Histology, ultrastructure and prevalence of Henneguya piaractus (Myxosporea) infecting the gills of Piaractus mesopotamicus (Characidae) cultivated in Brazil

The histopathological and ultrastructural characteristics of Henneguya piaractus, a parasite of the gill lamellae of Piaractus mesopotamicus, are reported here. Histological analysis showed that the plasmodia were of the intralamellar type. The development of the plasmodia resulted in marked dilatation of the infected lamellae, with the neighbouring lamellae being displaced laterally. Discreet epithelial hyperplasia was observed, but there was no inflammatory reaction. Ultrastructural analysis showed that the plasmodium had a single thin wall that was in direct contact with the host cells. Pinocytic canals and points of phagocytosis were observed in the wall. The prevalence of the parasite varied according to host size, with the lowest prevalence occurring in hosts up to 10 cm long.