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To date, there is only one named species of Helicosporidia; Helicosporidium parasiticum. It was initially described and named by Keilin (1921), who detected this protist in larvae of Dasyhelea obscura Winnertz (Diptera: Ceratopogonidae) collected in England. He examined the new parasite thoroughly and attempted to infer its life history from his observations. He described a vegetative growth characterized by very active multiplications of helicosporidial cells inside the body cavity of the host, and noticed that these "schizogonic multiplications" were followed by the formation of what he called spores. Keilin noted that the spores (cysts) are very easily recognized: they consist of the assembly, inside an external membrane, of three ovoid cells (named by Keilin "sporozoites") and one peripheral, spiral, filamentous cell. These features, especially the highly characteristic filamentous cell, have since remained the principal diagnostic for identification of a Helicosporidium sp. Keilin was able to describe and characterize structurally the new genus Helicosporidium, and the new species H. parasiticum. He was also able to present a hypothetical life cycle of this protist based on his microscopy observations. He suggested that the spores (or cysts) break open in the host hemocoel, releasing the filamentous cell and the three "sporozoites", which he proposed are the infective forms of H. parasiticum. Keilin believed that H. parasiticum belonged to the Protozoa, and compared this isolate with members of various clades: Cnidiosporidia (which, at that time, included Microsporidia such as Nosema bombycis Naegeli), Haplosporidia, Serumsporidia and Mycetozoa. He concluded that the genus Helicosporidium differed markedly not only from all these groups, but also from all the protists known at that time. He finally proposed that Helicosporidium "forms a new group, which may be temporarily included in the group of the Sporozoa".

Introduction | 1 | 2 | 3