The myxozoan Henneguya friderici is a parasite of the gills, intestine, kidney and liver of Leporinus friderici, a characiform fish belonging to the family Anostomidae. Forty-two specimens of L. friderici that had been caught in the Mogi Guaçú River, state of São Paulo, were studied. Elongated white plasmodia were found in the gill filaments of 10 host specimens (24%). The mature spores had an ellipsoidal body with polar capsules of equal size and caudal length greater than body length. This study also described 18S rDNA sequencing of H. friderici infecting the gill filaments. This produced a sequence of 1050 bp that demonstrated significant genetic differences with previously described species of Henneguya. Similarity analysis using sequences from species that clustered closest to those produced by this study showed that the species with greatest genetic similarity to H. friderici was H. leporinicola, with 94% similarity.

During a parasitological survey of Astyanax altiparanae captured in the Mogi Guaçú river, Pirassununga, state of São Paulo, Brazil, a new species of myxozoa was observed infecting the kidney of 30 of 42 specimens. Plasmodia of the new myxozoa, referred to herein as Unicauda whippsi n. sp. were spherical and varied from 450 to 1000 μm in length. The mature spore was elongated and measured 11 ± 1.2 (10-12) μm long by 5.4 ± 0.6 (4.7-7.2) μm wide. Polar capsules of unequal length with the filament coiled 7-9 times and a unique caudal process characteristic of the genus Unicauda, were recorded. Phylogenetic analysis demonstrated that U. whippsi n. sp. clustered with members of the genus Unicauda and Myxobolus, confirming it's placement within the family Myxobolidae. This is the first record of a species of Unicauda infecting characiform fish, as well as the first registered occurrence of species of this genus in South America.

Una sinopsis de los registros de las especies válidas de mixozoários (Myxozoa: Myxosporea) descritas en las Américas se proporciona con base en un levantamiento bibliográfico desde 1893, cuando la primera especie de mixosporídeo fue descrita, hasta diciembre de 2016. Esta es una sinopsis de 495 especies, Distribuidas en 36 géneros, 15 familias y asociadas a 286 especies de hospedadores. En términos de número de asociaciones hospedador-parasito, los peces fueron el grupo más representativo (97% del número total de asociaciones hospedador-parasito, mientras que los registros en las aves representaron el 0,2%, los crustáceos 0,2%, helmintos 0,2, 0,8% reptiles y 1,6% fueron en los anfibios. El número de descripciones de mixozoarios fue mayor en la Región Neártica en relación a la Región Neotropical. La sinopsis incluye una lista parásito-hospedador con datos sobre el hábitat del hospedador, el sitio de infección, la localidad, el tamaño y el formato del quiste, las medidas de las esporas y los especímenes en las colecciones y una lista de los parásitos.

Pseudoplatystoma (Bleeker, 1862) species are of commercial importance in the region of Rondônia, however studies with the species are scarce, so the objective of the research was to carry out a diagnosis of the composition and structure of the parasite communities of Pseudoplatystoma fasciatum (Linnaeus, 1766) and P. tigrinum (Valenciennes, 1840) of the river basin Jamari, Ariquemes - RO, Brazil. During the period from November / 2016 to February / 2018, 50 specimens of P. tigrinum and 51 specimens of P. fasciatum were collected. Of the total sampled fish, 45 specimens of P. tigrinum and 51 specimens P. fasciatum were parasitized by at least one species of parasite presenting a prevalence level of 90% and 100%, respectively. In the parasitic infrapopulations of P. tigrinum, the groups with the highest prevalence rates were nematodes, with larvae being the largest representative of the group, followed by monogenean Vancleaveus ciccinus Kritsky, Thatcher e Boeger, 1986 and for P. fasciatum the parasite helminths that presented the highest prevalence level were the Cestoda groups: Megathylacus sp. Woodland, 1934; Harriscolex sp. Rego, 1987; Monticellia sp. La Rue, 1911; Nominoscolex sp. Woodland, 1934; Nematoda: larvae; Eustrongylides sp. Jägerskiöld, 1909; Cucullanus sp. Müller, 1777; Contracaecum sp. Railliet Henry, 1912 and Monogenea: Vancleaveus ciccinus One species showed a negative correlation between total length and average abundance for P. tigrinum, while P. fasciatum correlation was positive for three species. The comparison of abundance averages by sex for P. tigrinum showed no significant difference between groups, for P. fasciatum a significant difference was observed among the individuals parasitized by Peltidocotyle sp., Monticellia sp., Nominoscolex sp. where females showed higher average abundance in relation to males.