Immunopathological Response, Histological Changes, Parasitic Burden, and Egg Output in Sheep Naturally Infected by Dicrocoelium dendriticum

SIMPLE SUMMARY: Dicrocoelium dendriticum, commonly known as the lancet liver fluke, is a liver parasite that affects ruminants and occasionally other species, including humans. The aim of this study was to investigate the histological modification and the phenotype of inflammatory cells in the livers of sheep naturally infected with D.dendriticum and the correlation of these variables with the parasitic burden, egg output, and gross appearance of the liver. We studied twenty-four sheep selected on the basis of positive D. dendriticum fecal egg counts. Gross and histological modifications of the liver and the number of adult D. dendriticum worms were examined. Macroscopically, the livers were swollen with thickened bile ducts, cholangitis, and fibrosis. Histologically, we observed leucocyte infiltration, bile duct hyperplasia, and fibrosis. Lesions were present in different degrees of severity and were scored. A significant positive association was observed between the number of adult worms recovered from the liver, egg per gram, macroscopic lesions, fibrosis, and bile duct hyperplasia. A significant negative association was observed among these variables and the degree of leukocyte infiltration. The immunohistochemical examination showed a CD3+ predominant cell population in all assessed animals. These findings allow us to better understand the complex mechanism of the host–parasite interaction, in relation to the egg output and parasitic burden in dicrocoeliosis. ABSTRACT: The aim of this study was to investigate the correlation between infection by Dicrocoelium dendriticum (class Trematoda) and the animal host response in terms of macroscopic lesions, the immunopathological response, and histological changes in the livers of naturally infected sheep. Twenty-four sheep were selected on the basis of positive D. dendriticum fecal egg counts (FECs). Gross and histological injuries were scored. A positive significant association was observed between the number of adult worms recovered from the liver, FEC, macroscopic lesions, fibrosis, and bile duct hyperplasia. A significant negative association was observed among these variables and the degree of leukocyte infiltration. In addition, immunophenotyping of the inflammatory cells was carried out using primary antibodies against T cell epitopes (CD3+, CD4+, and CD8+), B cell epitopes (CD79α), and the ionized calcium-binding adapter molecule 1 (IBA-1) antigen. Independently of the severity of the D. dendriticum infection, the predominant cell population was CD3-positive and associated with lesser numbers of CD79α- and Iba-I-positive cells. An increase in Iba-1-positive cells was observed in the livers of animals with a high worm burden. Our results provide a reference basis to better understand the local immune response in sheep naturally infected by D. dendriticum in relation to the FEC and parasitic burden.