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Concept: Myxozoa

247

The Myxozoa comprise over 2,000 species of microscopic obligate parasites that use both invertebrate and vertebrate hosts as part of their life cycle. Although the evolutionary origin of myxozoans has been elusive, a close relationship with cnidarians, a group that includes corals, sea anemones, jellyfish, and hydroids, is supported by some phylogenetic studies and the observation that the distinctive myxozoan structure, the polar capsule, is remarkably similar to the stinging structures (nematocysts) in cnidarians. To gain insight into the extreme evolutionary transition from a free-living cnidarian to a microscopic endoparasite, we analyzed genomic and transcriptomic assemblies from two distantly related myxozoan species, Kudoa iwatai and Myxobolus cerebralis, and compared these to the transcriptome and genome of the less reduced cnidarian parasite, Polypodium hydriforme. A phylogenomic analysis, using for the first time to our knowledge, a taxonomic sampling that represents the breadth of myxozoan diversity, including four newly generated myxozoan assemblies, confirms that myxozoans are cnidarians and are a sister taxon to P. hydriforme. Estimations of genome size reveal that myxozoans have one of the smallest reported animal genomes. Gene enrichment analyses show depletion of expressed genes in categories related to development, cell differentiation, and cell-cell communication. In addition, a search for candidate genes indicates that myxozoans lack key elements of signaling pathways and transcriptional factors important for multicellular development. Our results suggest that the degeneration of the myxozoan body plan from a free-living cnidarian to a microscopic parasitic cnidarian was accompanied by extreme reduction in genome size and gene content.

Concepts: DNA, Gene, Genetics, Gene expression, Genome, Animal, Cnidaria, Myxozoa

169

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.

Concepts: Fish, Salmo, Rainbow trout, Brown trout, Brook trout, Myxozoa, Tetracapsuloides bryosalmonae, Myxobolus cerebralis

144

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.

Concepts: Experiment, Myxozoa, Tetracapsuloides bryosalmonae, Myxobolus cerebralis, Myxobolus, Lineage III, Myxosporea

28

Myxobolus cerebralis is a microscopic metazoan parasite (Phylum Myxozoa: Myxosporea) associated with salmonid whirling disease. There are currently no vaccines to minimise the serious negative economical and ecological impacts of whirling disease among populations of salmonid fish worldwide. UV irradiation has been shown to effectively inactivate the waterborne infective stages or triactinomyxons of M. cerbralis in experimental and hatchery settings but the mechanisms by which the parasite is compromised are unknown. Treatments of triactinomyxons with UV irradiation at doses from 10 to 80 mJ/cm(2) either prevented (20-80 mJ/cm(2)) or significantly inhibited (10 mJ/cm(2)) completion of the parasite life cycle in experimentally exposed juvenile rainbow trout (Oncorhynchus mykiss). However, even the highest doses of UV irradiation examined (80 mJ/cm(2)) did not prevent key steps in the initiation of parasite infection, including attachment and penetration of the epidermis of juvenile rainbow trout as demonstrated by scanning electron and light microscopy. Furthermore, replication of UV-treated parasites within the first 24h following invasion of the caudal fin was suggested by the detection of concentrations of parasite DNA by quantitative PCR comparable to that among fish exposed to an equal concentration of untreated triactinomyxons. Subsequent development of parasites treated with an 80 mJ/cm(2) dose of UV irradiation however, was impaired as demonstrated by the decline and then lack of detection of parasite DNA; a trend beginning at 10 days and continuing thereafter until the end of the study at 46 days post parasite exposure. Treatments of triactinomyxons with a lower dose of UV irradiation (20 mJ/cm(2)) resulted in a more prolonged survival with parasite DNA detected, although at very low concentrations, in fish up to 49 days post parasite exposure. The successful invasion but only short-term survival of parasites treated with UV in rainbow trout resulted in a protective response to challenges with fully infective triactinomyxons. Prior treatments of juvenile rainbow trout with UV-treated triactinomyxons (10 and 20 mJ/cm(2)) resulted in a reduced prevalence of infection and significantly lower concentrations of cranial myxospores (two direct measures of the severity of whirling disease) compared with trout receiving no prior treatments when assessed 5 months post parasite exposure to fully infective triactinomyxons.

Concepts: Salmon, Oncorhynchus, Salmonidae, Rainbow trout, Cutthroat trout, Myxozoa, Myxobolus cerebralis, Myxosporea

10

Proliferative kidney disease (PKD) is a major threat to wild and farmed salmonid populations because of its lethal effect at high water temperatures. Its causative agent, the myxozoan Tetracapsuloides bryosalmonae, has a complex lifecycle exploiting freshwater bryozoans as primary hosts and salmonids as secondary hosts. We carried out an integrated study of PKD in a prealpine Swiss river (the Wigger). During a 3-year period, data on fish abundance, disease prevalence, concentration of primary hosts' DNA in environmental samples [environmental DNA (eDNA)], hydrological variables, and water temperatures gathered at various locations within the catchment were integrated into a newly developed metacommunity model, which includes ecological and epidemiological dynamics of fish and bryozoans, connectivity effects, and hydrothermal drivers. Infection dynamics were captured well by the epidemiological model, especially with regard to the spatial prevalence patterns. PKD prevalence in the sampled sites for both young-of-the-year (YOY) and adult brown trout attained 100% at the end of summer, while seasonal population decay was higher in YOY than in adults. We introduce a method based on decay distance of eDNA signal predicting local species' density, accounting for variation in environmental drivers (such as morphology and geology). The model provides a whole-network overview of the disease prevalence. In this study, we show how spatial and environmental characteristics of river networks can be used to study epidemiology and disease dynamics of waterborne diseases.

