified parasite DNA with high specificity and a detection limit of 10(-5) dilution. The LAMP assay involves incubation of the target DNA with a set of 6 primers and Bst DNA polymerase for 60 min at 65 degrees C in a water bath or heating block, followed by visualisation of the reaction products with the SYBR Green I stain; sensitivity of visual detection with SYBR Green I is equivalent to that with agarose gel electrophoresis. The LAMP assay can detect T. contejeani DNA to a dilution of 10(-7). The LAMP assay is 100 times more sensitive than the PCR test and is the method we recommend as an alternative to traditional means of diagnosing T. contejeani.

Reverse transcription loop-mediated isothermal amplification (RT-LAMP) for rapid detection of viral hemorrhagic septicaemia virus (VHS)

A one step reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed for detection of viral hemorrhagic septicaemia virus (VHS). A set of six primers were designed, based on the G-protein sequence of the VHS virus serotypes (He, F1, 23.75, Klapmolle and Rindsholm). The assay was optimised to amplify VHS RNA by incubation at 63 °C for only 1 h, and required only a simple water bath or heating block to provide a constant temperature of 63 °C. RT-LAMP amplification products were detected by visual inspection using SYBR Green I stain and had a ladder-like appearance when electrophoresed on an agarose gel. The detection limit of the RT-LAMP assay was found to be similar to the commonly used RT-PCR method: both methods detected VHS RNA at a dilution of 10⁶. The assay was evaluated using clinical samples and the results indicated the suitability and simplicity of the test as a rapid, field diagnostic tool for VHS virus.

Development of a rapid assay for the diagnosis of Myxobolus cerebralis in fish and oligochaetes using loop-mediated isothermal amplification

A loop-mediated isothermal amplification assay was developed for the rapid detection of Myxobolus cerebralis in both fish and oligochaete hosts. The assay was optimized to amplify parasitic DNA by incubation with Bst DNA polymerase and a set of six specially constructed primers at 65 degrees C for 60 min. The amplification products were detected visually using SYBR Green I dye which gave identical results to gel electrophoresis analysis. Parasite DNA was detected from infected oligochaetes, and from the anal fin, caudal fin, dorsal fin and operculum of clinically infected fish. This 'Myxo-LAMP' assay has a detection limit similar to that of a polymerase chain reaction assay (10(-6)), but is more rapid and only requires a water bath for amplification and is therefore practical for simple and rapid diagnosis of infected tissue.

Polar filament discharge of Myxobolus cerebralis actinospores is triggered by combined non-specific mechanical and chemical cues

This study presents initial evidence for the requirement of both chemical and mechanical stimuli to discharge polar capsules of Myxobolus cerebralis actinospores, the causative agent of salmonid whirling disease. The obligate need for combined discharge triggers was concluded from data obtained in a before/after experimental set-up carried out with individual locally immobilized actinospores. Homogenized rainbow trout mucus as chemostimulus and tangency of the apical region of the spores to achieve mechanical stimulation were applied subsequently. The actinospores showed discharged polar filaments exclusively when mucus substrate application was followed by touching the polar capsule-bearing region, but not when either stimulus was offered solely to the same individuals. We measured filament discharge rates to mucus preparations in a microscopic assay using supplementary vibration stimuli to ensure mechanical excitation. The actinospores responded similarly to different frequencies, which suggested a touch-sensitive recognition mechanism. Discharge specificity for salmonid mucus could not be confirmed, as mucus of common carp and bream could trigger similar filament expulsion rates. To a lesser extent homogenized frog epidermis and bovine submaxillary mucin could also stimulate the attachment reaction. In contrast, mucus of a pulmonate freshwater snail elicited no response.

The life cycle of Henneguya nuesslini Schuberg & Schroder, 1905 (Myxozoa) involves a triactinomyxon-type actinospore

The life cycle of the histozoic myxozoan parasite Henneguya nuesslini was investigated in two salmonid host species. Naive brown trout, Salmo trutta, and brook trout, Salvelinus fontinalis, were experimentally infected in two trials by triactinomyxon type actinospores from naturally infected Tubifex tubifex. In exposed common carp, Cyprinus carpio, no myxospore production was detected. The parasite formed cysts with mature myxospores in the connective tissue of the fish 102 days post-exposure. The morphology of both actinosporean and myxosporean stages was described by light microscopy and a 1417-bp fragment of the 18S rDNA gene was sequenced. Sequence analysis confirmed the absolute congruence of the two developmental stages and assisted in determining species identity. Host range, tissue specificity and myxospore measurements provided sufficiently distinctive features to confirm species validity and were thus crucial for identification. The triactinomyxon spores had 16 secondary germ cells, unique dimensions, a very opaque sporoplasm matrix and three conspicuously protruding, pyriform polar capsules. This is the first record of a Henneguya sp. life cycle with a triactinomyxon-type actinospore, which suggests a close relationship with the Myxobolus group and a polyphyletic origin of the genus Henneguya.

