First record of facultative parasitism of Chilodonella uncinata based on goldfish (Carassius auratus) infection model

Abstract Ciliate parasites can lead to great economic losses to aquaculture. Among them facultative parasites have not emerged yet as important pathogens. In this study, the facultative ciliate parasite Chilodonella uncinata was isolated from the gills of goldfish. Its morphological characters were investigated by observations of both living cells and specimens stained with silver nitrate and protargol methods. Sequence alignments revealed that the present ITS gene sequences were consistent with C. uncinata strain ATCC 50194 and the SSU rDNA gene sequences were similar to the sequences of existing C. uncinata in NCBI website with 99.28% of similarity. Using phylogenetic analyses based on SSU rDNA gene sequences, ciliates collected in this study were confirmed to be C. uncinata. To further validate the infectivity of C. uncinata, an infection model of goldfish was established by adding the C. uncinata isolates from cultures to the goldfish's gills. Microscopic examination and pathological analysis were also used to identify the success establishment of the model, which have shown the presence of C. uncinata, infiltration of inflammatory cells and epithelial hyperplasia on gill tissues. This model has the potential to be used in subsequent diagnostic test and drug screening, which may be beneficial to control chilodonellosis infections in aquaculture. Moreover, this is the first record that C. uncinata demonstrated to be parasitic to fish.