Myxobolus cerebralis (Myxozoa), the causative agent of whirling disease, reduces fecundity and feeding activity of Tubifex tubifex (Oligochaeta)

Effect of substrate on the proliferation of Myxobolus cerebralis in the mitochondrial lineages of the Tubifex tubifex host

The study goal was to examine the effects of sand and mud on the propagation of Myxobolus cerebralis, the whirling disease agent, in four mitochondrial 16S ribosomal DNA lineages (I, III, V, VI) of its oligochaete host, Tubifex tubifex (Tt). In all the lineage groups held continuously in either substrate (non-shifted) or transferred from sand to mud (shifted), substrate influenced parasite proliferation only in lineage III. Sporogenesis and release of triactinomyxon spores (TAMs) were more prevalent in lineage III Tt in mud compared to sand. Low-infection prevalence and lack of parasite development in lineage I is associated with the greater number of resistant worms and were not affected by substrate type. Substrate did not impact Tt from lineages V and VI that failed to develop any parasite stages in either substrate even after shifting from sand to mud. The relationship between the microbial community in the substrate and parasite proliferation in lineage III was described but not analyzed due to small sample size. Substrate-associated bacteria were hypothesized as essential dietary source for the oligochaete host feeding selectively on fine (mud)-microflora. Progeny was produced by all lineage groups shifted to mud with disparate survival profiles in lineage V and VI and high mortalities in lineage III. Our study demonstrates that substrate type can alter parasite proliferation in lineage III. Conversely, parasite development and infectivity were not altered in lineage V and VI that are refractory to the parasite nor among the more resistant phenotypes (I), regardless of substrate type.

Survival of metazoan parasites in fish: Putting into context the protective immune responses of teleost fish

Defence mechanisms of fish can be divided into specific and non-specific that act in concert and are often interdependent. Most fish in both wild and cultured populations are vulnerable to metazoan parasites. Endoparasitic helminths include several species of digeneans, cestodes, nematodes, and acanthocephalans. Although they may occur in large numbers, helminth infections rarely result in fish mortality. Conversely, some ectoparasites cause mass mortality in farmed fish. Given the importance of fish innate immunity, this review addresses non-specific defence mechanisms of fish against metazoan parasites, with emphasis on granulocyte responses involving mast cells, neutrophils, macrophages, rodlet cells, and mucous cells. Metazoan parasites are important disease agents that affect wild and farmed fish and can induce high economic loss and, as pathogen organisms, deserve considerable attention. The paper will provide our light and transmission electron microscopy data on metazoan parasites-fish innate immune and neuroendocrine systems. Insights about the structure and functions of the cell types listed above and a brief account of the effects and harms of each metazoan taxon to specific fish apparati/organs will be presented.

Transcriptomic analysis of oligochaete immune responses to myxosporeans infection: Branchiura sowerbyi infected with Myxobolus cultus

The Myxozoa are endoparasites characterized by a two-host life cycle that typically involves invertebrates and vertebrates as definitive and intermediate hosts, respectively. However, little is known about invertebrate-myxosporean interactions, particularly about patterns of host immune defense. We used RNA-sequencing to identify genes that are possibly involved in the immune responses of the oligochaete Branchiura sowerbyi naturally infected with Myxobolus cultus. De novo assembly of the B. sowerbyi transcriptome yielded 119,031 unigenes, with an average length of 896 bp and an N50 length of 1754 bp. Comparative transcriptome analysis revealed 4059 differentially expressed genes (DEGs) between M. cultus-infected and uninfected B. sowerbyi groups, including 3802 upregulated genes and 257 downregulated genes. Among the B. sowerbyi immune factors implicated in the responses to M. cultus infection, DEGs related to lectins, ubiquitin-mediated proteolysis, phagocytosis, oxidative-antioxidative responses, proteases, and protease inhibitors were upregulated. The expression of some immune-related molecules such as calmodulin, heat shock proteins, antimicrobial peptides, lysenin, and serum amyoid A protein were also significantly upregulated. The expression patterns of 14 immune-related DEGs identified by RNA-seq were validated by quantitative real-time polymerase chain reaction. This study is the first attempt to characterize the B. sowerbyi transcriptome and identify immune-related molecules possibly associated with M. cultus infection. It is also the first report of invertebrate host-myxosporean interactions at the transcriptomic level. Our results will facilitate the elucidation of adaptive evolution mechanisms of myxosporean parasites in the definitive host and the genetic basis for differences in resistance of invertebrate hosts of different genotypes to a myxosporean species.

