Dr. Kassai Tibor - Dr. Murai Éva szerk.: Parasitologia Hungarica 9. (Budapest, 1976)

As to the size, the smallest protozoon I observed was 3 n, its purplish-red nucleus was 2, 5 ja in diameter. In the strikingly blue cytoplasma, next to the nucleus', there is a deep purplish-red dotlike granule which turnes into a pinlike thin stick in the protozoa of about 4 ju in size. This stick stains blue at both ends and purplish-red in the middle (Figs, lb, and 2-3). In protozoa of 5-6 u (max. up to 11 ju) the stick becomes spindleshaped, is already 1 n in width and reaches through the cell almost in the total lenght of the diameter (Figs, le, d, e and 5). The spindle-shaped developmental form looks dark blue at both ends, the nucleus is purplish-red in its centre, and there is a tiny pale blue nucleolus in it (Fig le). In protozoa of about 10 u in size the spindle-shaped structure becomes spherical (Figs. If and 6), and begins to divide (Figs, lg, and 7). In the course of division the nucleus stretches, performs bipartition in the middle and soon the dividing nucleus is bridged merely by a purplish-red threadlike part. The division of the nucleus is followed by that of the cytoplasma as well. Repeated divisions within the protozoon result in parasites containing 2-4-6-8 spindle-shaped or rounded forms (Figs, lh-1 and 8-13) as well as in transitional forms (Fig. 18), When the parasites contain 6-8 divisional products and grow up to 10 to 15 m in size, the internal elements become rounded their inner structure undergoes transformation and the cell becomes similar to the smallest form described first. During the course of internal divisions, besides inner developmental forms also residual bodies appear in the plasma of the protozoon. They are deep purplish-red in colour, their number being 2, 3 and 4 in cells containing 6, 7 and 8 divisional products, respectively (Figs. Ik1 and 11-13). In the next stage of development the parasites containing 8 elements fall into pieces (Figs, lm and 16) releasing the developmental forms of 3 ja, described first, and the whole cycle begins again. In heavily infected fish many immature red blood-cells and an increase in the number of white blood-cells can be observed by microscopic examination. Discussion As judged by the type of its division the observed protozoon seems to be a sporozoan parasite, however, the question whether this is a vegetative or a sporogonic process remains to be cleared. In case of Haemosporidia generally gametogonic forms appear in the blood (KUDO, 1954). The division of the parasites in the present study rather resembles the vegetative phase at the start, however, during the last divisions (in cells containing 6-8 divisional products) residual bodies appear which are rather characteristic of the sporogonic processes. This parasite differs from all known sporozoa of fish as far as its development takes place extracellularly. Similar parasites had been described by SMIRNOVA (1971), these however, developed in white and red blood-cells and were considered to be schizogonic forms, the resulting schizonts falling into max. 16 pieces depending on their size. In the present study merely cells that fell into max. 8 units were found. Besides, in the blood of the same fish SMIRNOVA observed the fusion of gametocytes as well, and discussed the division of the zygota in red blood-cells. In the course of my observations no similar sexual forms have occurred. The protozoon I found resembles at least to a certain extent (in case of more mature developmental forms) a white blood-cell attacked by parasites. However, the clear sequence of successive developmental stages and the increase in their size from 3 to 15 u appear to prove that these formulae, situated in the blood plasma, are by no means of host cell origin but independent extracellular protozoa. The developmental form attacking the nucleus of erythrocytes described by SMIRNOVA (1971) and erythrocyte necrosis recorded by LAIRD and BULLOCK (1969) were not present. The accurate taxonomic identification on the protozoon described here still requires further examinations. Figs. 11-13: Sphaerical inner structures and residual bodies. Figs. 14-16: Cells falling into 8 pieces. Figs. 17-18: Transitional developmental forms. Fig. 19: The parasite in phagocytic ceUs. (Photo Csaba) 2 3