A list of myxozoan genera is presented in the current taxonomical scheme. These genera are defined; their type species and most important pathogens along with their hosts are listed. Simultaneously, definitions of actinospore stages representing sexual stages of the myxosporean life cycle are given; altogether, 17 actinospore collective groups with 180 types have been described. Life cycles of the two classes of the phylum Myxozoa, Malacosporea and Myxosporea, are briefly outlined with specification of the appropriate terms. Up to now, 4 malacosporean and 2,180 myxosporean species assigned to a total of 62 genera, have been established. The surviving classification of myxosporeans, based on spore morphology, is discussed in the context of the still fragmentary data resulting from SSU rDNA sequence analyses. The main task for the future is a rigorous, detailed morphological description combined with molecular techniques in establishment of new species and in revision of the existing ones. Establishment of a classification acceptable from morphological, biological and phylogenetical viewpoints is necessary.

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Myxozoan genera: definition and notes on taxonomy, life-cycle terminology and pathogenic species.

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.

https://bioone.org/journals/the-journal-of-eukaryotic-microbiology/volume-48/issue-4/j.1550-7408.2001.tb00173.x/Recent-Advances-in-Our-Knowledge-of-the-Myxozoa/10.1111/j.1550-7408.2001.tb00173.x.pdf

Recent Advances in Our Knowledge of the Myxozoa

Infectious disease can be a concern for several aspects of conservation biology, such as determin- ing threats to species, estimating population viability, and designing reserves, captive breeding, and recovery programs. Several measures are useful for describing infectious diseases in host populations, but it is not straightforward to determine the degree to which a particular disease may affect a host population. The most basic epidemiological theory suggests that populations should be least subject to host-specific infectious dis- ease when they are at low abundance ( paradoxically, the state at which they are in most need of conserva- tion action). There are important exceptions, however, such as when a reservoir host exists or when Allee or stochastic effects occur. Several of the key threats to biodiversity—habitat alteration, introduced species, pol- lution, resource exploitation, and climate change—can facilitate and/or impair transmission of infectious disease. Common management tools such as population viability analysis rarely address infectious disease explicitly. We suggest that such an inclusion is both possible and warranted. Considerations of infectious dis- ease may influence the way we determine whether a species is in need of protection and how we might design reserves and captive breeding programs. Examples from the literature suggest that (1) introduced pathogens can make abundant species rare and (2) diseases of domestic animals can dramatically affect rare species. For both scenarios, conditions that cause stress or reduce genetic variation may increase susceptibility to dis- ease, whereas crowding and cross-species contact can increase transmission. Southern sea otters ( Enhydra lu- tris nereis ) make an interesting case study for consideration of the intersection of epidemiology and conserva- tion because disease may be an important factor limiting the growth of otter populations. We conclude that pathogens are of increasing concern for conservation. Because many newly emerging pathogen dynamics of- ten do not conform to the simplifying assumptions used in classic epidemiology, a detailed understanding of pathogen life history will illuminate the intersection of epidemiology and conservation theory.

Good Medicine for Conservation Biology: the Intersection of Epidemiology and Conservation Theory

A review of the myxosporean genus Kudoa Meglitsch, 1947, and its impact on the international aquaculture industry and commercial fisheries

While investigating the parasite fauna of wild coho salmon. Oncorhynchus kisutch (Walbaum, 1792), histological examination provided evidence of a new species of Myxobolus (Myxozoa: Myxosporea) infecting nerves of skeletal muscle. Spores were morphologically similar to those of the intramuscular Myxobolus insidiosus Wyatt and Pratt, 1963, both having pyriform spores with clavate polar capsules. However, the former developed exclusively in the nerves of skeletal muscle rather than in myocytes. We examined both species of Myxobolus derived from coho salmon; Chinook salmon, Oncorhynchus tshawytscha (Walbaum, 1792); cutthroat trout, Oncorhynchus clarkii (Richardson, 1836); and rainbow trout Oncorhynchus mykiss (Walbaum, 1792) from freshwater in Oregon. Spore morphology, small subunit ribosomal RNA gene (rDNA) sequences, and site of infection were compared. Myxobolus arcticus Pugachev and Khokhlov, 1979 has pyriform spores, infects the central nervous system of many salmonids, and is found in the Pacific Northwest. It was therefore included in the analyses to rule out conspecificity with the new species. Together, these data show that the Myxobolus sp. from peripheral nerves in the skeletal musculature of coho salmon, rainbow trout, and cutthroat trout is a new species, described herein as Myxobolus fryeri n. sp.