Seasonal occurrence of actinosporeans (Myxozoa) and oligochaetes (Annelida) at a trout hatchery in Bavaria, Germany

A systematic inventory of actinosporeans and oligochaetes conducted over 3 years at a trout fish farm in Bavaria, Germany, allowed the identification of 12 actinosporeans from five collective groups: four Triactinomyxon (Triactinomyxon nov. types 1-4), two Raabeia (Raabeia nov. types 1, 2), two Echinactinomyxon ( E. radiatum, Echinactinomyxon nov. type 1), two Aurantiactinomyxon ( A. pavinsis, Aurantiactinomyxon nov. type 1) and two Neoactinomyxum (Neoactinomyxum nov. types 1, 2). Nine forms are novel but can be placed within existing collective groups. All 12 forms were detected in the laboratory in aquarium water associated with farm sediment. However, only four of these could be linked with an oligochaete host. Three families of oligochaetes were identified from the sediment: Tubificidae, represented by eight species, Lumbriculidae with one species and Naididae with two species. Only tubificid oligochaetes were found to host actinosporeans.

Early developmental stages of two actinosporeans, Raabeia and Aurantiactinomyxon (Myxozoa), as detected by light and electron microscopy

The development of actinosporeans in their oligochaete host proceeding pansporocyst formation is relatively well documented, however, phases preceding it are not as well known. The initial stages in the development of two actinosporeans, Raabeia type 1 of Oumouna et al. [Parasitol. Res. 2002] and Aurantiactinomyxon pavinsis (Ormières, 1968) Marquès [Languedoc, Universite des Sciences et Techniques, Dissertation, 1984] from schizogony to gametogony and sporogony are described. Both actinosporeans begin their development as multinucleate stages near the basal lamina of the oligochaete intestine. Proximal to these stages and between the host epithelium cells are uninucleate cells whose nuclei divide to produce binucleate cells. These divide mitotically to produce cells with four nuclei which then undergo plasmotomy to yield a tetracellular stage and the first phase in pansporocyst formation. From the uninucleate stage to the tetranucleate stage, the cell membrane of the parasite is associated closely via finger-like projections with the intestinal epithelial and glandular cells of the host.

Recent advances in our knowledge of the Myxozoa

In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis. and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete. Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that: 1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.

Electron microscopic study of a new microsporean Microsporidium epithelialis sp. n. infecting Tubifex sp. (Oligochaeta)

The cytology of a new microsporean parasite Microsporidium epithelialis sp. n. from the intestinal epithelial cells of the freshwater oligochaete Tubifex sp. (Tubificidae) is described. The microsporean occurred together with an actinosporean of the genus Triactinomyxon, which was found between the epithelial cells. The merogonic and sporogonic stages (mature spores included) of the microsporean parasite are monokaryotic. An individual sporophorous vesicle surrounds each spore. The fixed and stained spore has an average dimension of 1.9-2.5 x 0.9-1.2 microm. The spores are oval with a characteristic surface layer, showing ornamentation-like projections, which are in close contact to the exospore. A short polar filament forming three to four coils traverses the polaroplast with two lamellar layers. The ultrastructure and other characteristic features of this microsporean parasite are distinct from those of the microsporean species described so far from oligochaetes.

Detection of early developmental stages of Myxobolus cerebralis in fish and tubificid oligochaete hosts by in situ hybridization

The myxosporean and actinosporean spores of Myxobolus cerebralis develop through many stages in their respective hosts, salmonid fishes and a tubificid oligochaete. Using a modified, non-radioactive in situ hybridization protocol, the parasite, which exhibits radically different structural forms during its development in each host, could be specifically detected in paraffin-embedded tissues of both fish and oligochaetes. Our study aims to demonstrate the application of the technique for detection of early stages of M. cerebralis in both hosts.

