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Summary In research of chemotherapy of fish parasitized by myxosporeans toltrazuril was tested in vivo against Henneguya sp. parasitizing the primary gill filaments in the tapir fish, Gnathonemus petersi. Naturally infected tapir fish were incubated at 22° C in water containing 0, 5, 10, or 20 µg toltrazuril/ml, or the pure solvent for 4 or 2 h, respectively. It was found that the drug caused severe damages in all developmental stages of Henneguya sp. except the mature spores. Starting with a dose of 5 µg/ml and 4 h exposure, a destruction of the mitochondria and a decrease in the number of ribosomes was observed in the uni- and multicellular stages. After treatment with 10 µg/ml, in the unicellular stages the karyoplasms were partially lyzed. In pansporoblasts with differentiated sporoplasm and beginning differentiation of the polar filament all organelles in the cytoplasm of the valvulogenic cells were destroyed after incubation in 10 µg toltrazuril/ml. In all cells the perinuclear space was enlarged. The grade of damage increased after incubation in 20 µg toltrazuril/ml for 2 h. In capsulogenic cells, in which the polar capsule formation had already started, their rough endoplasmic reticulum was dilated. In premature pansporoblasts when valve production has started, a shrinking of the sporoplasm and of the capsulogenic cells was only observed after treatment with the highest dose (20 µg/ml; 2 h) : in some cases the pansporoblasts burst. The pure solvent had no effects on developmental stages of Henneguya sp. From these results it is suggested that chemotherapy against Henneguya sp. should be accomplished by bathing the fish in aerated containers with 10 µg toltrazuril/l for 4 h. This treatment will decrease considerably the output of spores. However, since the mature spores are not affected, the treatment has to be repeated within several months. Fish with extensive skin lesions, caused by net-catching to infections by fungi, should be treated under careful observation, because these factors decrease the tolerance of the drug. |
