Myxosporidians, members of the phylum Myxozoa, are common multicellular parasites of cold‐blooded vertebrates, primarily fishes. They have a complex life cycle and many species are highly pathogenic in commercially important fishes, particularly in aquaculture.

Keywords: myxozoa; actinosporeans; fish; parasites

Figure 1.

Generalized life cycle of myxozoa. 1–7; Development in the fish host (myxosporean phase). 1, Actinosporean contacts surface of fish and discharges sporoplasm; 2, free unicellular sporoplasms in fish skin shortly after infection; 3, extrasporogonic multiplication; 4, trophozoites develop into sporogonic plasmodia, vegetative nuceli (v) and internal generative cells (g) formed by endogeny (internal budding); 5, sporogenesis: internal daughter cells form sporoblasts; s, sporogonic cell; 6, sporoblasts differentiate into the cells comprising the spore; c, capsulogenic cell; x, sporoplasm; v, valvulogenic cells; 7, fully‐formed myxospore (Sphaerospora sp.), v, valve; pc, polar capsule; x, sporoplasm.

8–20: Actinosporean phase in the annelid worm host (actinosporean phase). 8, Myxospore is ingested by worm and released sporoplasm penetrates epithelium; 9, sporoplasms form multiple schizogonic stages; 10, plasmotomy (fusion) of two uninucleate cells to produce one binucleate cell; 11, division to form cell with four nuclei; 12, division of ‘11’ into four unicellular cells; 13, formation of early pansporocysts with two somatic cells (S) and two internal generative cells; 14, gametocytes form within pansporocysts; 15, meiosis of gametocytes results in transformation of diploid to haploid cells and polar bodies (pb); 16, fusion of gametocytes to form diploid ‘zygotes’. 17–20: Sporoblasts develops into actinospores, which are then released from the worm. Adapted from Moser and Kent , El‐Matbouli and Hoffmann , El‐Matbouli et al..

Figure 2.

Wet mounts of Myxobolus arcticus, a myxozoon infecting the brain of salmonid fishes. (a) Myxospore from brain. Bar, 10 μm. (b) Triactinomyxon actinospore from oligochaete worm host. Bar, 50 μm; pc, polar capsule.

Figure 3.

Phylogeny of the Myxozoa based on analysis of small subunit ribosomal DNA (SSU rDNA) sequences. Maximum parsimony analysis of the Myxozoa inferred from complete SSU rDNA sequences with a bootstrap resampling of the data set. The numbers at the forks represent the percentage of times the group occurred out of 100 trees. Branch lengths are proportional to the scale given in substitutions per sequence position.



Anderson CL, Canning EU and Okamura B (1998) A triploblast origin for Myxozoa? Nature 392: 346.

Anderson CL, Canning EU and Okamura B (1999) 18S rDNA sequences indicate that PKX organism parasitizes bryozoa. Bulletin of the European Association of Fish Pathologists 19: 94–97.

Andree KB, Székely C, Molnár K, Gresoviac SJ and Hedrick RP (1998) Relationships among members of the genus Myxobolus (Myxozoa: Bivalvulida) based on small subunit ribsomal RNA sequences. Journal of Parasitology 85: 68–74.

Bartholomew JL, Whipple MJ, Stevens DG and Fryer JL (1997) The life cycle of Ceratomyxa shasta, a myxosporean parasite of salmonids, requires a freshwater polychaete as an alternate host. Journal of Parasitology 83: 859–868.

Diamant A (1997) Fish‐to‐fish transmission of a marine myxosporean. Diseases of Aquatic Organisms 30: 99–105.

El‐Matbouli M, Hoffmann RW and Mandok C (1995) Light and electron microscopic observations on the route of the triactinomyxon‐sporoplasm of Myxobolus cerebralis from epidermis into rainbow trout cartilage. Journal of Fish Biology 46: 919–935.

El‐Matbouli M and Hoffman RW (1998) Light and electron microscopic studies on the chronological development of Myxobolus cerebralis to the actinosporean stage in Tubifex tubifex. International Journal of Parasitology 28: 195–217.

Kent ML, Margolis L and Corliss JO (1994) The demise of a class of protists: taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grassé, 1970. Canadian Journal of Zoology 72: 932–937.

Kent ML, Khattra J, Hervio DML and Devlin RH (1998) Ribosomal DNA sequence analysis of isolates of the PKX Myxosporean and their relationship to members of the genus Sphaerospora. Journal of Aquatic Animal Health 10: 12–21.

Lom J and Noble ER (1984) Revised classification of the Myxosporea Bütschli, 1881. Folia Parasitologia 31: 193–205.

Longshaw MSW, Feist SW, Canning EU and Okamura B (1999) First identification of PKX in bryozoans from the United Kingdom molecular evidence. Bulletin of the European Association of Fish Pathologists 19: 146–148.

Moser M and Kent ML (1994) Myxosporea. In: Kreier JP (ed.) Parasitic Protozoa, vol. 8, pp. 265–318. New York: Academic Press.

Shulman SS (1966) Myxosporidii fauny SSR. Moscow‐Leningrad: Izdatel'stvo Nauka. English translation 1988: Myxosporidia of the USSR. New Delhi: Amerind.

Siddall ME, Martin DS, Bridge D, Desser SS and Cone DK (1995) The demise of a phylum of protists: Phylogeny of myxozoa and other parasitic cnidaria. Journal of Parasitology 81: 961–967.

Smothers JF, von Dohlen CD, Smith LH Jr. and Spall RD (1994) Molecular evidence that the myxozoan protists are metazoans. Science 265: 1719–1721.

Further Reading

Lom J and Dyková I (1992) Protozoan Parasites of Fish. Amsterdam: Elsevier.

Lom J and Dyková I (1995) Myxosporea (phylum Myxozoa). In: Woo PTK (ed) Fish Diseases and Disorders, vol. 1, Protozoan and Metazoan Infections, pp. 97–148. Wallington, UK: CAB International.

Hedrick RP, El‐Matbouli M, Adkison MA and MacConnell E (1998) Whirling disease: re‐emergence among wild trout. Immunological Reviews 166: 365–376.

Moran JDW, Whitaker DJ and Kent ML (1999) A review of myxosporean genus Kudoa Meglitsch, 1947, and its impact on the international aquaculture industry and commercial fisheries. Aquaculture 172: 163–196.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Kent, Michael L, and Palenzuela, Oswaldo(Dec 2001) Myxozoa. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001983]