Effect of water temperature on the development, release and survival of the triactinomyxon stage of Myxobolus cerebralis in its oligochaete host

The development of the triactinomyxon stage of Myxobolus cerebralis and release of mature spores from Tubifex tubifex were shown to be temperature dependent. In the present work, the effect of temperature over a range of 5-30 degrees C on the development and release of the triactinomyxon stages of M. cerebralis was studied. Infected T. tubifex stopped releasing triactinomyxon spores 4 days after transfer from 15 degrees C to 25 degrees C or 30 degrees C. Transmission electron microscopic examinations of the tubificids held at 25 degrees C and 30 degrees C for 3 days showed that all developmental stages degenerated and transformed to electron-dense clusters between the gut epithelial cells of T. tubifex. In contrast, tubificid worms held at 5 degrees C and 10 degrees C examined at the same time were heavily infected with many early developmental stages of triactinomyxon. At 15 degrees C, the optimal temperature for development, maturing and mature stages of the parasite were evident. Infected T. tubifex transferred from 15 degrees C to 20 degrees C stopped producing triactinomyxon spores after 15 days. However, 15 days at 20 degrees C was not sufficient to destroy all developmental stages of the parasite. When the tubificid worms were returned to 15 degrees C, the one-cell stages and the binucleate-cell stages resumed normal growth. It was also demonstrated that T. tubifex cured of infection by holding at 30 degrees C for 3 weeks and shifted to 15 degrees C could be re-infected with M. cerebralis spores. The waterborne triactinomyxon spores of M. cerebralis did not appear to be as short-lived as previously reported. More than 60% of experimentally produced waterborne triactinomyxon spores survived and maintained their infectivity for rainbow trout for 15 days at water temperatures up to 15 degrees C. In natural aquatic systems, the triactinomyxon spores may survive and keep their infectivity for periods even longer than 15 days.

Determination of nuclear DNA concentration in cells of Myxobolus cerebralis and triactinomyxon spores, the causative agent of whirling disease

Myxobolus cerebralis (Myxozoa: Myxosporea) has a complex two-host life cycle, which begins when waterborne triactinomyxon spores released from the infected oligochaete Tubifex tubifex contact a susceptible trout. Upon contact the triactinomyxon spores attach to the fish and release their sporoplasm cells into the epidermis. At approximately 50 days postinfection, sporogenesis begins, resulting in a large number of M. cerebralis spores in the cartilage of infected fish 6 weeks later. The spores of M. cerebralis can be released from infected fish only after the fish die or are eaten by predators. In both cases, spores released into the aquatic environment can be ingested by oligochaete worms of the species T. tubifex and then develop into the actinosporean triactinomyxon stage in the intestine within about 3 months. The triactinomyxon is the only stage infectious for salmonid fish. We determined the DNA concentration in sporoplasm cells, capsulogenic cells, and valvogenic cells of M. cerebralis spore stages from the trout and of triactinomyxon spore stages from T. tubifex. DNA was visualized using the DNA-specific fluorescent stain DAPI. Our results demonstrate that meiosis occurs only once in the developmental cycle of M. cerebralis in contrast to the previously published hypothesis. This takes place within the pansporocyst found in T. tubifex. Thereafter, the sporoplasm cells of the triactinomyxon spores in T. tubifex and M. cerebralis in trout are diploid.

Light and electron microscopic studies on the chronological development of Myxobolus cerebralis to the actinosporean stage in Tubifex tubifex

Whirling disease caused by Myxobolus cerebralis has become the most widely known disease of salmonids in the 1990s. In the last 5 years we have studied many aspects regarding the host-pathogen relationship of this parasite. The parasite's histozoic development causes significant damage to cartilage and induces CNS symptoms by pressure on the brain and spinal cord. Myxobolus cerebralis has a two-host life-cycle involving a salmonid fish and a tubificid oligochaete. Two different stages of sporogony occur, one in each host. Early developmental stages in the fish can be found multiplying in the epidermis and peripheral and central nervous systems. The presporogenic stages then migrate to vertebral and cranial cartilages, where the first sporogonic phase occurs. Mature M. cerebralis spores found in fish cartilage are infectious for T. tubifex when ingested by the oligochaete after destruction of the infected fish. In the gut lumen of the tubificid, the spores extrude their polar capsules and attach to the gut epithelium by polar filaments. The shell valves then open along the suture line and the sporoplasm penetrates between the gut epithelial cells. The binucleate sporoplasm multiplies by schizogony, producing many one-cell stages which begin gamogonic development. As a result of the multiplication process, the intercellular space of the epithelial cells in more than 10 neighbouring worm segments may become infected. At this time (60-90 days p.i.), pansporocysts with eight zygotes start the sporogonic phase. The final stage of this development is a pansporocyst containing eight folded triactinomyxon spores. Shortly afterwards, the spores are liberated into the gut lumen. The spores reach the water either by egestion or following the death of the infected tubificids. Infected tubificids can release triactinomyxons for at least 1 year. The ultrastructure of all four phases, schizogony, gametogony, gametogamy and sporogony, is demonstrated and discussed.