3.1. Histological changes
The biliary system consists of two main parts: the intrahepatic and extrahepatic bile ducts including the gall bladder. In this study, the intrahepatic bile ducts of the hamster were classified as the first and second order bile ducts. The second order bile ducts were large bile ducts and their main branches, starting from the hepatic hilus, are characterised by the presence of a fibrous ductal wall and lined by low columnar to columnar epithelium. They corresponded to the left or right hepatic ducts, segmental ducts and area ducts in human [16] and [17]. The first order bile ducts were small bile ducts corresponded to the septal and interlobular bile ducts, and bile ductules. Normally, the adult liver flukes inhabited the second order bile ducts and were not found in the first order bile ducts.
No obvious pathological changes were observed in any specimens in the control group. In infected hamsters, the liver changes were similar to those previously described [4]. A distinct inflammatory response was observed around the juvenile flukes in the intrahepatic second order bile duct at the hepatic hilus and extrahepatic bile duct by day 3 p.i. Mild inflammation without flukes was also seen in the periportal areas and first order bile ducts. The cell types mainly comprised mononuclear cells and eosinophils. The severity of the inflammation gradually increased from day 7 p.i. onwards. Heavy mononuclear cell and eosinophil infiltration around the intrahepatic bile ducts was noted in the areas with epithelial desquamation. Focal liver cell necrosis was also observed early in the infection. Maximal inflammation was established by 1 month p.i.
From the second month onwards, active inflammatory changes around the bile ducts (both the first and second order bile ducts) regressed slightly and lymphoid follicles, as well as plasma cell infiltration, were predominant. Active germinal centers were seen in most large lymphoid follicles of the liver, extrahepatic bile duct and gall bladder. The most obvious histological changes in the liver in chronic infection were periductal fibrosis, ductal dilatation and mononuclear cell infiltration with lymphoid aggregation. The pathological changes were more severe in the liver and extrahepatic bile duct than that in the gall bladder. Granulomatous inflammation around fluke eggs was frequently seen in the intrahepatic periductal areas and protruding from the mucosal infolding within the bile duct lumen.