Concepts: Epidemiology, Disease, Infectious disease, Biology, Demography, Prevalence, Myxozoa, Tetracapsuloides bryosalmonae

8

Parasitic protozoan infections represent a major health burden in the developing world and contribute significantly to morbidity and mortality. These infections are often associated with considerable variability in clinical presentation. An emerging body of work suggests that the intestinal microbiota may help to explain some of these differences in disease expression. The objective of this minireview is to synthesize recent progress in this rapidly advancing field. Studies in humans, animal models and in vitro concerning the contribution of the intestinal microbiota to infectious disease will be discussed. We hope to provide an understanding of the human-protozoal pathogen-microbiome interaction and to speculate on how that might be leveraged for treatment.

Concepts: Epidemiology, Infectious disease, Bacteria, Microbiology, Infection, Protozoa, Myxozoa

4

In the present study, a new species Myxobolus dermiscalis n. sp. infecting scales of Labeo rohita, an Indian major carp from Harike Wetland in Punjab, India has been described on the basis of spore morphology and amplification of a part of 18S rDNA gene. The pseudocysts of M. dermiscalis n. sp. are milky white with irregular outline, 0.5-3.6 mm in diameter embedded within the dermal scale in the form of a cavity. The spores 5.84-7.98 × 3.98-5.98 μm in size, having two equal polar capsules 3.98-5.98 × 1.85-3.85 μm in size. The most differentiating feature from closely related species, Myxobolus saugati (Kaur and Singh, 2011) is the presence of two parietal folds at the posterior - lateral margins of the shell valves. The present species is regarded as host, organ and tissue specific in nature. The partial sequence of SSU gene of M. dermiscalis n. sp. clustered with other Myxobolus species infecting cyprinids available in the GenBank. Blast search revealed 98% homogeneity with Myxobolus sp (KM401439) infecting scales of L. rohita in Myanmar (unpubl. data). The present myxobolid parasite has been recorded to cause serious, highly symptomatic disease of the scales, causing their loosening from the skin of L. rohita. It rendered the host fish unsightly giving it cloudy appearance with white patches and mucoid body surface. Scale pseudocyst Index (SPI) has been provided to record the intensity of infection.

Concepts: Skin, India, Uttar Pradesh, Myxozoa, Myxobolus cerebralis, Labeo, Rohu, Myxosporea

1

Myxozoan parasites pose emerging health issues for wild and farmed salmonid fish. Rainbow trout (Oncorhynchus mykiss) is a particularly susceptible species to Tetracapsuloides bryosalmonae (Malacosporea), the etiological agent of Proliferative Kidney Disease (PKD), and to Myxobolus cerebralis (Myxosporea), the etiological agent of Whirling Disease (WD). The objective of this study was to investigate the impact of myxozoan co-infections on the pathogenesis of PKD and WD in the rainbow trout.

Concepts: Salmon, Oncorhynchus, Salmonidae, Rainbow trout, Myxozoa, Tetracapsuloides bryosalmonae, Myxobolus cerebralis, Myxosporea

1

Fredericella sultana is an invertebrate host of Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonids. The bryozoan produces seed-like statoblasts to facilitate its persistence during unfavourable conditions. Statoblasts from infected bryozoans can harbor T. bryosalmonae and give rise to infected bryozoan colonies when conditions improve. We aimed in the present study to evaluate the integrity and viability of T. bryosalmonae-infected statoblasts after a range of harsh treatment conditions. We tested if statoblasts could survive ingestion by either brown trout or common carp. After ingestion, the fish faeces was collected at different time points. We also tested physical stressors: statoblasts collected from infected colonies were desiccated at room temperature, or frozen with and without Bryozoan Medium C (BMC). After treatments, statoblasts were assessed for physical integrity before being incubated on BMC to allow them to hatch. After 4 weeks, hatched and unhatched statoblasts were tested by PCR for the presence of the parasite. We found that statoblasts ingested by brown trout and those frozen in BMC were completely broken. In contrast, statoblasts ingested by common carp and those subjected to dry freezing were able to survive and hatch. T. bryosalmonae was detected by PCR in both hatched and unhatched infected statoblasts, but neither from broken nor uninfected statoblasts. Our results confirmed for the first time the ability of infected statoblasts to survive passage through a fish, and freezing. These findings suggest potential pathways for both persistence and spread of T. bryosalmonae-infected statoblasts in natural aquatic systems.

Concepts: Coprophagia, Animal, Salmo, Rainbow trout, Bryozoa, Myxozoa, Tetracapsuloides bryosalmonae, Myxobolus cerebralis

1

Myxozoa is a speciose group of endoparasitic cnidarians that can cause severe ecological and economic effects. Although highly reduced compared to free-living cnidarians, myxozoans have retained the phylum-defining stinging organelles, known as cnidae or polar capsules, which are essential to initiating host infection. To explore the adaptations of myxozoan polar capsules, we compared the structure, firing process and content release mechanism of polar tubules in myxospores of three Myxobolus species including M. cerebralis, the causative agent of whirling disease.

Concepts: Immune system, Animal, Cnidaria, Myxozoa, Tetracapsuloides bryosalmonae, Myxobolus cerebralis, Myxosporea, Radiata