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Myxobolus cerebralis
''Myxobolus cerebralis'' is a myxosporean parasite of salmonids (salmon, trout, and their allies) that causes whirling disease in farmed salmon and trout and also in wild fish populations. It was first described in rainbow trout in Germany a century ago, but its range has spread and it has appeared in most of Europe (including Russia), the United States, South Africa〔Bartholomew, J.L. and Reno, P.W. (2002). "The history and dissemination of whirling disease." ''American Fisheries Society Symposium'' 29: 3–24.〕 and other countries. In the 1980s, ''M. cerebralis'' was found to require a tubificid oligochaete (a kind of segmented worm) to complete its life cycle.〔Markiw, M.E. (1992). "Salmonid Whirling disease." ''Fish and Wildlife Leaflet'' 17: 1–3. ()〕 The parasite infects its hosts with its cells after piercing them with polar filaments ejected from nematocyst-like capsules.
Whirling disease afflicts juvenile fish (fingerlings and fry) and causes skeletal deformation and neurological damage. Fish "whirl" forward in an awkward, corkscrew-like pattern instead of swimming normally, find feeding difficult, and are more vulnerable to predators. The mortality rate is high for fingerlings, up to 90% of infected populations, and those that do survive are deformed by the parasites residing in their cartilage and bone. They act as a reservoir for the parasite, which is released into water following the fish's death. ''M. cerebralis'' is one of the most economically important myxozoans in fish, as well as one of the most pathogenic. It was the first myxosporean whose pathology and symptoms were described scientifically. The parasite is not transmissible to humans.
==Taxonomy==
The taxonomy and naming of both ''M. cerebralis'', and of myxozoans in general, have complicated histories. It was originally thought to infect fish brains (hence the specific epithet ''cerebralis'') and nervous systems, though it soon was found to primarily infect cartilage and skeletal tissue. Attempts to change the name to ''Myxobolus chondrophagus'', which would more accurately describe the organism, failed because of nomenclature rules.〔 Later, the organisms previously called ''Triactinomyxon dubium'' and ''T. gyrosalmo'' (class Actinosporea) were found to be, in fact, triactinomyxon stages of ''M. cerebralis'', the life cycle of which was expanded to include the triactinomyxon stage.〔Kent, M. L., Margolis, L. & Corliss, J.O. (1994). "The demise of a class of protists: taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grasse, 1970." ''Canadian Journal of Zoology'' 508: 932–937.〕 Similarly, other actinosporeans were folded into the life cycles of various myxosporeans.
Today, the myxozoans, previously thought to be multicellular protozoans, are considered animals by many scientists, though their status has not officially changed. Recent molecular studies suggest they are related to Bilateria or Cnidaria, with Cnidaria being closer morphologically because both groups have extrusive filaments,.〔Hedrick, R. P. & El–Matbouli, M. (2002). "Recent advances with taxonomy, life cycle, and development of ''Myxobolus cerebralis'' in the fish and oligochaete hosts." ''American Fisheries Society Symposium'' 29: 45–53.〕 Bilateria are somewhat closer in some genetic studies.〔Monteiro, A. S., Okamura, B., and P. W. H. Holland. (2002). "Orphan worm finds a home: Buddenbrockia is a Myxozoan." ''Molecular Biology and Evolution'' 19: 968–971. ()〕
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