Historic Perspective on the Microsporidia: Expanding Horizons

The polar filament is the defining character of the microsporidia, and its function has attracted the attention of many investigators. Microsporidian species have been reported to infect nearly all of the invertebrate phyla, including such unicellular organisms as ciliates and gregarines, myxozoans, cnidarians, platyhelminths, nematodes, rotifers, annelids, molluscs, bryozoans, and arthropods, as well as all five classes of vertebrates; the greatest numbers of species infect arthropods and fish. Microsporidia are obligate intracellular parasites of eukaryotes, the transmittable stage being a resistant spore which is usually small, possesses a thick wall, and contains a characteristic polar tube apparatus. Glugea anomala, which causes subcutaneous cysts in fish (e.g., stickleback), was the first microsporidian parasite recognized to cause disease in vertebrates. Subsequently, other species such as Pleistophora spp and Nucleospora (Enterocytozoon) salmonis were implicated in sporadic but serious disease outbreaks among fish populations. These outbreaks have had an important economic impact on tropical freshwater fish, commercial fish farming, and the sport fishing industry. Once it became clear that human microsporidiosis was largely a disease of immuno-incompetent individuals, particularly those with AIDS, additional species were recognized that also caused disseminated infection. In addition, protozoan thymidylic synthetase and dihydrofolic acid reductase, which are present in protozoa as separate epitopes on the same protein, are found as separate proteins in the microsporidia as they are in yeast.