Amphipod parasites may bias results of ecotoxicological research

Amphipods are commonly used test organisms in ecotoxicological studies. Nevertheless, their naturally occurring parasites have mostly been neglected in these investigations, even though several groups of parasites can have a multitude of effects, e.g. on host survival, physiology, or behavior. In the present review, we summarize the knowledge on the effects of Microsporidia and Acanthocephala, 2 common and abundant groups of parasites in amphipods, on the outcome of ecotoxicological studies. Parasites can have significant effects on toxicological endpoints (e.g. mortality, biochemical markers) that are unexpected in some cases (e.g. down-regulation of heat shock protein 70 response in infected individuals). Therefore, parasites can bias the interpretation of results, for example if populations with different parasite profiles are compared, or if toxicological effects are masked by parasite effects. With the present review, we would like to encourage ecotoxicologists to consider parasites as an additional factor if field-collected test organisms are analyzed for biomarkers. Additionally, we suggest intensification of research activities on the effects of parasites in amphipods in connection with other stressors to disentangle parasite and pollution effects and to improve our understanding of parasite effects in this host taxon.

Persistence, impacts and environmental drivers of covert infections in invertebrate hosts

Background

Persistent covert infections of the myxozoan, Tetracapsuloides bryosalmonae, in primary invertebrate hosts (the freshwater bryozoan, Fredericella sultana) have been proposed to represent a reservoir for proliferative kidney disease in secondary fish hosts. However, we have limited understanding of how covert infections persist and vary in bryozoan populations over time and space and how they may impact these populations. In addition, previous studies have likely underestimated covert infection prevalence. To improve our understanding of the dynamics, impacts and implications of covert infections we employed a highly sensitive polymerase chain reaction (PCR) assay and undertook the first investigation of covert infections in the field over an annual period by sampling bryozoans every 45 days from three populations within each of three rivers.

Results

Covert infections persisted throughout the year and prevalence varied within and between rivers, but were often > 50%. Variation in temperature and water chemistry were linked with changes in prevalence in a manner consistent with the maintenance of covert infections during periods of low productivity and thus poor growth conditions for both bryozoans and T. bryosalmonae. The presence and increased severity of covert infections reduced host growth but only when bryozoans were also investing in the production of overwintering propagules (statoblasts). However, because statoblast production is transitory, this effect is unlikely to greatly impact the capacity of bryozoan populations to act as persistent sources of infections and hence potential disease outbreaks in farmed and wild fish populations.

Conclusions

We demonstrate that covert infections are widespread and persist over space and time in bryozoan populations. To our knowledge, this is the first long-term study of covert infections in a field setting. Review of the results of this and previous studies enables us to identify key questions related to the ecology and evolution of covert infection strategies and associated host-parasite interactions.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2495-8) contains supplementary material, which is available to authorized users.

