Abstract
The presence of intestinal helminths in the alimentary canal of brown trout, Salmo trutta L., can alter the number of cells that synthesize modulatory peptides. A total of 167 brown trout were collected from tributaries of the River Brenta (northern Italy), of which 119 (71.3%) specimens were infected with enteric helminths, 28 with the acanthocephalan Pomphorhynchus laevis Müller, 1776 with intensity of infection ranging from 1 to 162 (18.57 ± 30.79) worms per host and 67 fish with the cestode Cyathocephalus truncatus Pallas, 1781. Intensity of infection with C. truncatus ranged from 1 to 85 (6.87 ± 12.59) per fish. In 24 fish there were concurrent infections of both species of helminths. The caecal and middle regions of the intestine were the most heavily parasitized. Immunohistochemical tests showed a decrease in endocrine cells (ECs) of the diffuse endocrine system (DES) positive to gastrin, cholecystokinin‐8, bombesin and secretin antisera in the intestine of the infected trout. The number of ECs immunoreactive to anti‐glucagon serum did not show differences in the digestive tract of uninfected brown trout and in conspecifics parasitized with P. laevis . The density of cells containing glucagon‐like material was low in the fish parasitized with C. truncatus . The results suggest that endoparasitic helminths induce alterations in the DES of infected S. trutta .