Oocyte degeneration in Pygocentrus nattereri induced by Myxobolus sp. (Cnidaria: Myxozoa) in the Brazilian Amazon.

The red piranha (Pygocentrus nattereri) is a carnivorous fish of significant ecological and economic value, widely distributed across tropical and neotropical regions of South America, particularly within the Amazon basin. This study investigated the presence of Myxobolus sp. infection in the ovaries of red piranhas collected in the municipality of Pracuúba, in the state of Amapá, Brazil. During necropsy, whitish cysts suggestive of parasitic infection were observed. Microscopic examination revealed spores with morphological characteristics consistent with Myxobolus sp., predominantly located in the ovarian parenchyma. These infections caused tissue lesions and disrupted the normal ovarian architecture, although no inflammatory infiltrates were detected. While Myxobolus sp. infections in reproductive organs are rare, they can significantly impact fish health and reproduction. This study is the first to report of Myxobolus sp. infection in the ovaries of P. nattereri, underscoring the importance of monitoring parasitic infections in this species, which plays a vital role in the food security of riverside communities in the Amazon.