Treatment of fish parasites: 5. The effects of sym. triazinone (Toltrazuril) on fish parasitic ciliophora (Ichthyophthirius multifiliis FOUQUET, 1876, Apiosoma amoebea GRENFELL, 1884, Trichodina sp. EHRENBERG, 1831).

For chemotherapy of fish parasitized by different Ciliophora (Protozoa) toltrazuril was tested in vivo and in vitro against skin and gill parasitizing species (e.g. Ichthyophthirius multifiliis, Apiosoma amoebea and Trichodina sp.). For in vitro treatment naturally infected fish were incubated at 20 °C for 0.3,1,2,4.5 h in water containing 0,1,5,10,20 or 50 μg toltrazuril/ml. Lethal damages already occurred in about one third of the trophozoites of I. multifiliis after incubation in 5 μg/ml for 4.5 h, and in more than two thirds of the trophozoites after incubation in 10 μ/ml for 2 h. A few trophozoites, however, were still able to leave their hosts, to encyst and to produce theronts. However, when treatment was done by intermittent therapy (1×10 μg/ml for 2 h, first day; 1 × 20 μg/ml for 1 h, second day; 1 × 20 μg/ml for 1 h, third day) all the trophozoites were lethally damaged. The damages consisted in the destruction of the outer cell membranes, of the cilia, and of the mitochondria, as well as in the complete abolishment of the ribosomes and in the enlargement of the nuclear space. After in vitro treatment (10 μg/ml, 2 h) all the trophozoites were lethally damaged. In contrast to the trophozoites, the free-swimming theronts of I. multifiliis were not affected by the drug. In vivo treatment starting with 20 μg/ml for 1 h led to severe damages in A. amoebea, which were intensified after treatment with 50 μg/ml for 0.3 h. The specimens were heavily contracted, and the oral cilia were redrawn. Furthermore, the pellicular pores seen in the surface of controls were not detectable. Despite the affections caused by the treatment the parasites did not drop off their hosts. In vivo treatment against Trichodina sp. led to a reduced motility in these parasites starting with 10 μg/ml for 2 h. Incubation with 20 μg/ml for 1 h caused a complete stop of motion in most of the specimens. The highest dose (50 μg/ml; 0.3 h) only caused a dropping off in about one third of the specimens from their hosts. Treated specimens of Trichodina sp. had a more flattened appearance compared to untreated controls, and the epistomial disc was drastically enlarged. From these experiments it is suggested that treatment against the trophozoites of I. multifiliis should be done by intermittent therapy according to the following scheme: 1st day: 10 μg/ml, 2 h, 2nd day: 20 μg/ml, 1 h; 3rd day: 20 μg/ml, 1 h. In the cases of Apiosoma spp. and Trichodina spp. infected fish incubating with 50 μg/ml for only 20 minutes is recommended.