Treatment of fish parasites. 4. Effects of sym. triazinone (toltrazuril) on Monogenea.

For chemotherapy in fish parasitized by monogeneans, toltrazuril was tested in vivo and in vitro against gill-parasitizing species (Dactylogyrus vastator, D. extensus, D. cornu, Pseudodactylogyrus anguillae, Diplozoon paradoxum, Dip. homoion) as well as the skin-parasitizing species Gyrodactylus arcuatus. Naturally infected fish were incubated at 20 degrees C for 0.3, 1, 2, 3, 4, and 6h in water containing 0.5, 10, 20, 30, and 50 microgram toltrazuril/ml. In general, the damage observed resulted in the vacuolization and lysis of the parasitic tegument. Toltrazuril caused irreversible lesions in the tegument of Dactylogyrus species and Pseudodactylogyrus bini, beginning at a dose of 5 microgram/ml (4-h exposure). In vitro treatment with 10 microgram/ml toltrazuril caused death in Dip. paradoxum and Dip. homoion after 4-80 min, depending on the age of the parasites. Subadult worms were more susceptible to the drug than adults. Specimens of G. arcuatus were severely affected after 1-h exposure to 20 microgram/ml. In all species, the prohaptor and peduncle regions of the specimens were most sensitive to the drug. Treatment of infections of Gyrodactylus species, D. vastator, D. extensus, C. cornu, and P. bini using a water bath with 10 micrograms toltrazuril/ml for 4 h (D. cornu, 15 degrees C; other species, 20 degrees C) is recommended, since then species diagnosis is not needed. Fish with extensive skin lesions caused by net catching or infection by Saprolegnia spp. should be carefully observed during treatment, as these factors decrease their drug tolerance.