“Characterization and host-parasite interactions of Myxobolus n. sp. (Cnidaria: Myxozoa) infecting the gills of a Brazilian bryconid (7169)”

Characterization and host-parasite interactions of Myxobolus n. sp. (Cnidaria: Myxozoa) infecting the gills of a Brazilian bryconid

Myxozoa are a highly diverse group of endoparasitic cnidarians, comprising approximately 2,600 species that infect various tissues and organs of vertebrates, primarily fish. These parasites affect economically important fish species in aquaculture and commercial fishing, such as Salminus brasiliensis (Cuvier, 1816), known as dourado, a carnivorous fish of significant economic importance in Brazil. This study aimed to describe Myxobolus n. sp. infecting S. brasiliensis in the Pantanal region of Brazil through morphological and molecular analyses (SSU rDNA sequencing) and to characterize the host-parasite interaction using histopathological analysis. Specimens were collected from the Miranda River, Mato Grosso do Sul, where eleven S. brasiliensis individuals were sampled, and seven (63.6%) were found to be parasitized. Myxobolus n. sp. was observed in the gill filaments, forming white and elongated plasmodia measuring 2.1 ± 0.5 mm in length. Morphological analysis under light microscopy revealed mature myxospores, pear-shaped in frontal view, measuring 10.1 ± 0.3 μm in length, 6.6 ± 0.2 μm in width, and 5.1 ± 0.2 μm in thickness. In the lateral view, the myxospores were biconvex with symmetrical valves. The polar capsules were elongated, asymmetrical, and occupied more than half of the spore body, measuring 4.9 ± 0.3 μm in length and 1.9 ± 0.2 μm in width. Molecular analysis based on SSU rDNA yielded a 1,954 bp fragment. BLASTn analysis identified Myxobolus filamentum, previously described infecting Brycon orthotaenia from the São Francisco River in Minas Gerais, Brazil, as the closest species, with 95.3% similarity. Histopathological analysis using hematoxylin and eosin staining revealed numerous plasmodia of Myxobolus n. sp. developing within the subepithelial connective tissue of the distal region of the gill filaments. The parasitic development caused compression of adjacent blood vessels, indicating structural disruption of the gill tissue. A layer of fibroblasts was observed surrounding the plasmodia, along with granulocytes in the connective tissue, suggesting an active host immune response to the infection. These findings demonstrate the pathological alterations associated with Myxobolus sp. infection and its potential impact on gill function. Myxobolus n. sp. is the sixth myxozoan species described in S. brasiliensis, and the results based on morphological, molecular, and histopathological analyses contribute to understanding how this new species interacts with its host.

Acknowledgments: This work was supported by the  Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) through a doctoral fellowship to C. M. Meira (grant #2023/15107-7), a Thematic Project (grant #2018/24980-8), and PIPE grants (Innovative Research in Small Business Program) to T. Milanin (grants #2024/01323-2 and #2023/12685-0).

1. MEIRA, CAROLINE MUNHOZ, UNIVERSIDADE DE SÃO PAULO – USP, Author
2. MILANIN, TIAGO, UNIVERSIDADE DE SÃO PAULO – USP, Presenter
3. ADRIANO, EDSON APARECIDO, UNIVERSIDADE FEDERAL DE SÃO PAULO – UNIFESP, Author
4. MAIA, ANTONIO AUGUSTO MENDES, UNIVERSIDADE DE SÃO PAULO – USP, Author