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Members of the Myxozoa are microscopic metazoan parasites with an extremely reduced body. The dimensions of the myxospore, the typical myxozoan stage in fish hosts, range usually between one hundredth and two hundredth of a millimetre. Myxospores consist of several cells, which are transformed to shell valves, nematocyst-like polar capsules with coiled extrudible polar filaments and amoeboid infective germs. Myxospores develop in plasmodia (trophozoites), which can be very large and polysporic (generally histozoic in host tissue) or small and mono- or disporic (coelozoic in organ cavities). Myxozoans are parasites of fish, worms (oligochaetes and polychaetes) and bryozoans. Few representatives were found as parasites of amphibians and reptiles, and recent findings confirmed the ability of myxozoans to infect mammals (Prunescu et al. 2007, Dyková et al. 2007) and birds (Bartholomew et al. 2008). Humans as potential hosts for myxosporea were also reported (Boreham et al. 1998, Moncada et al. 2001), however, myxospores were detected in faecal samples and probably just passed through the digestive tract.
Myxozoa Grassé, 1970 contains two classes: Malacosporea Canning, Curry, Feist, Longshaw et Okamura, 2000 and Myxosporea Bütschli, 1881. Malacosporea includes only two genera (Tetracapsuloides and Buddenbrockia) with a total of three described species. Myxosporea includes about 2200 species in 60 genera.
Wolf and Markiw (1984) discovered myxosporean life cycles altering between two host species – fish and annelid worm. The myxospore is ingested by annelids and then the myxosporean undergoes a schizogony and a gametogony. Finally, the parasite develops into an actinospore, a triradiate myxosporean spore, which infects the vertebrate host. Here, the sporoplasm released from the actinospore divides by endogony, and then presporogenic multiplication of the myxosporean occurs. The life cycle is completed with the development of mature myxospores in sporogonic plasmodia. The annelids are definitive hosts whereas vertebrates are intermediate hosts for Myxosporea.
Myxozoans were considered to be protists for more then one hundred years until the early nineties of 20th century. Then, the phylogenetic analysis of the primal myxosporean SSU rDNA sequence (Smothers et al. 1994) confirmed earlier hypotheses that myxozoans are multicellular organisms (tolc 1899, Weill 1938) and placed Myxozoa inside Metazoa. However, SSU rDNA data failed to find the correct position of the Myxozoa within metazoan taxa. Myxozoan SSU rDNA appeared to be a fast-evolving sequence resulting in long-branches in phylogenetic trees. Therefore, the SSU rDNA data are insufficient to decide whether Myxozoa are closely related either to Bilateria, Cnidaria (including Polypodium hydriforme) or other taxa (Smothers et al. 1994, Siddall et al. 1995, Hanelt et al. 1996, Siddall and Whiting 1999).
The rediscovery of Buddenbrockia plumatellae, a worm-like animal, as a myxozoan species was an important clue to the origin of Myxozoa (Monteiro et al. 2002). SSU rDNA of this enigmatic worm showed its close relationship to Tetracapsuloides bryosalmonae, and B. plumatellae was assigned to Malacosporea, the sister group to Myxosporea. Consequently Myxozoa were considered to be bilaterians or their close relatives (Monteiro et al. 2002). However, phylogenetic analysis based on sequences of numerous protein-coding genes (Jimenez-Guri et al. 2007) excluded a bilaterian origin of B. plumatellae and suggested Cnidaria as the most closely related taxon to Myxozoa.
The malacosporean Tetracapsuloides bryosalmonae and some myxosporean species are of economic importance causing significant losses of farm-reared fish. T. bryosalmonae causes dangerous proliferative kidney disease of salmonids (Kent et al. 1994). Probably the most serious myxosporean pathogen is Myxobolus cerebralis (whirling disease) which infects cartilage of freshwater salmonids and causes vast losses due to death (Nehring and Walker 1996). The other economically important species are e. g. Enteromyxum leei (enteromyxosis) (Palenzuela et al. 2002), Henneguya ictaluri (proliferative gill diseases) (Pote et al. 2000) and Kudoa thyrsites (post-harvest soft flesh) (Kent et al. 1994).