Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 27. (Budapest, 1994)

INTRODUCTION Fish-parasitic myxosporeans comprise an extraordinarily large number of species. The genus Myxobolus alone, which is the best studied genus of Myxosporea, was reported to contain 444 species (Landsberg and Lorn 1991). Accurate determination of the number of species is rendered difficult by the fact that taxonomic classification is based on the morphology of spores showingonly slight structural differences, and that the development and the species, organ and tissue specificity of the parasites are little studied. Reliable information on the extrapiscine development of myxosporeans has existed only since the studies of Markiw and Wolf (1983) who demonstrated the involvement of Oligochaeta alternative hosts in the development of Myxobolus cerebralis. The stages formerly called Actinosporea develop in these oligochaetes. The studies of Markiw (1989) revealed that infection by myxosporeans can take place also via pathways other than the alimentary route, and that Myxobolus cerebralis infection may occur also in the manner observed by Daniels et al. (1976), i.e. through the percutaneous entry of sporoplasms released from tr'actinomyxons. El-Matbouli et al. (accepted for publication) demonstrated that after some divisions the sporoplasms that have entered the epithelial cells of the skin, gills and fins actively travel along the nerve tracts to the final site of colonization specific of the given species. However, there still are only few species whose life cycle is known in every detail. Precisely the inadequate knowledge and poor reproducibility of the life cycle of myxosporeans account for the scarcity of data available on the species, tissue and organ specificity of these parasites. DISCUSSION Relying on my own studies spanning a period of 25 years and on data of the literature, in this work I will attempt to determine the species-specificity of different myxosporeans, to demonstrate their strict tissue specificity and their occasional organ specificity arising from the former. The same studies enable me to describe the three basic models of the intrapiscine development of myxosporeans. The task undertaken is very difficult, as numerous excellent books have been published on the subject, which contain a detailed description of the complex intrapiscine developmental processes of these parasites. At the same time, these sources still contain some erroneous statements while failing to mention some general characteristics of myxosporeans. Of the latter, here I would like to point out the following: 1. The host specificity of different myxosporean species is different; however, it is always restricted to a well-definable circle of related hosts. 2. Myxosporeans are characterized by a very pronounced tissue specificity. 3. The organ specificity of myxosporeans depends primarily on whether the cell type necessary for the development of the given myxosporean species is available in the organ concerned.