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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. Keywords: Amazon, Tambaqui, Parasite, Myxozoa, Myxosporidians

The present study describes lesions caused by Kudoa sp. in the esophageal tissue of mapará catfishes (Hypophthalmus marginatus) collected from Cametá, Tocantins River in the northern Brazil. Fifty refrigerated H. marginatus specimens were dissected and tissue samples were for embedded in paraffin and stained with hematoxylin-eosi, Masson’s trichrome and Giemsa. Pseudocysts of whitish color were observed in the esophagus region. The parasites had four valves and other morphological characteristics of the genus Kudoa. Microscopic analysis revealed lesions in the mucosa, submucosa, and muscular layers. The Masson staining highlighted the presence of myxosporean cysts in the cytoplasm of the muscular fibers, and marked fibrosis of the muscular and mucous layers. The macro and microscopic findings of the present study confirm the presence of necrotic esophagitis in H. marginatus associated with infection by Kudoa.