Accelerated deactivation of Myxobolus cerebralis myxospores by susceptible and non-susceptible Tubifex tubifex

In the 1990s, the Tubifex tubifex aquatic oligochaete species complex was parsed into 6 separate lineages differing in susceptibility to Myxobolus cerebralis, the myxozoan parasite that can cause whirling disease (WD). Lineage III T. tubifex oligochaetes are highly susceptible to M. cerebralis infection. Lineage I, IV, V and VI oligochaetes are highly resistant or refractory to infection and may function as biological filters by deactivating M. cerebralis myxospores. We designed a 2-phased laboratory experiment using triactinomyxon (TAM) production as the response variable to test that hypothesis. A separate study conducted concurrently demonstrated that M. cerebralis myxospores held in sand and water at temperatures ≤15°C degrade rapidly, becoming almost completely non-viable after 180 d. Those results provided the baseline to assess deactivation of M. cerebralis myxospores by replicates of mixed lineage (I, III, V and VI) and refractory lineage (V and VI) oligochaetes. TAM production was zero among 7 of 8 Lineage V and Lineage VI T. tubifex oligochaete groups exposed to 12500 M. cerebralis myxospores for 15, 45, 90 and 135 d. Among 4 mixed lineage exposure groups, TAM production averaged 14641 compared with 2202495 among 12 groups of Lineage III oligochaetes. Among the 6 unexposed Lineage III experimental groups seeded into original Phase 1 substrates for the 45, 90 and 135 d treatments during the Phase 2 portion of the study, TAM production was reduced by 98.9, 99.9 and 99.9%, respectively, compared with the average for the 15 d exposure groups. These results are congruent with the hypothesis that Lineage V and Lineage VI T. tubifex oligochaetes can deactivate and destroy M. cerebralis myxospores.

Susceptibility to Myxobolus cerebralis among Tubifex tubifex populations from ten major drainage basins in Colorado where Cutthroat Trout are endemic

Establishment of Myxobolus cerebralis (Mc) resulted in declines of wild Rainbow Trout Oncorhynchus mykiss populations in streams across Colorado during the 1990s. However, the risk for establishment and spread of this parasite into high-elevation habitats occupied by native Cutthroat Trout O. clarkii was unknown. Beginning in 2003, tubificid worms were collected from all major drainages where Cutthroat Trout were endemic and were assayed by quantitative PCR to determine the occurrence and distribution of the various lineages of Tubifex tubifex (Tt) oligochaetes. Over a 5-year period, 40 groups of Tt oligochaetes collected from 27 streams, 3 natural lakes, 2 private ponds, and a reservoir were evaluated for their relative susceptibility to Mc. Exposure groups were drawn from populations of pure lineage III Tt, mixed-lineage populations where one or more of the highly resistant (lineage I) or nonsusceptible lineages (V or VI) were the dominant oligochaete and susceptible lineage III worms were the subdominant worm, or pure lineage VI Tt. Experimental replicates of 250 oligochaetes were exposed to 50 Mc myxospores per worm. The parasite amplification ratio (total triactinomyxons [TAMs] produced / total myxospore exposure) was very high among all pure lineage III Colorado exposure groups, averaging 363 compared with 8.24 among the mixed-lineage exposure groups. Lineage III oligochaetes from Mt. Whitney Hatchery in California, which served as the laboratory standard for comparative purposes, had an average parasite amplification ratio of 933 among 10 exposed replicates over a 5-year period. Lineage I oligochaetes were highly resistant to infection and did not produce any TAMs. Lineages V and VI Tt did not become infected and did not produce any TAMs. These results suggest that the risk of establishment of Mc is high for aquatic habitats in Colorado where Cutthroat Trout and lineage III Tt are sympatric.

Whirling disease dynamics: an analysis of intervention strategies

Whirling disease (WD), a severe and widespread disease of salmonids, is caused by the myxosporean parasite Myxobolus cerebralis. It is further characterized by a unique two-host life cycle, utilizing the oligochaete Tubifex tubifex as an intermediate host. M. cerebralis is an invasive species that has been affecting populations in the United States including epidemics that killed in excess of 90% of populations in Colorado and Montana streams within the past 20 years. Currently, there is no known cure for WD, and the accepted method of control is removal of infected fish from the population. We have created a compartmental model of the WD system in order to assess more efficient means of control and management of the disease. Using data gathered from the literature, we used Bayesian model fitting to estimate model parameters and estimated that R0≈1.51 (95% CI: 1.39, 1.72), a value which implies that WD can be controlled using available strategies. To this end, we posit several parameters that we expect to be most influential to WD propagation, namely: release of triactinomyxons by T. tubifex, release of spores by salmonids, and infectious particle loads in each respective host. Based on currently available control strategies, approaches targeting the infectious particles and the oligochaete host appear the most effective alternative strategies for management and control of WD.

