ABSTRACT
The microsporidia constitute a phylum of small, spore-forming unicellular parasites. New species are being described in rapid succession, and it is obvious that about 1 000 species known today are only a minor fraction. The real number probably exceeds one million. Their history started in 1857 with Nosema bombycis, the famous agent of the ‘pébrine’ disease of the silk moth. However, before long it was apparent that microsporidia were not restricted to insect hosts. At present microsporidia have been found in all major groups of animals. They are especially common and important parasites of insects, crustaceans, and fish. Approximately 10% of the species are parasites of vertebrates, with about 100 species from fish. Thus, far a little more than 10 species have been described from homoiothermous vertebrates (Canning and Lom 1986). Many of the species, at least of invertebrates, appear to be host specific.
