Treatment of fish-parasites: 7. Effects of sym. triazinone (toltrazuril) on developmental stages of Myxobolus sp. Bütschli, 1882 (myxosporea, myxozoa): A light and electron microscopic study.

In research for chemotherapy of fish parasitized by myxosporeans toltrazuril(1) was tested in vivo against Myxobolus sp., parasitizing the connective tissue in gills of the bream, Abramis brama. Naturally infected breams were incubated at 20°C in water containing 0, 5, 10 or 20 μg toltrazuril/ml or the pure solvent for 4 or 2 h, respectively. After replacing the fish into fresh water, they were sacrificed on day 2 after the treatment and the pseudocysts were studied by means of transmission electron microscopy. It was found that the drug caused severe damages in all developmental stages of Myxobolus sp. except for the mature spores. Starting with a dose of 5 ug/ml and 4 h exposure, a destruction of the mitochondria and a decrease in the number of ribosomes were observed in the uni- and multicellular stages. In sporoblasts while developing the polar capsules the perinuclear space was enlarged in most of the specimens. The karyoplasms were more electron lucent compared to those of untreated controls. A focal lysis of the karyoplasm was observed in early pansporoblasts (characterized by primordia of the polar tubes). Treatment with 10 μ/ml for 4 h had more pronounced effects. The shape of these unicellular and multicellular stages was, however, not changed after treatment with 5 or 10 μg toltrazuril for 4 h, whereas 20 μg toltrazuril/ml for 2 h altered the surface of both stages and led to considerable degenerations of the cytoplasm. In premature pansporoblasts an extrusion of the polar capsules (leaving the capsulogenic cells) was caused when incubated in 20 μm toltrazuril/ml. The pure solvent had no effects on developmental stages of Myxobolus sp.. From these experiments it is suggested that chemotherapy against Myxobolus sp. should be accomplished by bathing the fish in aerated containers with 10 μg toltrazuril/ml 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 extended skin lesions, caused by net-catching or infections by fungi, should be treated under careful observation, because these factors decrease the tolerance to the drug.