Mortality of fish fry as a result of specific and aspecific cercarial invasion under experimental conditions

Majoros, G. (1999) Mortality of fish fry as a result of specific and aspecific cercarial invasion under experimental conditions. Acta Veterinaria Hungarica, 47 (4). pp. 433-450. ISSN 0236-6290

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Abstract

Although the effect exerted on fish by fluke cercariae developing in fish as intermediate hosts is fairly well known, cercariae of flukes requiring intermediate hosts other than fish occur in fish ponds in much higher numbers. To study the effect of these latter cercariae on fish fry, a few days old common carp fry (Cyprinus carpio) were exposed to cohorts of xiphidiocercariae or echinocercariae shed by one or two snails acting as intermediate hosts. The observed pathogenicity of these cercariae was compared to the well-known pathogenic effect ofDiplostomumcercariae. AlthoughDiplostomumcercariae proved to be the most pathogenic, occasionally also the other two types of cercariae could kill the common carp fry by invading their body. Xiphidiocercariae ingested by the fish penetrated the intestinal wall and developed into metacercariae within the body of the fry. They could also get into the musculature through the skin. The much larger echinocercariae invaded the gill cavity where they became encapsulated, thus hampering gill function and leading to fish mortality due to oxygen deficiency.Diplostomumcercariae were more pathogenic than the cercariae of the other two flukes in that they were shed by the intermediate host in very large numbers. Although much less xiphidiocercariae and echinocercariae emerged from the intermediate host snails thanDiplostomumcercariae, the former could kill the fish fry already in lower numbers. These findings prove that fluke cercariae nonspecific to fish may exert pathogenic effects on fish, and presumably contribute to the early mortality of young fry placed out into fish ponds.

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