Dr. Murai Éva szerk.: Parasitologia Hungarica 25. (Budapest, 1992)

to disease. The oligochaete exerted its pathogenic activity by disturbing the development of larvae, by mechanically inhibiting their first breath-taking, swimming movements and feed intake. By actively moving on the body surface, chiefly in the first third of the body, the helminth gives rise to epithelial lesions which make the fish vulnerable to secondary infection by water-moulds. We made efforts to eliminate the problem by medication and by developing preventive methods. The only feasible solution was medication by exposure to solutions containing active ingredients effective against the pathogen. Only the short exposure could be used, as at the density typical of the technology (3000-5000 larvae/tank) the sterlet larvae would not have tolerated without an injury to health the interruption of water throughflow for more than 1-2 hours, which is absolutely necessary for exposing the fish to a precisely adjusted dose. The usual solutions of formaldehyde not yet toxic to the larvae (40 mg/1) proved ineffective against the helminths. At the same time, two other compounds, mebendazole and trichlorphon, were found to be effective. The use of mebendazole alone in the form of exposure was first reported by Székely and Molnár (1987) who found that modern anthelmintic, applied at a dose of 10 mg/1 as an one-hour exposure, effective against Pseudodactylogyrus gill parasites which endangered the profitability of intensive eel culture. Choosing a similar dose and mode of application, we successfully treated sterlet fry infected by Chaetogaster limnaei. One day after the treatment, the larvae resumed normal swimming activity, started to breathe and took up the feed. Having got rid of helminths, the larvae were actively looking for food on the bottom of the tanks and took up also the oligochaetes that had fallen off as a result of medication. Following the treatment, the commensalists disappeared from the snails as well. Exposure to trichlorphon-containing solutions has long-standing traditions in pisciculture. It is used for selective plankton control when preparing ponds for fish culture, and it is also applied for treating fish against various ectoparasites. Its applicability was proved also in the present case: when used at a dose of 4 mg/1 as a 30-min exposure, it eliminated the helminths from the larvae. A contraindication may be that the use of this active ingredient in the form of Flibol is not permitted; at the same time, its inexpensiveness, water-solubility and easy applicability are arguments that support its use. The preparations Masothen and Neguvon are more indicated for veterinary use. These preparations also contain trichlorphon as active ingredient; however, in chemically purer form than does Flibol. The best solution to problems like this is still the preventive approach. It is absolutely necessary to eradicate snails from intensive acipenserid cultures; namely, snails can carry the oligochaete Chaetogaster limnaei which can excessively multiply when water temperature rises in the spring. The risk of infection of fish larvae by any pathogen can be reduced further if the fish farms base the reproductive and fry-rearing phases of fish culture on well-water which is surely free from fish-pathogenic organisms.