An outbreak of myxozoan parasites in farmed freshwater fish Colossoma macropomum (Cuvier, 1818) (Characidae, Serrasalminae) in the Amazon region, Brazil

• Out of the 60 specimens of tambaquis analyzed, 83.3% (50/60) were parasitized in different organs for Myxobolus, Ellipsomyxa, Henneguya and Thelohanellus.
• This is the first report of occurrences of the genera Ellipsomyxa and Thelohanellus for this species.
• This report suggests that the mortality among C. macropomum specimens was caused by the outbreak of myxosporidians.

The tambaqui (Colossoma macropomum) is a native fish species that is farmed most frequently and in the largest quantities throughout Brazil. The high production of this species from fish farms has contributed to the occurrence of emerging parasites, which may compromise fish health and productivity. In a batch of 2500 tambaqui fry acquired for experimental farming procedures in Brazil, a mortality rate of 80% was observed, with the fish swimming erratically and gasping for air at the water surface. From among the specimens that were still alive, 60 individuals were selected at random. Organs or fragments of organs containing lesions and/or cysts were examined under an optical microscope to investigate for the presence of parasitic spores. Of the 60 specimens of tambaqui analyzed, 83.3% were found to be infected in different organs, such as the gills, liver, and gallbladder with myxosporidian species belonging to four genera, namely, Myxobolus, Ellipsomyxa, Henneguya and Thelohanellus. The parasite with the greatest prevalence was Myxobolus sp., located in the gills (70%), followed by Henneguya sp. in the gills region (68.3%), Myxobolus sp. in the liver (63.3%), Thelohanellus sp. in the liver (58.3%), and Ellipsomyxa in the gallbladder (50%). This is the first report of parasitic infection caused by the genera Ellipsomyxa and Thelohanellus in C. macropomum. The present study reported the second incidence of the occurrence of the genus Thelohanellus in South America. This study suggested that the mortality among C. macropomum specimens was caused by the outbreak of myxosporidians.