Co-occurrence of Euclinostomum heterostomum (Trematoda: Clinostomatidae) and bacterial pathogens in the fish Hyphessobrycon herbertaxelrodi. A case report

This paper presents a case report of the occurrence of metacercariae of the fluke Euclinostomum heterostomum in the subcutaneous tissue of the fish Hyphessobrycon herbertaxelrodi and the co-occurrence of bacteria in its internal organs. In the affected fish, two cysts were observed in the muscles, accompanied by other symptoms such as skin darkening, swimming disturbances, and petechiae at the base of the pectoral and pelvic fins. The cysts contained two elongated, oval metacercariae of the parasite, measuring 3 mm and 5 mm in length. Morphological and molecular analyses confirmed that these parasites belong to the species E. heterostomum, marking the first report of this parasite in H. herbertaxelrodi and its first occurrence in fish in Poland. The bacteria Aeromonas veronii and Acinetobacter johnsonii were identified in the liver of the studied fish, suggesting a co-occurrence of parasitic and bacterial infections. These findings significantly contribute to the understanding of parasite occurrences in ornamental fish and their potential interactions with bacteria, expanding current knowledge on fish diseases.