Microsporidian xenomas in fish seen in wider perspective.

The history of understanding xenoparasitic complexes or xenomas provoked in the host cell by various protists and especially by microsporidia is outlined. Microsporidia have been known to produce xenomas in oligochaetes (e.g., genera Bacillidium, Burkea, Hrabyeia, Jirovecia, species of the collective group Microsporidium), crustaceans (e.g., Abelspora, Mrazekia), insects (e.g., Polydispyrenia, Thelohania) and poikilothermic vertebrates, mostly fish (Alloglugea, Amazonspora, Glugea, Ichthyosporidium, Loma, Microfilum, Microgemma, Neonosemoides, Pseudoloma, Spraguea, Tetramicra). An overview of characters of xenomas caused by species of these genera is presented. The study of microsporidia causing xenomas in fish offers an insight into cell pathology and is of interest since many of these species are important agents of diseases in commercial fish. Xenomas produced from a few types of target cell display a complete change of organisation of the host cell and differ, according to the agent, in their structure. Recent data show that proliferation of the parasite may have already started in the cells transporting the parasites to the final site of xenoma formation. However, these are preliminary revelations and most of the facets of the life cycle are still to be clarified. Curiously, xenoma-forming microsporidia do not seem to be strictly host specific. The salient features of fish microsporidian xenomas are discussed, such as role of the xenoma, whether its features are host- or microsporidium-dependent, development and demise of the xenoma in the course of time, and host reaction phenomena. The need of further research is emphasised.