Triaenophorus nodulosus (Cestoda: Bothriocephalidea) in Northern Pike (Esox lucius) and Pumpkinseed (Lepomis gibbosus) from the Mrežnica River: Pathological Effects of Adult Tapeworms and Intestinal Immune Response

Populations of northern pike (Esox lucius) and pumpkinseed (Lepomis gibbosus) from the Mre & zcaron;nica River were found to be infected with the tapeworm Triaenophorus nodulosus. In both species, the mean intensity of infection was low, suggesting a well-balanced host-parasite relationship. This study investigates pathological changes caused by adult T. nodulosus in its definitive host, the northern pike, and the associated intestinal immune response. The infection had no detectable adverse effects on either the northern pike population or the host's body condition index. Histological examination revealed lesions both at the site of tapeworm attachment and in areas adjacent to the free strobila, involving the lamina propria and submucosa. A moderate, multifocal, ulcerative, and necrotizing enteritis was observed, accompanied by an increased number of mast cells (MCs), which were identified as the predominant immune cells involved in the E. lucius-T. nodulosus interaction. MCs, mostly degranulated, were found in the lamina propria and superficial submucosa at the attachment site. Immunofluorescence revealed a subpopulation of piscidin 1-positive MCs in the same layers, with a higher number at the attachment site compared to unaffected intestinal areas. This represents the first evidence of piscidin 1 involvement in intestinal host defence against cestode infections in teleosts.