Hepatic fibrosis in experimental schistosoma japonicum infection in pigs : a histopathological and immunohistochemical study

Abstract: Schistosomiasis japonica is a zoonotic, parasitic disease caused by the trematode Schistosoma japonicum, with Oncomelania snails serving as the intermediate hosts.Schistosomiasis japonica is endemic in China, the Philippines, and Indonesia and is a major public health problem. The inflammatory response to the schistosome eggs in host tissues leads to formation of perioval granulomas, especially in the liver and intestine. In the liver,this eventually results in chronic portal fibrosis referred to as pipe-stem fibrosis. The pig has many biological similarities with man and is also a natural host for S. japonicum, which has led to exploration of this species as a large animal model of human schistosomiasis japonica. In pigs, there is marked portal and septal fibrosis of the liver in the early stage of patent infection, whereafter the fibrosis gradually resolves due to spontaneous recoveryfrom the infection. The pig can thus serve as a model of the development and regression of schistosomal liver fibrosis.In the present study, twenty-six pigs were divided into six groups, A to F. Groups A to D (n = 5/group) were infected with Schistosoma japonicum cercariae and groups E and F (n =3/group) served as uninfected controls. The degree of fibrosis was assessed at 3 different time-points upto 21 weeks after a single infection and after a primary infection followed by a challenge infection, using semi-quantitative histopathological scores as well asquantitative image analysis on liver sections and the two methods were compared. A possible correlation between fibrosis and liver tissue egg counts (TEC) was investigatedand the fibrotic lesion was characterised by immunohistochemical detection of α-smooth muscle actin (α-SMA), desmin and collagen type-1.Major histopathological lesions were egg granulomas, diffuse portal and septal inflammatory cell infiltration, and fibrosis. Granulomatous obstruction of portal veins wascommon. The degree of fibrosis varied from mild to marked in all four groups of infected pigs and there were no significant differences in liver fibrosis scores or area of fibrosis between the groups. The measurement of the area of fibrosis by image analysis was found to be well correlated with the semi-quantitative histopathological scores for total fibrosis i.e. the sum of portal and septal fibrosis scores, as well as for portal and septal fibrosisrespectively. There was a positive correlation between liver tissue egg counts (TEC) and septal and total fibrosis scores as well as the area of fibrosis.Expression of α-SMA was detected in connective tissue cells in portal and septal areas of normal and infected livers, but the numbers were increased proportionally to increases of the degree of fibrosis in infected pigs. Similar expression was observed in thickened portal veins of infected pigs and in hepatic stellate cells (HSC), smooth muscle cells of vessel walls, pericytes, and sub-epithelial cells of bile ducts of all pigs. Desmin expression wasdetected in HSC and in smooth muscle cells of portal veins, hepatic arteries and bile ducts in both normal and infected pigs. In areas with portal vein destruction, scattered desminpositive cells identified as smooth muscle cells were often found. Collagen type-1 was present in increased amounts in portal and septal areas in infected pigs. In granulomas, there was a peripheral network of α-SMA-positive flattened, fibroblast-like cells andconcentric collagen type-1-positive fibres, whereas desmin-positive cells were not observed.In conclusion, this study demonstrated a correlation between liver fibrosis and liver TEC. Good correlation was also found between the area of fibrosis as measured by imageanalysis and the semi-quantitative histopathological scores, making image analysis a useful additional tool for assessment of liver fibrosis in the pig model. In fibrotic portal and septal areas α-SMA-expressing connective tissue cells and collagen type-1 were increased.Collagen type-1 and α-SMA, but not desmin, were also detected in granulomas. Desmin could be used as a marker of portal vein destruction.