In several experiments it was shown that praziquantel is effective against Monogenea (Dactylogyrus vastator, Dactylogyrus extensus) parasitizing the gills of pond-reared carp (Cyprinus carpio). Also, the effects of praziquantel onDiplozoon paradoxum from chub (Squalius cephalus) were investigated. Infested carp were incubated for 30, 60, 90, 120 or 180 min at 22° C in water containing 0 (control), 1, 5, 10, 100 μg praziquantel/ml. It was found that the drug caused irreversible alterations in the tegument of Monogenea within 30 min after exposure to at least 1 μg praziquantel/ml. The damage found in vitro resulted in vacuolisation of the parasites' teguments. The three species showed little differences in the grade of tegumental alternation:Dactylogyrus vastator showed vacuolisation of its tegument in all regions of its body, which was very marked in the opisthaptor and in the peduncle region. Parasites did not drop from the gills even when killed by treatment with praziquantel. InDactylogyrus extensus the opisthaptor region was especially affected. The worms usually lost their marginal hooklets as well as the large hooks. In contrast toDactylogyrus vastator, treated worms often dropped from gills.Diplozoon paradoxum was lesser sensitive to the drug. After incubation in 1 μg praziquantel/ml for 7 h the worms had a reduced motility. Doses of 10μg/ml and 150 μg/ml for 1.5 h led to severe damage of the tegument, especially in the region of the oral sucker. However, the drug is effective in treatment of this parasite.
It is suggested that therapy againstDactylogyrus vastator andDactylogyrus extensus in carp fingerlings may be done with 10 mg praziquantel/l for 3 h at 22° C during storage in smaller tanks.