Reduced Myxobolus cerebralis actinospore production in a Colorado reservoir may be linked to changes in Tubifex tubifex population structure

Elucidating the dynamics of a parasitic infection requiring two hosts in a natural ecosystem can be a daunting task. Myxobolus cerebralis (Mc), the myxozoan parasite that causes whirling disease in some salmonids, was detected in the Colorado River upstream of Windy Gap Reservoir (WGR) in 1988. Subsequently, whirling disease was implicated in the decline of wild Rainbow Trout Oncorhynchus mykiss in the river when WGR was identified as a point source of Mc triactinomyxons (TAMs). Between 1997 and 2004, numerous investigations began to elucidate the etiology of Mc in WGR. During this period, Mc TAM production in WGR declined more than 90%. Explanations for the decline have included differences in stream discharge between years, changes in the thermal regime of the lake, severe drought, changes in the fish population structure in WGR, and reductions in the prevalence and severity of Mc infection in salmonids in the Colorado and Fraser rivers upstream of WGR. All of these have been discredited as explanations for the reduced TAM production. In 2005, a new study was conducted to replicate the studies completed in 1998. In this paper, the results of a new real-time polymerase chain reaction assay utilized to quantify the mitochondrial 16S rDNA specific to each of four lineages of Tubifex tubifex in pooled samples of 50 oligochaetes are presented. These results suggest that compared with 1998, the densities of aquatic oligochaetes and T. tubifex have increased, TAM production has been greatly reduced, and the decline is congruent with the dominance of lineages I, V, and VI of T. tubifex-three lineages that are refractory or highly resistant to Mc infection-in the oligochaete population. While it is possible that the resistant lineages function as biofilters that deactivate Mc myxospores, the reason for the decline in TAM production in WGR remains an enigma.

The susceptibility of diverse species of cultured oligochaetes to the fish parasite Myxobolus pseudodispar Gorbunova (Myxozoa)

This study provides detailed information on the invertebrate hosts of Myxobolus pseudodispar (Myxozoa) and explores the susceptibility range of several species and analyses the relevance of the species composition of an oligochaete population. Our findings demonstrate that the oligochaete host range of M. pseudodispar is similarly wide as the number of vertebrate host species. Besides Tubifex tubifex and Limnodrilus hoffmeisteri, Psammoryctides barbatus and Psammoryctides moravicus were also found to be susceptible invertebrate hosts. The genetic characterization of the mitochondrial 16S rDNA of T. tubifex sensu lato revealed that lineages I, II and III are susceptible to M. pseudodispar, whereas T. tubifex lineage VI seems to be non-susceptible. T. tubifex lineage V and L. hoffmeisteri specimens were positive in a M. pseudodispar-specific PCR, but in most cases, the release of mature actinospores could not be detected. Hence, these non-susceptible oligochaetes likely serve as `biological filters` as they remove myxospores from the sediment without producing actinospores. Together with the phylogenetic analysis of the susceptible and non-susceptible oligochaete hosts on the basis of mt 16S rDNA sequences, the route of the development of M. pseudodispar in the oligochaete hosts was tracked by in situ hybridization. According to our findings, the gut epithelia seem to be a portal of entry of the sporoplasms, where the development of the parasite also takes place. The basal lamina seems to be involved in the migration of the parasite, and the worm's cellular immune response is activated by the infection.

