Diagnosis and Histopathological Damage of Myxobolus koi of Mirror Carp (Cyprinus carpio L.)

PurposeTo identify the pathogen of diseased mirror carp (Cyprinus carpio L.) characterized by white neoplasm on gills in a farm of Sichuan Province, and to explore the pathological effects on gills, providing a scientific knowledge for diagnosis and prevention of this disease.

MethodsFish with typical symptoms were sampled and dissected. Pathogens (including bacteria, fungi and parasites) were detected using normal methods and identified by molecular methods. Gills were further used for histopathological observation to reveal the pathogenicity.

ResultsNo bacteria were isolated from the spleen and kidney of diseased fish, while fungal hyphae were observed in the gill filament. A large number of parasite spores could be seen in the gill filament tablet. Two obvious “eight” shaped symmetrical polar cysts were observed in the spores, which were similar to the myxozoans. The similarity of 18S rRNA between Myxobolus reported and the present (GenBank accession number: KT240127) species was from 92% to 97% and clustered into one family with Myxobolus koi (GenBank accession numbers were FJ841887 and KJ725077). Histopathologically, sporocysts were only found on the gill arch and gill filament, but not in other organs. A large number of mature spores compacted in the parasitic cysts were observed. The gill lamellae exhibited hyperplasia and necrosis, with significant infiltration of inflammatory cells, manifesting as parasitic, proliferative, and necrotic gill disease.

ConclusionM. koi is the pathogen of the present case. The parasitic type and pathological damage mechanism of M. koi in C. carpio are explored. All results will provide theoretical support for the study of the pathogenic mechanism of M. koi of C. carpio.