https://bioone.org/journals/journal-of-parasitology/volume-94/issue-6/GE-1606.1/Molecular-and-Morphological-Analysis-of-Myxobolus-Spp-of-Salmonid-Fishes/10.1645/GE-1606.1.pdf

Molecular and morphological analysis of Myxobolus spp. of salmonid fishes with the description of a new Myxobolus species.

Several myxosporean parasites are of importance in fisheries and aquaculture in British Columbia. The PKX organism and Ceratomyxa shasta Noble, 1950 cause disease and mortality, Kudoa thyrsites (Gilchrist, 1924) and Henneguya salminicola Ward, 1919 are of importance because they infect somatic muscle, cause unsightly cysts and soft flesh, and thus reduce the market value of the fish. Myxobolus arcticus Pugachev et Khokhlov, 1979, an apparently non-pathogenic species, along with H. salminicola, is used as a biological tag in fishery management. Myxobolus arcticus has also been used in our laboratory as a model for the study of myxosporean life cycles. Other myxosporeans that have been found in salmonids in British Columbia include Myxobolus squamalis (Iverson, 1954), Myxobolus insidiosus Wyatt et Pratt, 1963, Myxidium truttae Leger, 1930, Myxidium salvelini Shulman et Konovalov, 1966, Chloromyxum sp., Parvicapsula sp., and Sphaerospora sp.

Review of Myxosporea of importance in salmonid fisheries and aquaculture in British Columbia.

The helminths found in the gastrointestinal tracts and gallbladders of 68 sea otters (Enhydra lutris) from Prince William Sound, Alaska, and their prevalences were: Corynosoma enhydri (Acanthocephala), intes- tine, 51.5%; Orthosplanchnus fraterculus (Trematoda), gallbladder, 50%; Diplogonoporus tetrapterus (Cestoda), intestine, 12%; and Pseudoterranova decipiens (Nematoda), stomach, 1.5%. One pup examined was free of helminth parasites. Throughout its range from the Kuril and Komandorski islands in Russia, across the Aleutian Islands, Alaska, and south to California, the sea otter harbors a total of 17 or 18 species of helminths, of which 5 or 6 are incidental infections with larval (2 species of Anisakis) or juvenile worms (3 or 4 species of Poly- morphus). The adult worms of sea otters (12 species) are derived primarily (9 species) from pinnipeds. One species, Microphallus pirum (Trematoda), found widely in sea otters from the Komandorski Islands to California, may be primarily a parasite of various shorebirds. Only one species, C. enhydri, is uniquely a parasite of sea otters, occurring throughout its range at prevalences greater than 50%. California populations of sea otters harbor only M. pirum, C. enhydri, and Polymorphus species, lacking any species known to be transmitted by fish; the diet of these sea otters consists almost entirely of invertebrates. Northern sea otter populations from Russia and Alaska have a more varied diet and are hosts for at least 5 fish-transmitted parasites. Among the commonly occurring parasites, the most striking difference between Russian and Alaskan populations of sea otters is the absence of O. fraterculus from Russian populations and its frequent occurrence in Alaskan populations.

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Helminth Parasites of Sea Otters (Enhydra lutris} from Prince William Sound, Alaska: Comparisons with Other Populations of Sea Otters and Comments on the Origin of Their Parasites

L. Margolis

Papers

Review of Myxosporea of importance in salmonid fisheries and aquaculture in British Columbia.

Helminth Parasites of Sea Otters (Enhydra lutris} from Prince William Sound, Alaska: Comparisons with Other Populations of Sea Otters and Comments on the Origin of Their Parasites