Dr. Murai Éva szerk.: Parasitologia Hungarica 25. (Budapest, 1992)
than normal, from time to time they tried to emerge to the surface but the first breath-taking did not take place. The few larvae that succeeded in taking a breath could not start feeding. In tanks in which the disease occurred usually all larvae died of the signs described above. Several hundred Physa acuta snails were consistently seen on the brim of the tanks, close the water surface. According to data obtained from the farm's staff, large numbers of this snail inhabit the water-conduits as well (Fig. 1). The oligochaete Chaetogaster limnaei (Fig. 3) was present in all larvae and snails examined. These 1.3-4.8 mm long helminths occurred on the larvae in numbers ranging from 1 to 14 (!), the majority being present and moving actively in the primitive gill cavity and in the cephalic region (Fig. 2). Live helminth specimens were often found in the intestinal tract of the moribund larvae. Soon after the larva's death, all helminths migrated to the body surface. They left the decomposing larvae covered by watermoulds by active movement. The helminths caused no visible changes in the snails. Their number ranged between 1 and 17 per snail. No other pathogen was found to be present by parasitological dissection. Three different active ingredients were tested for efficacy against the helminths. Of them, formaldehyde failed to kill the worms even at doses toxic to the larvae. At the same time, a one-hour exposure to a mebendazole (Vermox R ) solution of 10 mg/1 concentration and a 30-min exposure to a trichlorphon (Flibol) solution of 4 mg/1 concentration effectively killed the helminths. On the day following the treatment, the larvae - if they had not become utterly exhausted - were swimming normally, in a way typical of the species, were breathing and took up the feed. Often the dewormed fish consumed from the bottom of the tank the helminths killed by the treatment, as indicated by the presence of numerous bristles in their intestinal content. Intensive infection occurred also in 1992; however, by treating every tank twice with Flibol (which had been found easier to use and cheaper than the other effective drug, Vermox) the disease signs could be prevented and no mortality occurred. DISCUSSION This is the first report in the literature on severe, epizootic-like mortality of fish fry caused by a snail commensalist helminth. Only few papers deal with the role played by Chaetogaster limnaei in the life of water snails. Far-Eastern authors (Dimatulac and Pinto 1983) reported a negative correlation between Chaetogaster infection of the snail Lymnaea philippinensis and the presence of trematode developmental stages in the snail. They established that the presence of minimum 4 Chaetogaster helminths protected the snail against invasion by the developmental stages of Fasciola and Plagiorchis species. Thus, instead of causing damage to the host, the commensalist Chaetogaster limnaei even provided it with indirect protection against the infective stages of trematodes. In contrast, the present studies have revealed that the snail commensalist oligochaete may be obligately pathogenic to intensively cultured sterlet fry, as it caused substantial mortality on stock level even in the absence of other factors predisposing