The development of Myxobolus pavlovskii (Myxozoa: Myxobolidae) includes an echinactinomyxon-type actinospore

Echinactinomyxon-type actinospores were found in a mixed-species oligochaete culture originating from the Temperate Water Fish Hatchery near Budapest, Hungary. On the basis of DNA sequence analysis, the actinospores were identified as Myxobolus pavlovskii (Akhmerov, 1954), the 18S rDNA sequence from myxospores of which is available in GenBank. Silver carp Hypophthalmichthys molitrix (Valenciennes) fry specimens were successfully infected by cohabitation with the echinactinomyxon-releasing oligochaetes, which confirmed the molecular data congruence. The echinactinomyxons and the myxospores that developed in the gills of exposed fish fry were analysed morphologically and on DNA basis. The infected gill tissue was examined histologically. As typical characters of M. pavlovskii, numerous small plasmodia were observed in the epithelia of gill lamellae. Plasmodia contained thousands of myxospores with polar capsules unequal in size and witl; large intercapsular processes. The 18S rDNA sequence from actinospores and those from myxospores originating from the experimentally infected fish were identical. The oligochaete species releasing actinospores was morphologically determined as Limnodrilus sp. This is the first record of an echinactinomyxon as an alternate stage within the genus Myxobolus.

The marine myxosporean Sigmomyxa sphaerica (Thélohan, 1895) gen. n., comb. n. (syn. Myxidium sphaericum) from garfish (Belone belone (L.)) uses the polychaete Nereis pelagica L. as invertebrate host

Sigmomyxa sphaerica (Thélohan, 1892) gen. n. (Myxozoa, Myxosporea) with myxosporean stages in the gall bladder of Belone belone (L.) (Teleostei, Belonidae) uses the polychaete Nereis pelagica L. (Nereidae) from shallow water in the northern Øresund, Denmark, as invertebrate host. The nearly spherical tetractinomyxon-type actinospores of S. sphaerica differ from those of two species of Ellipsomyxa which also use Nereis spp. as invertebrate host. Pansporocysts of S. sphaerica were not seen. S. sphaerica is redescribed on the basis of myxospore stages from B. belone and actinospores from N. pelagica, and the phylogenetic affinities examined on the basis of ribosomal small subunit gene sequences. S. sphaerica is closest related to Ellipsomyxa spp., and is not congeneric with morphologically similar Myxidium spp. from gadids. This is the fifth elucidated two-host life cycle of a marine myxozoan.

Comparative susceptibility of Deschutes River, Oregon, Tubifex tubifex populations to Myxobolus cerebralis

Dams along the Deschutes River (DR) in central Oregon have blocked fish migration for over 40 years. Reestablishment of anadromous fish runs above the dams as part of a fish passage plan may introduce fish pathogens, such as Myxobolus cerebralis, the myxozoan parasite that causes salmonid whirling disease. This parasite is carried by adult salmon that stray into the DR system during their return to enzootic areas of the upper Columbia River basin, and it is now known to be established in at least one lower DR tributary. The life cycle of M. cerebralis involves two obligate hosts: a salmonid and the oligochaete worm Tubifex tubifex. To determine the likelihood of parasite establishment above the DR dams, we conducted benthic sediment surveys between 1999 and 2007 and found that T. tubifex had a patchy distribution and low relative abundance. Mitochondrial 16S ribosomal DNA gene analysis indicated that two lineages of T. tubifex (III and VI) were present both above and below the dams. Laboratory susceptibility studies to characterize differences in infection prevalence and parasite production between nine T. tubifex populations revealed that production varied considerably among exposed groups and was proportional to the number of lineage III worms present. Our results suggest that M. cerebralis could become established above the dams if infected fish are allowed passage into the upper DR system, but not all areas of the DR basin can be classified as having the same likelihood for parasite establishment, and the potential impact will be location dependent.

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.