Histopathological and Molecular Data on Myxobolus spp. Infecting Bryconid Fish From the Brazilian Pantanal Wetland

Previous studies described Myxobolus umidus infecting the spleen of Brycon hilarii, whereas Myxobolus pantanalis and Myxobolus salminus infecting the gill filaments of Salminus brasiliensis. In this study, we provided supplementary morphological and molecular data on these three Myxobolus species and histopathological analyses of the tissues infected by M. umidus and M. pantanalis in bryconid fishes caught from the Brazilian Pantanal wetland. A total of four B. hilarii and eleven S. brasiliensis specimens were examined. Among the four B. hilarii, one specimen exhibited several M. umidus plasmodia surrounded by a thin capsule of connective tissue in the spleen, but no inflammatory process was noted. Whereas two of the eleven S. brasiliensis individuals presented plasmodia of M. pantanalis. Fish infected with M. pantanalis were characterized by pronounced inflammatory reaction, blood congestion, hyperplasia of lamellar epithelium, lamellar fusion, and focal necrosis of chondrocytes in the gill filaments. Myxobolus species confirmation was achieved through morphological assessment and small subunit ribosomal DNA (SSU rDNA) sequencing performed on myxospores isolated from a single plasmodium of each species (M. umidus, M. pantanalis, and M. salminus). Phylogenetic analysis based on the newly generated SSU rDNA sequence of M. salminus, placed this species as sister to Henneguya rotunda, reinforcing the strong tendency for Myxobolus and Henneguya species to cluster according to the phylogenetic relationships of their fish hosts.