The histopathology of Fasciola hepatica infections in sheep

Abstract

Experimental infections of lambs with Fasciola hepatica are described. The growth rate of the parasite, time of entry to the bile ducts, and time of patency are recorded and a preferential migration of the parasite in the liver parenchyma noted. The gross and histological lesions produced in the liver from 1 to 40 weeks after infection are described and compared with previous observations in cattle.The parenchymal migration of the parasite is shown to consist of two phases, a free migrating phase up to the 6th week, and a localized phase after the 6th week prior to entry into the bile ducts. Hepatic cell regeneration is observed and hepatic fibrosis is minimal. The localized phase of migration is associated with a unique peripheral palisade of giant cells in the fluke tracts and with the formation of pseudofollicular aggregation of lymphocytes. The presence of flukes in the bile ducts produces fibrosis of the duct walls. The walls, however, remain pliable and expanded to accommodate the parasites and calcification was never observed.