Glandular cell products in adult cestode: A new tale of tapeworm interaction with fish innate immune response

The caryophyllidean tapeworm <i>Caryophyllaeus brachycollis</i> (Janiszewska, 1953) is indigenous to the Lake Blidinje in the west-central part of Bosnia-Herzegovina where it infects chub <i>Squalius tenellus</i> (Heckel, 1843). Of 22 chubs examined, 45% were infected with <i>C. brachycollis</i> and a total of 912 specimens of this worm were counted. Histopathological and ultrastructural investigations were conducted on interface region between chub intestine and cestode scolex. Different sizes of lipid droplets in cestode tegument, in interface region and in chub enterocytes were observed. <i>C. brachycollis</i> lacks any specialized attachment organs and with an expanded, flattened scolex goes deep in mucosal folds and firmly attaches to them. In the epithelium of fish intestine, near the site of worm attachment, a high number of mucous cells and several rodlet cells were noticed. Indeed, within the intestinal tunica propria-submucosa, beneath the site of scolex attachment, numerous neutrophils and mast cells were encountered. Transmission electron microscopy of the apical part of the scolex of <i>C. brachycollis</i> showed the occurrence of a multicellular, syncytial glandular complex, the scolex produced membrane-bound secretory granules and their fibrillar contents discharged by merocrine and apocrine secretion onto the host-parasite interface. Our results are among the first to provide evidence on the sophisticated relationship between fish intestine and amorphous-undefinable substance produced by scolex glandular complex.