Pathological effects of Thaparocleidus vistulensis (Siwak, 1932) infection on the gills of Silurus glanis Linnaeus, 1758.

Histopathological alterations caused by Thaparocleidus vistulensis (Siwak, 1932) on the European catfish Silurus glanis Linnaeus, 1758 were investigated by histopathology and scanning electron microscopy (SEM). The pathological effects of T. vistulensis on the gills of the European catfish were mainly related to the attachment sites of the parasite to its host, but damage also affected adjacent structures. The primary attachment relies on the anchors at the opisthaptor which commonly adheres both superficially and deeply, into the basal region between adjacent secondary lamellae. At the attachment sites, the haptoral disc of the parasites formed deep concave cup-like depressions on the surface of the gill lamellae. Deep anchor penetration occasionally distorted the extracellular cartilaginous matrix and induced a marked proliferation of epithelial tissue. Epithelial hyperplasia leading to lamellar fusion and subsequent extravasated erythrocytes in the gill lamellae was also observed. The damage caused by the parasites also led to the fusion of lamellae at the tips of the heavily infected gill filaments, forming club-like structures. Epithelial eosinophilic granular cells were frequently observed at the attachment sites. The pathological changes caused by this monopisthocotylan parasite frame the need to implement effective management strategies for controlling T. vistulensis infections in farmed European catfish populations.